Tire size and FE with a Jeep
 

So I split this off my XJ thread.

When I bought my Grand Cherokee it had 235/75R-15s on it. While I was able to go most everywhere I wanted to go. But despite the factory 1" lift, I wanted a little more clearance. Mindful that I hadn't become concerned about FE yet. For reference, this Jeep has the 4.0 inline 6, 4 speed automatic, and 3.73 gears as part of a towing package. I sought this particular Jeep because of the lift, factory skidplates and tow hooks. I wanted to go offroad, didn't care much about FE, but then again, I was since I knew this got better mileage than a Wrangler.

Which leads up to now. The Jeep already had 235/75R-15s. They were All Terains. Last year I swapped up to some 30" tires, I got the set of tires and wheels for $80 and they made it over a year. Money well spent. They were also an All terrain type. I recently swapped to a different set of wheels with 31" tires, though they are more mud terrain tire this time. They were inexpensive, and I am cheap ($100 fro the wheels, $100 for the tires).

Pleasant surprise, as far as the taller tires for my ZJ, I noticed today on the freeway, I am spinning lower RPMs for the same speed than I did with the 30" tires. More important, it spins even lower than when I ran 235/75R-15s... Downside, the 31s are wider than either of the other tires, and they are just barely inside the fenders.

Progression, started with 235/75R-15s, about a 28" tire. Didn't like these wheels on my ZJ and they don't fit real well under my daughter's XJ, they rub when turning...
http://i178.photobucket.com/albums/w...21807RS001.jpg
Yes, I was the lowrider of this group, and the smallest tires. While I made it through this set of trails this day, I realized I needed more clearance. Either bigger tires or some additional lift. Or both.


30s, these are the tires I was on to achieve Hypermiler status with the ZJ.
http://i178.photobucket.com/albums/w...ckrock0013.jpg
These weren't too bad in the snow, but not as good as the previous tires. But, they were great while they lasted for FE.


31s, Only been on for about 6 weeks, my kids were using the ZJ and aren't good for at FE. Though they do try.
http://i178.photobucket.com/albums/w...reeride008.jpg
Yes, I was stuck in this pic, and yes, it is a purpose built offroad park. Resting on a skidplate. It hangs too low, and it will be modified to be more flat and tucked up closer to the bottom of the Jeep. But first, it will be getting a 2" lift, replacing the front control arms too. I believe I have mentioned this lift frequently. Again, I do not need massive amounts of lift, or humongous tires. This should be sufficient...

So, I will see if I can get similar mileage. The bigger tires create a little more rolling resistance, I have them currently at 35 PSI. But they also reduce RPMs, so, will they offset each other? I'll see what I can do to smooth the bottom of the Jeep this year. I still want to build a front bumper that might slice the air better and provide solid protection as well (we have a little deer problem in the mountain pass I drive daily...)...

Anyone familiar with Totegotes...
http://i178.photobucket.com/albums/w...otogote001.jpg
Winter project...

nice mini-bike, looks like a fun project :thumbup:

on my explorer sport (yes, 2wd), i stepped up from a 235/75/15 to a 31x10.5 heavy, off-brand mud tire. got them real cheap because "mudstar" is an off-brand of "buckshot" which is an offbrand of "procomp". plus they are "blemish" tires (minor cosmetic flaws ONLY).

i have also lifted it a little, but that was after the mud tires (they fit with no lift)

also, you mentioned more lift in the plans. are bigger tires (than 31's) in the plans? staying with all-terrains?

pros:
-i like lifted trucks
-pot holes and other road "irregularities" are a piece of cake
-rpms are lowered
-i cleared a dead deer on the highway that would have really messed up a car. (night driving, and i only swerve for children)

cons:
-mpg has suffered (breaking over 20 is really hard)
-i think the mud tread has a lot to do with this
-transmission will downshift out of OD a lot easier

from my experience, they will offset each other somewhat. better than running tsl's at 25 psi. but not back to stock.

i think you are on the right track with the underbelly smoothing. i think it may be more beneficial overall (road and trail) to put the "lift" money into smoothing out the belly. get everything up out of the way, and start making skidplates. you'll probably go with a little stronger materials than the "norm" on this site. but its dual purpose: less air turbulence (street), and better clearance/protection (trail)


that all being said (sorry, this post is going to be even longer), i am planning a "B-A" test of my "offroad" modifications. with my trail rig almost finished, the explorer will be retired from bad roads, camping, and fishing.

i have kept an excel file for 1 year now with my mpg almost everytime i fill up. my vehicle has not really changed in that time (except little stuff to try and get better FE)

i'm taking the lift off, throwing on stock sized street tires, and possibly lowering it a little. plus some little things like a grill block and maybe some smoothing underneath.

once i do that, i can compare my FE with the same month of the previous year. after i make the change, in my fuel logs (here at ecomodder), i'll post my previous year FE with it. should be interesting....

I worked pretty hard to get nearly 22 out of the ZJ a couple tanks. My kids had the ZJ for the last couple months while I fixed my daughter's Cherokee. And I drove my Neon. And along the way, I didn't update the mileage logs. Shame on me. I'll fix that.

Most of the lift parts were free, friends upgrade all the time, I just need to replace bushings in the control arms. And 2" is all I'm doing. I've said here and with my local 4x4 forum that 2" lift and 31" tires is all I'm doing to this Jeep. And staying with All Terrains. I will also add some armor to the bottom and keep it tucked. Like you said dual purpose, protection and smoothing the airflow. And well, a little more stout than coroplast.

Now, I might be getting a 93 explorer 4x4 with a dead transmission. Really cheap. I've got a friend that is a tranny tech that will walk me through a rebuild. I've always dreaded the thought of a tranny rebuild. I'm just trying to decide what direction to take the Ex. Leave it alone and drive it and make my jeep bigger, or lift it bigger than my Jeep with 35" tires as a more trail oriented rig and keep the Jeep as the daily driver. Immaterial if I don't get it. But fun if I do. I have everything needed to lift the Explorer, and the 35" tires too. Just some stuff I yanked off the Ranger when we changed the lift under it...
http://i178.photobucket.com/albums/w...ogPaint005.jpg

I know that some folks here don't agree with my hobby and to each their own. I am an advocate for land access and responsible use. But I do try to save gas where I can. And even though gas is half the price it was this summer, or even cheaper than its been in three years, I still intend to save where I can. Once I'm on the trail and in 4lo, there isn't much I can do to save though.

The Totegote minibikes were made here in Utah from 1958-1970. I got the bike for free from my good friend Scott. He recently rehabbed one too and he got this one free, and passed it on to me. It needs some help but I'll get it ready by spring. (Though riding one through the snow sounds like fun too!) Then Scott and I can head for the hills and go where our Jeeps can't. Just not very fast... 7HP only goes so fast.

Lots of entertaining stuff here:

http://users.infoconex.com/~ramrod/tgland.htm

My 86 cj7 4cyl with the stock tires would barely get 20mpg if driven easy and if I can remember was around 18mpg average. I went with a 3 inch lift and 35x12.50 tires with the stock 2.73 gears and my mileage really sucked(speedo read 40mph I was doing 70). I was lucky to get 15mpg driving it easy. I had to carry a 5 gallon gas can just to get enough range to drive to school. I went to 5.38 gears and that made the speedometer almost right. It read 5mph over at 55mph so it was pretty close but I had a bit higher rpms than stock. With that gear and tire combination I was back to 20mpg if I drove it under 55. Over 55 it really sucked with the 4spd trans, after swapping to a 5spd it didn't pick up any additional mileage, but at least I could drive 60mph without having to replace the motor every 5000 miles. Lowering the rpms isn't always good unless the motor is still in a range that it has good power and is able to hold your speed without a lot of throttle I guess.

With the bigger tires as long as I was real easy accelerating and drove slower than I did with the small tires the mileage was near stock. The biggest problem with the bigger tires is it exposes more tire to the wind and the weight makes accelerating take a lot more gas. So lower speed driving at a constant speed might gain mileage but anything else is worse for mileage at least in my case. Hopefully with the 6 cyl engine yours can handle the rpm drop and the weight of the tires and gain some mileage.

I have not driven that jeep in years and will probably sell it soon though. Anyone want to buy a slightly modified jeep :rolleyes:

well, you've got a friend here! i love to explorer the outdoors. i may use gas when i do it (truck or ATV), but i also enjoy hiking, canoeing, and mountain biking. but i know what you mean. irresponsible 'wheelers always grab the spotlight, even though they are the minority. makes the responsible majority (like us) look bad, no matter how hard we try.

My intro shows my Bronco II project (which includes quite a few explorer parts). my buddies all say to swap in a v-8, but the little 171 ci v-6 has enough power to get where i want to go. i'm actually "downgrading" it from its former glory to be more fuel efficient and drivable on the street.

love the ranger, and love the idea of an explorer project. check out BroncoII.org. we have lots of info for rangers, explorers and bronco 2's. a rebuild isn't your only option for that transmission. an upgrade may be more beneficial in the long run. and a small lift and a sawzall will get 35's under it.

i say build up the explorer, the real frame is stronger (of course, i'm a little bias). but if you 'wheel with a lot of jeeps, parts for the grand will be easier/cheaper to come by.


since we are on the subject of mini bikes, this is what i'm trying to convince my father-in-law to give me
http://i2.photobucket.com/albums/y46...e/IMG_1023.jpg

i love mini trail bikes. easy to transport, go anywhere, and cheaper (on gas and other prices). going slow is just fine with me. being in a hurry just makes the fun end faster.

Already bookmarked that Frank! Its fun to watch locals eyes glaze over remembering their Totegotes in their youth. They were quite popular in the early 60s as a ready built way to get to hunting areas and such. They were heavily marketed to the hunters. But they proved to be as much fun as they were workhorses. I'm not from Utah, nor am I of the local faith, but someone jokingly told me if I get the thing running and use it, it almost makes me a native... I've already got plans to build a slide in rack for the trailer receiver on the Jeep so I can take it places.

Wow, 4 banger, 35s, 4 speed and 2.73 gears? A wonder it even got rolling! I never understood putting the 4 in the CJ7. On our Ranger 4x4, its got the 2.3 liter 4 with a 5 speed. On the 31" tires it does great around town and on the trails. But the highway, its doing good to make it 80. 65 nets around 28 MPG when I was driving it to work daily though(through a mountain pass 50 miles each way), and that is even with the 6" lift and flat front these things have. So the next phase for the Ranger is a set of 33" tires and a move to 4.56 gears. I'd prefer 4.88 but they aren't made for the rear axle we are using.

The 6 is plenty strong 1500 to 3000, so maintaining speed isn't a big problem. I haven't floored it to get to highway speeds in a long time, though sometimes onramps make me consider it. The truck was built and programmed for the old 55 MPH speed limit, so getting what I have gotten at the slightly higher speeds is pretty cool.

Yes, the bigger tires have more weight, but I see that as an advantage to help maintain speed. And I am pretty easy accelerating. The tire and wheel package has the tires just inside the fender line, so they aren't out in the breeze. There will be a little more tire exposed with the lift, but that is still a couple weeks away. I can get some numbers before then though.

Oh, and on a daily basis I'm not carrying near as much stuff with me, but I do have a floor jack, straps, tools, water, emergency food, first aid kit, blankets and the like. I drive a mountain pass and its geting to be winter and snow, and have used all the above to help others. Not for myself yet. Hope not to, but, never can tell. For offroading and extended offhighway travel I carry a lot more of the above and all fluids I might need, spare parts, and spill kits. But I digress...

what rear axle are you using? still got the stock 7.5? an 8.8 out of an explorer is an easy upgrade, with 4.88's available.

Tony, land access and land use issues are of great importnace to me... I'm wearing a tshirt right now supporting a land access group, Utah Four Wheel Drive Association. I also support Blue Ribbon coalition, participate in fundraising/membership drives, trail maintenance and cleanups. I don't care if I ever get to all the trails I want to run (and there are plenty), but keeping public lands open for the public is very important. YES, the bad apples get all the attention. We work with private land owners, Forest Service and BLM, and even state and county officials. One of the clubs I participate in gets notice from the BLM for our efforts. We have adopted a couple of trails here in Utah, and wish we could adopt more. Maybe in time. I just hate seeing folks riding their ATVs and dirt bikes off trail and I try to talk with them where I can. For most, if they KNOW what they are doing is wrong, they change their ways. Another group I'm with is very active at recovering stuck vehicles that are in places they shouldn't be, or tried to get somewhere in a vehcile not capable of being where they got it. Excellent opportunity for education and a very humbling experience for them.

I like that site, I also am over The Ranger Station, Explorer Forum, BroncoII-Ranger, Rough Rangers, and a couple other RBV forums. I see some upgrade potential with the
A4LD trans, even bringing up to a more modern standard internally. I've discussed this with my transmission expert buddy... I wheel with Jeep guys, and a few Explorer guys. I will say there is a big size difference between the early Explorers and my ZJ. There are some places tight enough that my ZJ is about the longest wheelbase I can take through areas. The Ranger is nimble enough that it would be fine, but the Ex wouldn't stand a chance. Though most areas it won't matter. It would be nice to have a different sort of arrow in the quiver. And the Totegote is certainly a different arrow too.

Some thoughts:

1) Differences between tires - meaning make and model - can have a HUGE effect on rolling resistance.

2) Increases in tire size improves rolling resistance, but it's effect is small compared to the difference between tires (See #1)

3) Aero effects caused by tire size differences are even smaller than differences in rolling resistance caused by tire size differences.

4) Changing from P metric to LT metric or Flotation sizing is going the wrong direction for rolling resistance. LT metric and Flotation tires are built out of similar materials and RR is hardly a consideration. If it ever becomes a consideration, they will always be worse than P metric tires because of the loads they are designed to carry.

I support all of that here:

Barry's Tire Tech

So if you are going to report on tire size changes, be aware that the largest effect in rolling resistance is going to be the tires themselves and not the size change. (Please note: I am not commenting on the effect tire size may have on engine rpm and the change in fuel economy derived from that change. I am not aware that there is any data on that!)

I was thinking that a good way to increase your FE in that jeep is to get class d tires i think they Are because there able to hit 80 psi and i figured that with LRR tires all they've done is increased tire pressures, i mean think about it this way my jeep is 5300 pounds ok 2750 in front 2550 in the back if you take though's and divide them by 2 and then your tire pressure it gives you square inches that your tire is on then you divide by tire width and you have inches from front to back and the higher the pressure the less tire contacting the road and less RR.

there you happy now Frank;):rolleyes:

dr vna gv ths gy sm vwls k? tnks
http://i146.photobucket.com/albums/r...cher/vanna.jpg

First, LRR tires are all about the materials used. You can test differing tires of the same size at the same load and inflation pressure and get HUGE differences in RR.

Clearly increasing the inflation pressure does decrease rolling resistance. But what are labeled as LRR tire is connected to the RR when you compare them at the same conditions - meaning size, load, and inflation pressure.

And, no, the formula you've provided is not valid. You can not calculate the area of the footprint just given the load and the inflation pressure. Put another way, the average ground pressure of a tire's footprint is NOT the same as the inflation pressure.

And in case you are thinking that the friction with the road is what RR is all about, that would be incorrect. RR is mostly about the internal friction the material in a tire generate.

Check out this explination on tires and there psi to weight figures http://auto.howstuffworks.com/tire4.htm
it says u can guess pretty close to the weight of your vehicle just by looking at the area of your contact patch and yes i understand that your not going to be exact or anything with that calculation but i think you would get pretty dang close. I don't think that friction with the road is all RR is about but if you increase the tire pressure not only are u decreasing the contact with the road but the amount of deforming the tire does therefore less energy converted to heat in the tire now if you could get class D tires with 80Psi max and with the low rolling resistance material then you'll have really good FE gains. i got the idea from watching mythbusters the other night they took tires that had a max psi of 35 and increased it to 40 and got a 6.5% increase in FE and thats 2 points better then the best LRR tire i've seen so far but if you find a better one then let me know ok.(and yes i know i'm not goin psi to psi ok but it steal works)

Sorry, it's an old wife's tale that a tire's contact pressure and inflation pressure are somehow related in a linear way. Here's a link that is a lot more detailed on the subject:

Fact or Fiction? Tire contact patch and air pressure.

As you can see, the contact pressure and the inflation pressure don't seem to be very close at all.

And as far as Mythbusters is concern. I saw that episode, too. They measured the fuel economy at steady state - and the effects on inflation pressure are a lot more easy to measure - which is why they did it.

But let's start off with this as a basis:

http://www.energy.ca.gov/transportat...%20Testing.pdf

Bruce Lambillotte of Smithers Scientific Services is reporting a 60% difference between the best tire and the worst tire in 2 different sizes - pages 11 and 12.

That is HUGE - and much bigger than any test results I have seen for inflation pressure increases. Don't get me wrong - I am not saying inflation pressure increases don't improve fuel economy, but compared to what the differences can be between tires, changes in tire size are small, and changes in inflation pressure are next, but differences in tires is the largest of them all.

And you have to be careful when you change tire types. P metric tires are constructed quite different than LT metric tires - and as a general rule, LT metric tires - even with their 80 psi pressure - can be completely out classed by an efficient P metric.

I did my own measurements of pressure vs contact area and indeed there was no direct correlation.

so then technically contact size has nothing to do with efficiency and only the deforming of the rubber on the tires and the internal friction but increasing tire pressure would reduce the tires deformation therefore increasing FE by reducing the heat generation right???(and also tire material for internal friction)man i hate when people publish bad info on the web but thanks for the redirect:D

Not exactly. The size of the contact patch does relate to rolling resistance. However, the size of the contact patch is not linearly related to the inflation pressure nor is rolling resistance.

Also, you will find many old wife's tales on the internet. It is no better than the average group discussion.

"Not exactly. The size of the contact patch does relate to rolling resistance. However, the size of the contact patch is not linearly related to the inflation pressure nor is rolling resistance."

Sorry i was trying to get to this conclusion but i was in a hurry. i was also just wondering if the bulg of the tread could have a pretty big factor in RR for example my dad has 31 inch tires with 1/4 inch bulg will i have 29's with 3/4 inch bulg both pairs are new?? Thanks agian

Let's start with how tires are tested for RR. All RR test procedures specify a load and inflation pressure for each test point. The standard test methods specify the load as a percent of the rated load and specify the inflation pressure based on the rated inflation pressure. Both of these are found in the load tables as published by the tire standardizing organizations.

Please note: The rated inflation pressure is NOT necessarily the sidewall inflation pressure - and this is particularly true for passenger car tires.

Some RR test procedures specify a single test condition, and some have multiple test conditions. The result is sometimes expressed as a single point and sometimes expressed as multiple points.

However, enough testing has been to be able to say that tires behave in similar ways regardless of the test procedure and the various test conditions - that is, the test procedures will give fairly good rank order when comparing different tires.

Needless to say, you can not compare different tires if the test procedures are different - without knowing more.

So if you want to know the difference in RR between your Dad's truck with 31"er's with a 1/4" bulge and your truck with 29"er's and a 3/4" bulge - well, that's not enough information.

Obviously a tire with a 1/4" bulge is more lightly loaded than the same tire with a 3/4" bulge - and by "more lightly loaded" I mean a percentage of the load at the inflation pressure being used. Notice that I am referencing both the load and the inflation pressure.

But your question involves different sized tires- and likely a different make/ model. If you've followed along, you would have seen that make/model has a HUGE effect on RR. There's also an effect caused by the difference in size.

So there are difference in tire size, load, inflation pressure, and make/model. Too much information to make an estimate.

ok sorry for the miniscul info but the 31's are 80 psi Goodyear workhorse and are on a 8300 pound truck my 29's are 50 psi mesa A/P and on a 5300 pound jeep i tried to get info on there RR but couldn't find anything cause the browser would keep showing stuff that wasn't what i wanted or even what i was looking for. and i was referring to the tread bulging not the side wall

Hmm, didn't exactly intend for this thread to last this long or become such a huge debate. Only real update here is that its winter, and we have mandated alcohol content in the fuel. Sucks. Current mileage is in the mid 18s. Still good considering its still mixed driving. Getting close to a new set of tires. And I really have to consider what I want. Giving a great deal of thought to moving to a 16" wheel so I can get a narrower tire. And that moves me into tires meant for trucks like F350 duallies.