Light Truck Tires - Width and Diameter

[LeanBurn]

Not to hi-jack this thread, but my GMC Sierra (as listed below) needs tires. Currently at 90kph or 55mph on the speedo (but 87kph or 54mph on the GPS), it turns at 1800prm. I have a running average of ~12L/100km or 19 mpgUS unloaded and not towing. I am going to still get Load E, LT tires (So I will retain my load carrying capabilities), but I want to change from the 245/75R16 to 235/85R16. So 10mm skinnier and 10mm taller.

So in making this tire size change I will correct my speedo to actual speed, reduce aero profile of the tires and lower my rpm at speed.

I did this tire sizing with the 1997 5.7L suburban I used to have and obtained some really decent mpg numbers for a nearly 20 year old truck. In contrast it loafed along at 1600 rpm at a GPS verified/speedo correct 90kph(55mph).

This truck has a lot more power and torque as well as a tow/haul mode I didn't have with the suburban (I had to used 3rd gear) (both have same 3.73 rear gearing, so I don't feel it will hurt me in the towing/hauling department a bit.

245/75R16 IMPERIAL METRIC
Section Width: 9.65 in 245.00 mm
Rim Diameter: 16 in 406.4 mm
Rim Width Range: 6.5 - 7.5 in
Overall Diameter: 30.47 in 773.90 mm
Sidewall Height: 7.23 in 183.75 mm
Radius: 15.23 in 386.95 mm
Circumference: 95.72 in 2,431.28 mm
Revs per Mile: 682.67/mi
Actual Speed: 62.14 mph 100.00 kph


235/85R16 IMPERIAL METRIC
Section Width: 9.25 in 235.00 mm
Rim Diameter: 16 in 406.4 mm
Rim Width Range: 6 - 7 in
Overall Diameter: 31.73 in 805.90 mm
Sidewall Height: 7.86 in 199.75 mm
Radius: 15.86 in 402.95 mm
Circumference: 99.68 in 2,531.81 mm
Revs per Mile: 655.57/mi
Actual Speed: 64.71 mph 104.13 kph
Speed Variance: 3.97% too fast.
Diameter Variance: 4.13%

Thoughts?

[wjohn]

Originally Posted by elhigh

Here's a situation where I don't think the "buy a trailer instead" suggestion is going to work. The Dakota is just starting to feel the load by the time a trailer-equipped Civic is at the outer edge of its capacity.

I completely agree. Probably 75% of folks don't actually need a truck, which is why you see so many "family haulers with open-air trunks" as I like to call them, i.e. crew cab trucks with dinky little beds.

For me, trying to tow anything with the Civic is absurd. Like I said, it's a great car and for 75% of my personal driving (to and from work, long drives to visit relatives, etc.) it does the job exceptionally well - I can get around at 40+ MPG. Trying to hook a trailer to it to move hay, tractor implements, or lumber? I've never seen any tow ratings for this generation of Civics and nobody makes a trailer hitch for them, either. The clutch and a lot of other components certainly weren't designed for much weight.

Having the 2000 pound payload capacity of the Dakota means I don't have to mess with a trailer at all at this point. I'd be hesitant to pull any more than 500 pounds with the Civic - once you deduct the weight of even a very small HF trailer, you're not left with much of anything.

[wjohn]

Originally Posted by CapriRacer

I think flotation sizes ARE LT tires.

If you look carefully, you will find that some folks include the letters "LT" AFTER the size - which is the proper way to designate the size. Those letters are frequently omitted - as is the letter P in front of some P metric sizes.

But as I said before, the range of RR within a given size is HUGE!! So a comparison of a flotation sized tire to a regular LT tire, while somewhat problematic, is small, small potatoes.

I'll admit to being slightly confused about current LT designations. General's site has a spec list for the Grabber that I will post in a minute after I've reached my magical "5" post count that allows me to post links. They have three different 235/75R15 tires, with one of them being designated as an "LT" that, most notably to me, weighs .6 pounds more than either of the other 235s. Max load is close to 2000 pounds for each of the tires. If I go with these tires, I'll probably go with the 109T-rated mainly for the higher PSI rating.

[wjohn]

The promised link:

Grabber HTS

[CapriRacer]

Originally Posted by wjohn

I'll admit to being slightly confused about current LT designations. General's site has a spec list for the Grabber that I will post in a minute after I've reached my magical "5" post count that allows me to post links. They have three different 235/75R15 tires, with one of them being designated as an "LT" that, most notably to me, weighs .6 pounds more than either of the other 235s. Max load is close to 2000 pounds for each of the tires. If I go with these tires, I'll probably go with the 109T-rated mainly for the higher PSI rating.

To explain the 3 different tires:

One is an LT tire. LT is a designation about the relationship between the load carrying capacity, tire size, and inflation pressure. As a general rule, LT tires use more inflation pressure to carry the same load compared to P type tire - BUT - P type tire do not carry as much load.

One is a Standard Load (SL) tire - and since they didn't put the "P" in front of the tire size and they are a European based company, I assume they are using the European tire standard.

And the third is an Extra Load (XL) tire. In theory, this XL will have ever so slightly more RR and the SL - because it has ever so slightly more mass.

And just so you know, the maximum tire pressure listed on the sidewall isn't a rating. It isn't anything other than an advisory not to exceed that value. It has no relationship to what pressure works best for your vehicle, because the tire manufacturer doesn't know the characteristics of your vehicle.

The vehicle tire placard, which lists the original tire size and the specified inflation pressure for that size, will be located on the door or doorframe (and since you vehicle is older than a 2008, it might be in the glove box). You should reference that as the vehicle manufacturer did all its testing at that pressure and set the vehicle up to perform a certain way. Changing the inflation pressure is going to change the handling characteristics of the truck, so be careful here.

And perhaps it might be appropriate to talk about the differences between an d"LT" tire and a "P" type tire.

You might think that these designations indicate the type of vehicle the tire is designed for - and you would be sort of right. But what those designations really mean is the tire standard the tire manufacturer is working with. P type tire designations are low pressure tires designed for lower load carrying capacity. In the same size, an LT tire can carry more load, albeit with a higher inflation pressure.

One quirk is that P type tires have to have theiir load carrying capacity derated by a factor of 1.1 to be used on a pickup truck, van, or trailer - where an LT already has that factor built in.

But LT tires are built out of different materials than P type tires - typically not good for rolling resistance, because LT tires carry much more load for a given size because of the higher inflation pressures used. This is more evident in that larger sizes - like an LT245/75R16 Load Range E which has a load carrying capacity of 3042# at 80 psi where a P245/75R15 SL (Standard Load) has a load carrying capacity of 2271# at 35 psi, which would be derated for pickup truck usage to 2065#.

Because of this higher loading, the materials used in an LT tire are more resistant to a phenomenon know has compression set - where the rubber permanently deforms due to load. What comes with that change of materials is a higher RR.

[wjohn]

CapriRacer, thanks for that explanation. I did not know about the derating factor of 1.1 for trucks. Looks like if I go with these tires, I'll definitely want the 109T/XL version since the de-rated load capacity of the 105T/SL tires is right at my 3650 GAWR. I also see that Dodge only specs XL tires for the 235/75R15 size in the owner's manual, and my door spec plate matches that.

I won't have 2,000 pounds in the bed of this thing at all times (like, say, if I were to have a utility bed full of tools or something) but it will definitely see that amount of weight now and then.

With the tire pressure, the spec plate does call out the old standard of 35 PSI front and rear. The manual says to add 3 PSI for high speed driving. I guess my experience with the older Hondas has led me to somewhat ignore the manufacturer's spec - for both my Civic sedan and hatch the pressure spec was 32 PSI front and rear. It's glaringly obvious that this was done for ride quality, as the fuel economy and the handling both noticeably improve with air pressure at 38 PSI or so. The sedan especially handles like a completely different car at higher pressure. I will run 38 PSI in this truck but may not go much above that as I doubt it'll help the handling of the Dakota as it does with the Civics.

The old Michelin LTX A/Ts that are currently on the truck were at 50 PSI when I got it (minus the leaking front left on the smashed up rim). I just went and checked and they are actual LT tires, though.

So, I think I don't need the max PSI rating of the 109T/XL Grabbers, but I do need the load capacity. Plus they should still be better than the LT version of the Grabbers and hopefully better than the Michelins for fuel economy.

[wjohn]

I filled up after 200 miles and observed 21.5 MPG from some very mixed driving with these old Michelins. That's not much of a sample size but I may not burn another tank before I get new tires and fix everything.

Overall, I'm pretty pleased. I think 25 MPG will be a good goal with lighter wheels and tires, removing the mud flaps, and fixing a bunch of things like the brakes. I'd be very happy to get that out of this 4WD truck with the drag coefficient of a brick

[trooper Tdiesel]

my isuzus use tire sizes in that same range.some years are OE with P rated tire others are OE with LT's

1984/1986 they have a 225/75/15P tire with a 26 psi rating

1987/1991 they have a 235/75/15LT with 4.55 gears or a 31/10.50/15LT with 4.77s making for about the same rpms with ether size...yes bolth tire sizes are on the door panel 87/91
the LTs have a OE psi rating in the upper 20s low 30s versus the P rated tires at 26.

other then the early years have lighter axils and carbed engines or a rare diesel burner all under 100hp. and the later ones have bigger axils with EFI or TBI engines over 120hp theres next to no difference in the autos....


the same rule applies to the 95 ford explorer i did an engine swap to, its OE with 235/75/15P tires and have a presser rating of around 26psi as well.

ive tried running P rated tires at 26 psi on my SUVs, other then having more grip on snow pack or off road, i can't stand it. as it feels like it wants to wander all over the road compared to 40 psi

[wjohn]

I dropped off the wheels and then picked up the new tires for my truck last weekend. I've been busy changing the timing chain, oil pump, rear main seal, CV axles, brakes, and piles of other stuff, but I did throw one of the new wheels on last night and took a picture. I ended up going with the Grabber HTS with the 109T rating in the stock 235/75R15 size.

The wheels are factory cast aluminum that were an option on the '87-'90 Dakotas. The old wheels stuck out too far for my liking, had an unknown load rating and some had wallered out lug holes. The front left wheel was smashed up a PO hitting a curb or a pothole.

I'm hoping to get this truck finished up and back on the road soon - I have some long-ish trips in the future that would be excellent for getting some highway mileage readings. When that happens, I'll report for the sake of tying up this thread.