Taller Tire Test (5.7% MPG increase observed on Honda CR-Z @ 55-65 MPH)
 

Some work I just completed...

Edit by admin: see attached spreadsheet for raw data and full text; some of the spreadsheet text is quoted below...

second wind,
Nice job!!!!

I have a thread with the exact same results.
stock 245/45/18 26"
now 245/50/18 27.7"
I run 45psi.
Always thought the stiffer tire would bother me. Not one bit, but I also have electronic suspension.

As a note: truck do not tend to have the same results.
Also: IMHO you need to have 50% of your driving at 'cruise speed' to achieve a payoff. Since a larger tire is slower to turn on start off.

EDIT: on the comparision that I did it was Kuhmo escta on both sizes.

First: Good work!

But there are a couple of extra variables that are clouding the picture. Besides the tire size difference, there is also the difference in brand and tire type.

Were the Dunlop's the OE tire? That's another issue as OE tires are typically better for RR.

Bottomline: Be careful about characterizing this test as about larger diameter tires. There are other things that might be affecting the results.

Nevertheless, thanks for doing the work and sharing.

*****Edit****

Since I had done some work on tire size vs RR, I was wondering if I could quantify what affect the tire size alone had on RR.

Here's where I got the formula:

http://http://www.barrystiretech.com/rrandfe2.html

Doing the math, the change in RR due to tire size is 5.4%. So maybe the affect the difference in diameter is much smaller than you think.

CapriRacer: Yes, the Dunlops are OE. I realise that there is a certain "apples and oranges" quality to my study. However, I was very limited in the selection of tires. In certain lines, there are very few sizes made. The current fad is very low profile, and this is what the majority of tire buyers are looking for. I chose the Michelin tire based on the fact that it was 1 of only three made that would give me the revs per mile I needed.

All the confounds CapriRacer mentions being good to notice, I agree with him that this is great nonetheless. Others have found it really hard to get ideal conditions for testing too. Lots of work in this. Your conclusions suggest confirmation of other tests. Useful. Many thanks.

Good work. I took the liberty of making a plot with error bars:
http://ecomodder.com/forum/member-di...modder-com.png
Weight Rev/Mile Diameter Circumference
Dunlop 19# 856 24.4” 77 1/4”
Michelin 22# 786 26.5” 83 5/8”

Statistically speaking, if you were to repeat the tests over and over 67% of the results would fall in the span of the error bars and 95% would fall into 2x the span of the error bars.

Sure there's more than just tire size involved, but it's clear that the one tire is better. One other factor to keep in mind is that tread depth also plays a roll--meaning that worn tires typically have lower RR.

Another key takeaway is that 10 mph increase costs 10+ mpg.

Thanks a lot for posting this, SecondWind.

I hope you don't mind I quoted some of the text of the spreadsheet in your first post of the thread.

Taller Tire Test: City Driving
 

This is an addition to my earlier post showing the effect of taller tires on highway mileage. The car is a 2011 Honda CR-Z 6MT. The tires tested are the OE Dunlop 195/55R16 vs Michelin 205/65R16.

To determine the effect of the larger tires on city driving, a loop was laid out in and around Delaware, Ohio. The loop consists of 25, 35, 45 and 50 mph zones, and both stop signs and traffic lights. The loop also includes pulling into and out of some businesses. The idea is to duplicate running a series of errands.

The conclusions are that the larger tires do not affect city driving mileage. I expected, based on the opinion of others, that the larger, heavier tires would result in lower city driving mileage. I found that if I drove slower (due to traffic, etc.), that my mileage went down. I think that must be the result of more time spent in lower gears. I also found it difficult to drive consistantly. My data is not grouped together nearly as tightly as I would have liked. I think the point here is that in-town driving is strongly influenced by driving style. Highway driving is influenced much less by driving style; just a matter of setting the cruise...

The loop was run 5 times with the OEM Dunlop tires, 5 times with the Michelins, and 5 times again with Dunlops. As measured by ScanGuage II, the loop is 13.0 miles. By Google Maps, it's 12.9 miles, but Google doesn't recognize driving into a shopping center.

I drove in “normal” mode, and also as normally as possible. I do not use P&G, EOC, or any other hypermiling techniques. I shift at about 2,500 1st to 2nd; and at 2,000 or less up through 3, 4, 5, and 6. I use the brakes lightly, since it re-charges the battery, and
try to anticipte the traffic lights when possible. I decelerate in gear, again to re-charge the battery.


Run Time MPG Gallons Max RPM Max MPH
1 36.23 42.0 0.31 2,830 51
2 35.93 45.3 0.28 2,702 50
3 38.84 43.5 0.30 2,794 51
4 34.48 44.0 0.34 2,853 50
5 35.55 44.3 0.31 2,785 50
6 37.93 46.3 0.29 2,537 52
7 35.38 48.6 0.27 2,395 50
8 41.07 45.2 0.30 2,405 53
9 36.54 45.6 0.29 2,608 54
10 36.57 49.3 0.27 2,187 51
11 36.84 45.6 0.29 2,658 50
12 39.52 45.5 0.29 2,253 50
13 37.85 46.4 0.28 2,519 50
14 38.72 44.1 0.29 2,233 50
15 37.42 44.9 0.29 2,576 57

This is interesting--the taller profile of the vehicle should make aero drag worse, but apparently that's minor compared to the gearing effect. Too bad it's not as easy to swap gears as it is on a bicycle.

I went to 5% higher od tires and I am feeling it may be high enough gearing. I'm sure when they design a car they gear a car to suit the majority of people.Still think there should be some options in the gearing when you buy a new car.

Taller tires it is then.

capriracer- on the rolling resistance coefficient chart you have on Barry's Tire Tech it seems generally, taller and wider is better.
however the exact order that the 14" tires in sizes i might switch to seems inconsistent 185/75, 185/70, 195/70, 185/65, 175/65, 175/70. i have 175/70-14 tires.

195/70 out performed by a 185/70, 175/70 outperformed by 175/65, and 215/75 outperformed by a 3 sizes smaller tire 185/75.

do you think that overall rule of thumb, wider, taller is better works, and the specific tires used caused this? or do you think that there are certain sizes that cause an inherent disadvantage somehow?

What I did was run a regression analysis on all the data to derive a generaized formula that covers the 3 dimensions used in the tire sizing. The formula has an r squared value of 66% - which is NOT very good - meaning the formula doesn't explain the numbers very well. I would need to add more factors to get a better fit.

However, the data is what it is. It is the ONLY data I have seen that compares tires only by size. I have my doubts that this is EXACTLY true - and that may be why the data doesn't fit very well. I am pretty sure there are OE tires in that mix, but they do not jump out as outliers.

So I stand by the analysis. It is the best information available.

A side note: While you looked at 14", the same principle ought to work in 15". That why I did the regression. It looks at ALL the data.

I almost forgot about the wiki page I made on this subject.
Consider this added to the forum links.

I drove a crz with stick and I think the IMA gives it a great win for such modifications. Maybe even better if you had more power to supplement the IMA charge?

I found the crz didnt mind 6th gear at 35 mph and it was difficult to really lug the engine. I was rather disappoint at the performance at the 50-70 mph range of accelerating for merging in 4-6th gear.

I drove the cvt model too and just didnt like how the cvt shifts or should I say slide like you got a clutch going bad.

It is amazing though how the Insight can do 55mph at under 2 grand.

Be careful when comparing the average tire or other LRR tire to the Energy Saver A/S. in my testing I have found the Energy Saver A/S 195/65/15 to be 2-3mpg more efficient than the Prius OE Yokohama S33D and aftermarket Yokohama AVID Ascend. All tires had similar tread depth and conditions were very similar. TireRack.com also tested the Energy Saver A\S and showed it to be up to 3.8mpg more efficient than non-LRR but efficient tires like the Michelin Hydroedge and ComforTread tires. It's and extremely efficient tire!

As Secondwind has brought up the valid point before (either here or in person), that the Standard is more so aimed at younger people as a sports/sporty car, with gearing being aimed at performance.

I have to say, the CRZ might just be my favorite production car right now. No doubt at all if you don't include front rears. A nice splitter, maybe some performance mods if she needs and, and taller tires. Perfect. (in my opinion)

Glad to hear you!! I noticed your 90 day avg same as mine, similar car and drivers I guess.

I have to add this. When I switched to smaller tires on my cavalier the mpg and the speedometer were effected. It showed me as getting better mpg and my speedometer shower me as going 2-4 mph faster.

So if a larger diameter tire actually shows mpg gains, the gains are also multiplicative because it should decrease your mileage because it shows as your speedometer registering a slightly lower speed.

Anyone understand what I am getting at?
Larger tire shows a decreased speed and decreased miles on odometer.
Smaller tire shows increased speed and increased miles on odometer.

Was this factored into the mileage gains?

If you are going to change tire size and then trying to use the odometer or speedo (w/o recalibration) is silly.
You have to calibrate for the difference in doing the math for mileage.

All of my posts concerning larger diameter tires are based on:
1. Garmin gps for true mileage
2. calibrated ScangaugeII
note: SGII is calibrated in the metric mode w/ gps in the metric mode. this gives a more accurate reading when switched back to miles.

Hmm. I was under the impression that increasing the size increase the speed, and usually increase the mileage.

I mean, the cars calculate by axle turns. If one had 1 axle turn with 15s, then they would go so far on one axle turn. If one increase size to 20s, the cae moves way further for one axle turn.

I think the quoted may not be accurate. :/

As I am the OP on this thread, I'll say that the ScanGauge was re-calibrated between testing the OEM tires and the "tall tires". As I recall, the OEM tires used -3% setting, and the "talls" +7%. In both cases, the ScanGauge error was less than 0.1 mile in 10 miles. AFAIK, an improperly adjusted ScanGauge will affect both MPH and MPG readings.

Did you use a tape measure and measure to compare the actual circumference of each tire?

The new tires are taller, giving greater circumference and therefore greater distance traveled per revolution.

Also, a new tire has thick tread depth, but an old tire has thin tread depth, further increasing the difference in diameters/circumferences/distances traveled per revolution.

OTOH, an old tire with thin tread may have less squirm and rolling resistance than a new one, or being smoother, less aero drag.

Could be, the difference in mileage is attributable to more variables than just differences in nominal height, or that once the new tires wear down, they won't be as fuel efficient.

Per the original post, the tires were measured with a tape measure at 40 PSI cold, just prior to mounting the wheels on the car for testing.

Yes, which means the stock gauges would be under-counting mileage and speed when a larger diameter tire/wheel is used. The dash shows one mile traveled, when perhaps 1.25 or 1.33 was actually traveled.

So for one to go from their current gauge to accurate, they would need to increase, right? It is greater, not lesser.

I got 7% better mileage after fitting Energy Savers, so the increase in mpg is likely due to the tyres' profile, rather than their size.

Larger tire showes your going slower. Less RPM per original tire size.
Smaller tire shows faster. More rpm compared to the original tire size.

Euromodder you are correct. Smaller profile less air drag, less surface area touching road. It could just be a better tire as well.

Well. since the newer tires are 2" larger in diameter, and a 1" difference in radius (obviously) Calculate in the flex of the sidewall with 40 psi.. The car maybe only about 3/4 inch taller. If there is any extra drag from the larger frontal area, I think the total circumference benefits will outweigh them.

But , good thinking about this. Its one of the overlooked variables.
Specially in the offroading scene. LOL

It is my belief that long term proper testing is the only way to really know how a tire will affect FE on any given car.

I think the tires load rating (and loading), PSI and aero drag likely play more of a role than their size in terms of FE. (although they are also related)

Here's my take (and experience), In the day to day reality world of a slight upsize on a car (trucks are another discussion),
1. the load rating is higher so it's not a factor.
2. the width is actually SLIGHTLY narrower, but even it slightly wide, aero gain or lose is non-mearsurable and not an issue.
3. PSI - you should run the desired PSI (40+) regaurdless of size. Since you would run the same psi, then psi is not an issue
4. Size - need atleast a 5% increase in diameter for it to statisticly matter.
5. If you go from a high performance tire to a LRR tire you will see an increase even w/ same size. That is why it's important for all posters to state the full size and type of tire.
6. Downsizing to gain areo is UNSAFE. Period. running narrower (defined as 90% or less) tires is just too risky
Let me give you an example. driver A is got his car all stripped down, scangauge in the ashtray and running donut tires. He is in an accident w/ driver B in a normal car. If I am driver B's attorney......I'm going to own driver A. I wont settle for the insurance claim. I'm going after criminal malice, neglect, ........ I'm sure you see where this can head. When you step out and 'mod' a vehicle uotside the standards, you are setting yourself up for truoble, ESPECIALLY if you didn't notify your ins company of the mods.....

Interesting. How might load rating affect FE?

It can go either way, if you run the tire at rated pressure at much less weight than it is rated its RR is lower than the same tire loaded up to its rating.

This can mean a smaller tire can reduce aero+RR on the rear of many FWD vehicles because the weight isn't there. (per experience)

can you be more specific?

Less sidewall flexing and less tyre deformation is good for FE - that's what you get with a higher LI tyre when it's only lightly loaded - provided most factors remain the same, i.e. profile, pattern, rubber composition, pressure.
Tyre weight will normally be a bit higher on the higher LI tyre though.
OTOH, higher LI tyres often also have higher allowed pressures to cope with the higher loads.

Wether the effect is measurable on the road when you're using say LI 94 tyres instead of 91 ? :confused:

That makes a lot of sense. (I take it "LI" is a way of referring to "load rating".)

Load Index, after the 195-65-15 there's a 89S or something 89 is Load Index number S is the speed rating.

89 good for 1278 lbs, 91 is 1356, 95 is 1521 lbs for that size, don't know if same for all sizes.

What are you doing for speedometer and trip odometor totals?
If your speedometer reads 60, wouldn't your actual speed be around 63ish +?
How do you calculate your fuel economy?

chris D, pls read post #22 or re-read it above.

This is an interesting thread. I too thought about increasing the size of my tires (needed to be replaced anyway), but oped to go with factory since the gearing isn't bad.

On past experence prior to ecomodder... my dad owned a 1992 Buick Century with a 3300 v6. His tires were about shot, and going on the cheap, a I had a 92 chevy lumina I was scrapping out with good tires. The lumina's tires were taller and quite a bit wider, enough to hit *something* when turning really shape around corners or parking lots. He claimed a MPG loss and it took off slower. He wasn't exactly an eco driver though, and I don't have any good tracking/numbers to give. He probably didn't count for the tire size change, and the mpg drop wasn't enough to bother getting different tires (like 2-3mpg drop, car normally got like 26mpg). I'm pretty sure the tires were 8-10% taller.