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.