Effect of Tires and Gears on MPG (Real Data)

Hey TW,

I have collected MPG data for a few years and thought it would be interesting analyze the data to put the myths to rest. Also, I have a lot of homework to do and I'm procrastinating.

Abstract:

Background: There is a lot of anecdotal evidence of the change in fuel economy caused by larger tires and changing gear ratios, but no data to support the claims. With an MPG data set collected over a period of about 3 years, the effects of the change in tire size and gear ratios can be analyzed.

Results/Conclusion: 33" tires on stock 4.10 gears caused a 5% loss in fuel economy. Changing the gears to 4.56 caused a negligible further decrease in fuel economy, but made the truck significantly more fun to drive.

Data: See attached Excel Spreadsheet

Results:



Discussion:

MPG loss due to the 33" tires was a little less than 1 MPG, as expected based on feedback from TW users. The tires I chose are very light compared to most other options, which are in the high 50 to 60lb range, which could explain why my MPG loss was less than what others experience going from 31 to 33 inch tires. Of course, the MPG loss is influenced by the amount of city or highway driving.

Driving habits influence results, and this is not a factor that could be controlled during the "experiment". The 33" tires were installed during Oct 13, a time when most of my miles are spent commuting to school (mostly highway). The 4.56 gears were installed in June '14, so it might be expected that the school commuting would end. Unfortunatel/Fortunately(for the data), I took classes during the entire summer break, which resulted in me commuting to school 4-5 days per week, which is about the same as what I had done through the regular Spring and Fall term. During late summer I did some wheeling trips which saw the fuel economy drop as low as 16 MPG.

It is common to see reports of MPG increasing when lower gears are installed to correct the powerband shift caused by installing bigger tires. There are many factors which can influence the change, such as transmission type, engine type and torque/VE vs RPM, change in driving habits after changing gears, and more. In my experience, the gears caused a very small decrease in fuel economy. In my opinion, my own driving habits have not changed significantly before/after the gear change, so the drop in fuel economy should be a reasonably close to the actual change caused by the gears alone.

The correction factors applied assume perfect 31 or 33 inch tires sizes. These numbers provide reasonable approximations, but of course do not reflect actual conditions. In reality, tire pressure, vehicle load, tire wear, and manufacturing tolerances make these figures nearly impossible to determine precisely, so the approximations will have to suffice. The manufacturer provided tire diameters (30.7, 32.5 inch, respectively) can be used in the approximation but the factors listed above likely outweigh this difference.

 

Cool to see your data. I'd lve to see the mpg difference for a hilly trip like a Tahoe run. That's where I noticed the biggest difference. I have to keep my truck floored for a lot more hills than I used to.

 

Nice work OP. Yet, I see no data equating speed in the data which will significantly alter the results.

 

The data is based on avg MPG per fill up. Over 3 years I've driven 70% Highway and 30% City if I had to guess. I spend a lot of time commuting on the highway to school with some road trips and 4x4 trails thrown in. Its not really possible to relate speed with economy based on the data I have but I'm running on the assumption there is enough data to see the trends caused by the changes in configuration.

I'd like to see it too. My truck was unbearably slow on hills when loaded down until I regeared. I don't have the means to control variables for such an experiment.

The bottom line is that the costs associated with increased fuel consumption (or savings from gears) are so small. With some rough calculations, it costs about $125 extra per year to run my 33" tires with 4.56 gears based on 14,000 mi/year and about $3.50 per gallon. Not a big deal.

 

Nice job. Been looking into doing something like this and am currently in the process.

My next step is that I'm trying to find a used 4.10 gear from a 2.7L. And install it for a test with my 31.6" tires. I am expecting an increase in city and a decrease in highway, but my hypothesis is that the loss on highway is not as much as people assume. I hope my results can be as conclusive as yours. Also, I may put the 3.73 back on if I don't like 4.10s.

 

Updated the results to include miles driven on each configuration.

I tend to agree that your city mileage should increase, but not by much. I certainly wouldn't go through all that trouble to try and save any money.

 

The data supports the fact that 33's are only 8 percent larger than 31's, it will vary depending on the width of the 33's, tall and skinny might effect mileage less. I went from 31's(stock) to 33's on my old Jimmy with no change in mileage and so far going from 31-10.50-15's to 285-75-16's seems to make little or no difference in my mileage but that is almost exclusively city driving. The gearing is another issue, I have a 3.4 and while a good engine the power band is in the wrong place, even for 31's, a cam change might help that but a gear change won't change it either in the 3.4 or 2.7. A gear change might make the truck it seem to be a bit more drivable but that is all, higher rev's equal more gas. These trucks are already pretty well maxed out with the stock equipment, personally I would love a GM 4.3 in my tacoma, much better engine for a 4x4 of this size.

 

I have similar data from 3.73s, to 4.56s with a mixture of 285/70, 35x12.5 and 315/70s. I have not logged it in excel but it wouldn't be hard to do.

 

Hi cmj, very detailed and systematic data! I am doing research on regearing to 4.56 for my FJ running 33s as well. From my reading, speedo is measured from wheel speed sensors on the wheels, not directly from engine RPM. So I think for your correction factor for 4.56 gear, you should not include the gear ratio, but only the tire size. If you do that, your 4.56 MPG is the highest over 20 MPG. That is very exciting data. Please correct me if I'm wrong.

 

It depends on the year and options.. My truck has no ABS, so there are no wheel speed sensors. The speed is picked up at the transmission if I remember correctly.

Also I noticed that with stock gears and 33" tires, my speedometer would read lower than actual speed. After the gears, it reads higher than actual speed, confirming the effect of the gear change on the odometer.

 

I see. That make sense. Thanks for sharing the data.

 

Not really for money. The mpg testing is just to see how much money I will lose by using 4.10s. My end goal is to make the truck a lot more fun to drive without sacrificing much.