235/75r15 to 31x10.5r15 - MPG Change?

[elcidsurfer]

Currently I have a stock '87 T4R 22RE 5-Speed with 270,000 miles and 235/75r15 Michelin LTX ATs. It is my DD and I put about 50-60 miles on it daily, averaging about 50mph. Right now I'm getting around 22mpg. If I move up to a 31x10.5r15 BFG AT KO will I see significant change in MPG?

 

[ChoochCharlie]

Only if you get the little box that fixes your speedo with the bigger tires.
Otherwise, with bigger tires you will travel further per tank, but it won't show in the numbers.
Then again, the taller tires will take more effort to turn, probably a wash in the long run.

 

[cometobanks]

my tank range on 235s was 315 miles with 31s its in the 280 neighborhood. so in my case, I lost approx 30-40 miles on each tank.

 

[eighty64runner]

Go to this website and select the "Tire Size Calculator" tool under the "TECH" tab:

1010TIRES.COM® — Tires and Wheels Absolute Authority.

Then, compare the two tires. What you will find is that the 31" tire has an approximate 7% (6.79% to be exact) greater diameter than your 235's. In turn, this results in your speedometer showing 7.266% too slow. So if you are traveling at 60 mph, your speedo will show that you are doing 55.9 mph. By the same token, this will throw off your odometer, since your tires will not be doing the same amount of revolutions (larger surface area) for the same distance traveled. Using the 1010tires.com calculator, you will find that your 235's will complete 720 revolutions per mile, whereas the larger 31's will only complete 671 revolutions per mile (for the same distance traveled).

Therefore, if you calculate mileage based on your odometer reading, it will appear as though you have burned more gas because your odometer will show less miles traveled than it actually did. Does that make sense?

Of course, you will lose some mpg's due to the difference in weight, albeit negligible. Just last week, I went from a 225/75/15 Good Year AT to the VERY MEATY and heavier 235/75/15 Good Year Wrangler Duratracs (which are similar to an MT, to be quite honest). It might be in my head, but I feel like the extra weight is noticeable, although not significant. It does make me wonder how the heck people are running 31's on stock gears though (those with the 4-cyl. 22RE).

 

[s2kpuddydad]

I run 31x10.5 tires and my spedo is right on the money I get 20 mpg

 

[elcidsurfer]

Ok so lets do this totally mathematically then...


235/75r15 = 720rev/mile

235/75r15 = 23mile/gal

Therefore you have 16,560 rev/gal....

So if you back calculate....

31x10.5r15 = 671rev/mile

31x10.5r15 = 25mile/gal

But then factor in the 7.266% to slow and you get....

31x10.5r15 = 23mile/gal


So theoretically, going from a 235/75r15 to a 31x10.5r15 is pretty much neglegable..... I guess I might loose 1-2 MPG due to weight difference.

 

[ChoochCharlie]

I run 31x10.5 tires and my spedo is right on the money I get 20 mpg

Yea, My speedo is off by 7mph, but that's because the old fella that had my 4runner before me down sized from the stock 31's.
I'm looking forward to putting 31's back on it. Just waiting for these to wear out.

 

[eighty64runner]

Ok so lets do this totally mathematically then...


235/75r15 = 720rev/mile

235/75r15 = 23mile/gal

Therefore you have 16,560 rev/gal....

So if you back calculate....

31x10.5r15 = 671rev/mile

31x10.5r15 = 25mile/gal

But then factor in the 7.266% to slow and you get....

31x10.5r15 = 23mile/gal


So theoretically, going from a 235/75r15 to a 31x10.5r15 is pretty much neglegable..... I guess I might loose 1-2 MPG due to weight difference.

To be honest, I didn't quite follow your methodology. . . I guess my original point was, that some people might respond that their MPG's were decreased significantly, when in fact, they simply didn't account for the odometer change (due to the now larger tires).

Using BFG AT KO's for example, a 235/75R15 tire weighs in at 36 lbs., whereas a 31/10.5R15 weighs in at 42 lbs. Whether that is significant or not with respect to MPG's and performance, I don't know.

In my searching of old posts, I found that most report that 31's don't rub (not sure if spacer is required), but that the larger tire affects performance without a re-gear.

Taking all of the above factors into consideration, I opted to go from a 225/75R15 to a 235/75R15, which is only 2% larger. Therefore, my speedo/odometer is for the most part accurate, and performance power is virtually unchanged. As a compromise, I went to a very aggressive AT (Good Year Duratrac), which in my opinion, LOOKS much bigger.

 

[99blackSE]

Without re-gearing you will lose a couple MPG; more rolling resistance/more weight to move around, and you'll be holding gears longer to obtain the same speed.

 

[elcidsurfer]

Yeah they way I do my math is a little weird it makes sence in my head though....

Anyway, all things coinsidered moving up to a 31" is gonna drop my MPG. So is it worth the cost of regearing just to save 2-3 MPG?

 

[gman007]

Tire Size

I've been running 31's on my 86 runner for years, yes you lose power especially at uphill highway speeds and suffer in he mileage dept. But I put Rancho heavy duty tortion bars to raise the front 1- 1/2"(keeping the adjustment at center) and Northwest soft ride springs in the rear also 1-1/2" lift. When your off road the extra height helps while in low range you won't notice the power loss.

 

[99blackSE]

Yeah they way I do my math is a little weird it makes sence in my head though....

Anyway, all things coinsidered moving up to a 31" is gonna drop my MPG. So is it worth the cost of regearing just to save 2-3 MPG?

Depends what you're going to do with it. If you're doing lots of daily driving, it could make it less of a pain. Off-road might not matter too much depending on what type of off-road.