CAEMI

Does anyone know what the effective ratio is with 31" tires and 4.11 gears?


Also, after switching from 235's to 31's I checked my mileage yesterday on a short 120 mile trip. It seemed like I dropped one mile per gallon instead of gaining. Is there a logical reason why or is this just an anomoly?

Donnie_K

because your engine is working harder to turn the larger tires, therefore requires more fuel.

djb383

+1, changing to a taller tire is just like changing to a taller gear on a 10 speed bicycle, it's harder to pedal, requires more effort. The OEM tire is a 225, not a 235 so, if I've done my arithmetic correctly based upon a OEM 3.55 ratio, you would need to have at least a 3.89 gear ratio with a 31" tall tire to get to the factory equivalent final ratio. A 4.11 ratio with 31's should feel nice and peppy and make going vertical somewhat easier.

jmm0122

Your lucky if you only lost 1 mpg. I used to average 17mpg's and after I installed 31's I get about 14 mpg's. Gotta love 3.07's.

CAEMI

I understand. But I thought that if I had, for example, 3.55's with 235 tires and switched to 4.11's, my mile would DECREASE because I was using way more RPMs to go the same speed.

So, being mathematically challenged (a hard thing to be for a CPA), I figured if I increased tire diameter the effective gear ratio would drop from 4.11 back to around 3.55 to 3.73 or something and the result would be lower RPMs to achieve the same speed/distance.

I do understand that the tire is harder to spin...but with the lower gear ratio of 4.11 it should have made them easier to spin.

There is something I am not getting. What am I missing??

Rock Toy

The effective ratio is the old tire diameter divided by the new tire diameter times the rear gear ratio.
By tire diameter I mean the actual diameter not the tire size!
So if your tire diameter was 28 inches and the new 31s are 31 your effective ratio
would be 28/31x4.11 or about 3.71 to 1

djb383

Yes, mathematically you should get better mpg, with a taller tire but, the mathematical formula doesn't take into consideration the extra load/effort placed on the engine/drive train, even though the rpm's are lower.

CAEMI

You, sir, are a genius. THIS is what I wanted to know how to do.

DJB383: I understand. I guess I should not complain about 17 MPG with 31's and 4.11's.

CodeXJ

RPMs does not directly map onto MPG, the amount of work the engine is doing does. If you are running lower RPMs but the motor has to work harder to maintain those RPMs, it'll use more fuel.

CAEMI

I've seen this now in my own gas mileage.

The heck of it is I got four brand new Goodyear Wrangler RT/S tires on a nice set of American Racing Outlaw II's for $200. I could not pass them up...even though I'd prefer to have 235/75R's. A $1000 worth of tire and wheel will have to cover my excess gas useage...

djb383

This may help........

http://www.izook.com/gearcalc.htm

I keep coming up a 3.89 ratio and 31" tires are equivalent to stock 225 tires and stock 3.55 ratio.