On my 91 p/u...I am getting super fuel mileage...with the 31's on... I actually got 275mi out of 13.2gal of gas...works out to 20.8 mpg.... The truck had 31's on for that trip...and 4.10's....My question is...Since this truck came with 225's... how much is my odom/speedo off? Also...I notice that with the 33's...I'm getting even better mileage...dont have a hard # yet...but def better....have I missed something? Shouldnt I be getting worse mileage with the 33's? And if the odom says I have traveled 275mi....how many have I really traveled, and what would my milage be? I am pretty good with math...but I have no idea what formula to use to figure this out. It would seem to me that if I'm gettin 21+/- with the 31's...that would mean i'm actually getting better than that?? And what about the 33's...I'm baffled...I was sure I'd get worse mileage with the 33's...but I am not????
T.I.A. for any help, or links.

 

What is your tire pressure at? when i had 32" tires i got a considerable increase in fuel economy, up to 25MPG. When i went back to 31" (actual size is 30.9) i lost fuel economy, down to 22MPG. Now awhile back i changed my rear diff cause there was shards of metal inside the diff housing, so i bought a used diff from a junk yard but i dont know if the diff has a different gear ratio than what i had before that.

Also, when i was using the 32" tires my speedo was 100% accurate, where as when i went back down to the 31" tires the speedo is now 5 miles faster than actual speed.

 

I run my tires(33's) at 30psi, the 31's I run at 35-40psi. If u swapped ur diff, and didnt get the same gears...DON'T use 4wd, u will likely crack the transfer case....I managed to get away with a 4.33 rear and 4.88 front for a couple months...but only used 4wd when absolutely necessary....and yep...uu guessed it...now I have driveline issues.

 

I have a complete 4.30 front available if you are interested pm me

 

Make sure you're checking your mileage against a known distance or by using a GPS...

 

Well, you said your original tires were 225's, but I don't know the sidewall height. Here is the math with my assumptions and you can plug in your real numbers if they are different:

225/75r15 means 225mm wide with 75% of that as sidewall height, or .75x225 = 168.75mm.

2 sidewalls = 168.75x2 = 337.5mm of sidewall or 337.5/25.4mm/in = 13.29in
rim diameter = 15in

total original tire height = 15+13.29 = 28.3in.

31" - 28.3 = 2.7" 2.7/28.3=9.5% increase in size

Since you went up in tire size, each revolution of the tire actually moves you further than the odometer actually reads by 9.5%. So if your odo reads 275 miles you actually traveled 275x1.095 = 301.125 miles.

 

so then by that formula...I got 22.8 mpg, with the 31's....hmmm, and i'm getting even better with the 33's....thnx...with this...i'll figure out my mpg with the 33's.
I'm amazed...my 93 runner(auto, add flanges) only get 17-19mpg...no matter what....my 94(5spd, aisin hubs) get about 22 depending on my driving habits, and the p/u is doing better than both...I expected much worse!! Maybe I need to drive this more often....might save me some dough!!

 

i have 31 10 fiddys and at 65mph my speedo is off by exactly 5. but ive heard its different and different speeds

 

well, the way I figure it...with a 9.5%(lets just call it an even 10) difference...I would take that to mean...at 30mph...speedo off 3mph, at 50...off by 5, 70...off by 7.
Now...with the 33's... the speedo is off by 16.6%....so, at 55, I'm actually doin 64.7mph.... and the 200mi, I drove was really 233.2 mi, and I did that on just about 9 gal of gas... this means I am getting approx....25mpg...wooohooo, not bad for a beater...damn...my runner dd, only get 22-23....I might need to start driving this thing more often.
I'm takin this to work today....my route is approx. 250mi....cant wait to see how much gas I use in the p/u as opposed to the runner, I will post up my mpg later today when I get home.

 

correct me if i'm wrong, but isn't the only reason for your increase in gas mileage with larger tires due to not changing your gear ratio? and wouldn't you have considerably less engine power? i can't imagine what it would be like driving with 33s and 4.10 gearing. i've got like zero grunt going up hills or from a stop with my 31s and 4.10 gears. bringing gears to 4.88 with 33s should bring the truck back to stock power with stock gas mileage, right? how about running 4.88s with 31s? I've been looking to upgrade to bigger tires/33s when my current 31s need to be replaced, but might do go to 4.88s and a locker before then - what ya think? i would love to still get above 20mpg.

 

hmmm....you have a 22re? My rig is a 3vze...so it does have a bit more grunt... my 94 has 4.56 and 31's, and is only a touch faster than the p/u with 33's...it does handle the hills a bit better tho( a bit being the operative).
If I had a 22re...yes I would certainly regear...but I think I'm happy with my setup...it crawls really really well, and gets excellent traction..
You are right tho...its the tires that increased my mpg's...well...that and the fact that I do mostly highway(45+mph) miles, and not much stop and go....if I was in town...I prolly would have got much worse mpg's.

I didnt take it today any way...I got to looking at the tires before I left, and noticed quite a bit of weather cracking....not interested in a blowout on the highway...so I took the runner instead, guess this will have to wait till I get me some new rubber.