K here is the deal........truck was stock with 225/75/15 (28.3").......I bought the truck and put 31x10.5x15 on it (haven't measured to see if they are true 31s but....)

According to my GPS the speedo is still dead on (tested at 55 and 75mph).......which makes no sense to me with that large of a tire size change.

So my question is, would the odometer be correct if the speedo is correct?

(ps - my GPS is right according to several other tests)

 

My 4runner was stock with 225/75/15, and I put 31x10.5x15 on it, and the speedo was off by about 5% compared to my GPS. 60mph on the speedo was actually 64mph with the GPS.

Maybe your truck actually came with 31x10.5x15s originally? then the previous owner put on 225/75/15?

 

What year is your truck?If 31's were an option for that year, it could have been geared for the 31's, but the wheels and tires changed at the port of entry or the dealer.

 

my speedo tells me the same thing when im running 225's and 31's. im curious to see why also....

 

91 stock size for 4wheel drive (i could be way off) was 31 1050 15. 225 75 15 was for 2wheel drive. is it automatic or standard?

 

Using the mile markers on the side of the highway, I calculated my odometer is off by about 9% running 31 in tires with stock gears. (22RE auto with 4.30 gears)

 

I can't speak for all years, but this should apply for most 3rd gen trucks:

The original tires on the 22RE 4WD trucks were P225/75/15 dunlop grandtrek's with a tire diameter of 28.3". When switching to 31x10.5 r15 (BFG AT KO) the tire diameter is 30.7", this results in a 8.5% delta in the spedometer and odometer readings.

I believe most all V6 4WD models came with the 31x10.5 r15 bridgestone desert dueler option, so for your rig the spedometer and odometer should not have changed.

 

I'm not convinced the mile markers are posted at exactly 5280 ft apart (state employees and whatnot).

I would trust the GPS over mile markers.

 

1991 model
manual
sticker on door says 225/75/15

IDK, I'm almost postive the speedo is right..........just was sure about the odometer.

 

Check your door sticker and see what gear ratio it has, both of mine say they came with the P225/75/R15 but they are geared for 31s (which is nice).

 

mile markers are remarkably close. I like to use them for accuracy checks.

If you are concerned try using several different locations to verify.

You can calculate the % difference in diameter and that should tell you how much you've changed.

If you go to tirerack.com you can look up your tires and check the specs. Including rolling diameter. and revolutions per mile on every tire.

 

I have 31x10.50s and every test i've done my speedo has been right on the dot... I bought my truck with 31x10.50s on it...

 

you got lucky if you bought it new, with 31's, because the other dude is right; 4wd-22re's came with 225's. If you go to 265's you get a diff in overall diameter. So lets go.
225/25.4x.75x2+15=28.287
265/25.4x.70x2+15=30.649 I used 70 here; I don't think there is such a thing as a 265/75.

a diff of almost 2.5 inches, that is for each revo of the tire.
that means at 65 on the old tires, you are now going 70.4275 miles per hour on the new.

for mpg's , lets go a mile is 63360 inches.63360/30.649=2067.277 diameter turns
63360/28.287=2239.898 diameter turns
that is slightly over a 10% amount in full turns, in a 1 mile distance.
So your gas mileage is going to be that far off as well...
and this absolutely has to happen, so your gps is not as accurate as you
think!

 

I don't think my question was answered though..........

If my speedo is right, does that mean the odometer is right?

 

They're both operated by the same mechanism, so I'm going to have to go with... yes.

 

my truck used to have 235/70/16s, and the speedo was fine till I swapped the dash cluster and the needle wasn't put back on straight, so it read 5-6mph faster than it was actually going. I put 265/75/16s on it and now it reads right again within about 1mph.

all tested with GPS. I bet mile markers are accurate, but I'd trust a GPS over my ability to hold exactly 60 mph for an entire minute, though that will give you a rough estimate.