recalibrate speedometer / odometer for bigger tires

[tomtoc]

i got new, bigger tires saturday (Dunlop Radial Rover AT - $150/ea installed)

old ones: stock 235/75/r15
new ones: 265/70/r15

by my calculations that's 2.5% different. can i recalibrate my own speedometer & odometer, or does the dealership need to do that? approx cost?

so, 2.5% larger means
-my speedometer reads slower than i'm going (60 is actually 61.5)
-my odometer will read less miles than i've traveled (100 miles is 102.5)
-my rpms at 65 *should be lower than they used to be

are those assumptions correct? if so, i'm not overly worried about changing stuff. for an oil change at 3,000 miles, it will only be off by 75 miles.

pics will come - i just washed it really well yesterday, and now it looks like it's going to pour all week. they look nice. most people wouldn't notice a difference than stock, but they do fill up the wheel wells a little better.

i love the way it drives. 1st gear seemed very short and almost useless before, and 2nd was too low to start in. now, i can start in 1st and drive a few hundred feet without quickly going to 2nd. i don't notice any power loss (but i never tow anything either).

 

[radialarm]

I'm trying to find the proceedure, Tom!!!

 

[tomtoc]

also, not sure if this is solve-able, but compared to my gps, the speedometer was a few mph off before. 65 on the speedo was showing 62ish on the gps.

 

[john112deere]

If I'm thinking right...those trucks get all that info from the speed sensor on the rear diff, so you'd need a dealer to do it, unless you can somehow tap into the computer yourself.

*Also...with the variability in tire sizing (different brands, etc.) and the inherent inaccuracy of your truck's speedometer...I'm not sure I'd bet much on it being accurate to within 2.5% now.

EDIT: That difference of a few MPH is pretty typical on speedometers, btw.

 

[radialarm]

Sent it to you in a PM!!!

 

[Skandocious]

Pardon the link, but this is how you do it on a 92-96. Now sure if it works on your 97 w/ OBD-II

http://tinyurl.com/dfr3sl

 

[tomtoc]

Sent it to you in a PM!!!

Pardon the link, but this is how you do it on a 92-96. Now sure if it works on your 97 w/ OBD-II

http://tinyurl.com/dfr3sl

you guys had the same idea. thanks

looks a little confusing, but doable if i really feel like it

 

[fatherdoug]

Would changing the driven gear on the transfer case correct the problem also?

 

[Skandocious]

Would changing the driven gear on the transfer case correct the problem also?

That's how they fixed it in older trucks. The newer ones (I believe starting in 92) don't have that gear anymore.

 

[Lost]

I have yet to change mine since swap . The tranny's shift points seem way better .Then with the smaller tires.

But I have a posom already programed ready to go in.

 

[Raptor22]

Haha. I have 31" on mine and it reads 55 when Im going like 70.

Does anyone have a guide for a Gen8?

 

[nobodyspecial]

There is another alternative, the Superlift TruSpeed. If you are sticking with one tire size, recalibrating the PSOM thingy works, but if you are changing tire sizes, you can only recal it so many times.

I have it on my 95 F150, it just wires in between and with potentiometers (little dial thingies) and you just adjust it. It has a little sheet that tells you what you need for different tire and gear combos, pretty handy.

My speedo is dead on, up to 85. Never gone faster in it, thats with 31s and stock gears, had it with the original tire size too, and it was still dead on, it works.

 

[tomtoc]

just to update this thread - mine was always a little off according to the GPS with my old tires. now it appears dead on, so i'm not worried about adjusting anything

 

[nobodyspecial]

isn't it nice when things work out that way? Usually for me, its the opposite.

 

[Skandocious]

I've got 33's and my speedometer is only off by about 2 minutes... I have to believe that the previous owner recalibrated it but I don't know if he really had the technical know-how... He gave me receipts for everything and it basically shows that he paid people to do everything.