Larger Tires and Speedo/ECM Problems???

[hammersley]

If you go to a tire that is say 33"+ or over 2" larger than stock - how easy is it to recalibrate the Speedometer? I note that on the Rickson Site they have 2 methods - http://www.ricksontruck.com/speedometer.html - However does this size difference effect the engine computer, performance etc... ... I know larger tires effectively changes your rear end ratio... .

 

[SRadke]

Originally posted by hammersley

Does this size difference effect the engine computer, performance etc... ... I know larger tires effectively changes your rear end ratio... .

The engine does/will not know the diffrence. As far as it's concerned everything is normal. The only diffrence is that you're rolling faster than you used to be at the same rpm in the same gear with the same speedo reading. The speedo will not be accureate, nor will the mileage computer etc. but if you figure out what percentage you have changed the gear ratio and multiply everything by that number you'll still have accurate figures. as for less power, if you're only going to a 33-34 inch tire you won't even notice.



-Scott

 

[tgbol]

The speedo can be recalibrated by the dealer with the DRBII.

It is done by going into the anti-lock brake program and entering the number of revolution of the tire per mile. It will also corrrect the overhead computer.

 

[BigDaddy]

Not to worry...

The ECM will never know the difference and it does not care. However, the PCM is what you want to think of with an auto.



Going to a 33” tire is not that big of a deal. I went from stock to 33” tires and my speedometer is off (maybe) 2 mph max. It’s the guys running those monster 44”+ tires that need to be concerned.



I tried to have the dealer reset my tire rev/mile and he could not. It has been awhile but, I remember something about he could not enter a value lower than 669(?).



Joe

 

[MPassaro]

THE DEALER DID MY 04 TRUCK I HAVE 315-70-17 BFGS ON IT AND THEY CALLED STAR AND GOT THE RIGHT NUMBER TO PUT IN THE #602 AND IT WAS OUT BY 7 MPH NOW IT IS RIGHT ON.

 

[jrobinson2]

Re: Not to worry...

Originally posted by BigDaddy

Going to a 33” tire is not that big of a deal. I went from stock to 33” tires and my speedometer is off (maybe) 2 mph max. It’s the guys running those monster 44”+ tires that need to be concerned.



I tried to have the dealer reset my tire rev/mile and he could not. It has been awhile but, I remember something about he could not enter a value lower than 669(?).



Joe

I went from the stock 245/75/16s to 295/75/16s that are 33. 2" tall and my speedometer is 10% off, 77 MPH @ 70 MPH indicated, even 31. 6" tall 265s were more than 2 MPH off @ 70 on my truck.

Jared

 

[BigDaddy]

jrobinson2,



I went from 265s @ 30. 9" dia. to 285s @ 33. 2" dia. and it was only 2 mph difference.



How did you determine your speed???

 

[BigMike]

I went from 245's to 265's. Took it into the dealer and the found out they couldn't recalibrate speedometer. Not sure why. So now I'm off about 4%. No big deal!

 

[DieselDoghouse]

Models prior to a certain year (2001??) have limited tire ranges, as I found out with my '98. You'll need a converter box to change the pulse train such as (but not solely) Tru-Speed unit. IF you use theirs cut off the AGC fuse holder and install a blade holder instead. Otherwise when the fuse starts to shake loose under way you'll loose your speedo reading!! (Or it will go full-scale) the PCM don't like gaps in the pulse train!!!

 

[jrobinson2]

Originally posted by BigDaddy

jrobinson2,



I went from 265s @ 30. 9" dia. to 285s @ 33. 2" dia. and it was only 2 mph difference.



How did you determine your speed???

I used a GPS and verified with a stopwatch.

Are you sure about the tire diameter? 30. 9 sounds like a 245? I just looked at a BFG tire chart and the 265/75/16 was 31. 8" and a 285/75/16 BFG TA KO was 32. 9", thats only 1. 1" difference and should track with what I've seen at about 2. 1 MPH @ 60 MPH. Its not to hard to figure out what the change will be, circumference of a 31. 8" tire is 99. 85", circumference of a 32. 9" tire is 103. 30" or about 3. 5% farther traveled on each revolution, 3. 5% of 60 MPH is 2. 1 MPH in my case the mathematical change is 9. 5%, not 10%, but I doubt the speedo was within . 5% when the truck was new and I don't know the true diameter of the OEM tires

DISCLAIMER: Your results may vary, I'm not that good at math, I don't know what size tires are on anyones truck but my own, I basically pulled most of this out of thin air as I typed it and its way past my bedtime so if I'm wrong feel free to really chew my butt out, I've been wrong before and will be again

Jared