I just put on 295/70/17 nitto's. Is there any place other than the dealers to get the speedo recalibrated?

 

I would like to know also, I am putting 315s on this week.

 

It only takes the dealer less than 10 minutes to do.

 

Go to the dealer, they plug in the DRB and change rev's per mile. 315's are anywhere from 605-615rpm depending on their true outside diameter if I remember correctly. Put a tape-measure around a fully inflated tire and measure how big it is, then divide 5,252 by that #, and that's your revs per mile (I think). If they can't find it, I don't remember all the menu screens but it was under ABS

 

You can also take a tape and measure the true loaded height of the rear tire (on the truck) multiply it by 3.1416 then divide by 5252. Compare John's Revs to my Revs and split the difference. That will account for a little "tire standup" if you drive like a Tire Hater.

 

315s require 601 revs per mile per BFG website, I just had mine done for 35s @ 598 revs a mile and what a difference it made.

 

I mean divide 5252 by that number. Whoops!

 

Make that 5280...darn Mondays.

 

Where did you get 5252 John?

I need to find a dealer around here who isn't a super-<EDIT> about changing the computer. So far they tell me "we can only set it to factory sizes, and it will cost you $89.95". <EDIT>. I'll do the 13% calc in my head



Reported Post Edited By Moderator

 

TCR,
You stated that the recalibration made quite a difference, what do you mean. I do see my shift points being higher than stock with the 315s, not by the odometer but actual MPH. What improvements did you see?

 

It shifts better, doesn't seem to lug as much, and the speedo is right.

4x4not, I got that number from my backside it was close enough

 

I was told by one dealer that if the speedo was very far off that it would effect the ABS.

 

The revs per mile can only be changed with a DRB. Any repair facility with a DRB can change it for you, but you will probably have a hard time finding one other than at the dealer (spendy units). Most mom-and-pop shops just have code readers or basic OBDII diagnostic tools.

Most all manufacturers give the revs per mile for their tires on their web site. I used 601 for my 315s as well per The Tire Rack web site and it is dead on accurate at all speeds against my GPS.

The dealer can change it in the parking lot with their DRBIII. Under the "Anti-Lock Brake" section, select "Pinion Factor", select "Other" (as your tire size will not be listed), manually enter revs per mile that you want (you will need to know this before you go as the dealer probably won't), save and exit. Total time on the clock is about 3 minutes.....but you will probably get stiffed for an hour labor. You will want a GPS or a buddy to drive along side of you to check their work. For some reason most dealers have a hard time "saving" the changes and it takes two visits to get-r-done correctly.

If I remember right, my trans shifted a little early after putting on 315s until I had my speedo redone.

On a side note, if you use one of the downloaders (BullyDog for example), this is a section of the ECM that is not messed with, so your speedo recalibration won't be lost. (common question)

 

The shift points "seem" to be a little higher in the rpm range, what I mean is it seems to shift a little harder and at a higher rpm. I knew the speedo and milage were off by about 10% but I didn't realize the how bad that was until the change. The tranny also hunts alot less in and out of OD. I dont know exactly what electronic changes actually occur in the recalibration, but I would for sure do it again and recommend others to get it done.

 

Measuring the circumference of a tire will not give you proper rollout. With the weight of the truck on it, you have a different rolling radius.