Going in tomorrow to have the speedo recal done at the dealer. Tried several different shops 'round these parts and with the exception of the auto electric place that wants $100.00 to do it, the dealer is my only option. So when I have this done it will recal the speedo as well as reset the shift points right?? I find that since going with different tires the tranny seems to hunt for gears at certain speeds. Any info you may have regaurding what improvements I should expect, how the dealer should be going about doing this, how long it should take, etc.. would be great. Any info at all could come in handy! I know that I could most likely get all the info I need by doing a search but I am at work in the middle of harvest and dont have time to poor over the pages upon pages of info on this topic. Thanks!

 

Dang it....I can't find the step by step instructions for resetting the revs per mile but it is actually quite simple to do.....When the tech has the scanner connected, he must access the ABS menus....within one of those menus is a provision to manually reset the revs per mile (pinion factor).....Sorry I can't be more specific but once you are in the ABS menus it isn't hard to find. You must know how many revolutions per mile your tire size is. Measure the static height of your tire and multiply by 3.14 (pie) to get the circumference.....then take 63,360 (inches per mile) divided by your tire circumference to give the number of times your tire rotates in 1 mile.

Example 31" tall tire has a circumference of 97.34".

63,360 divided by 97.34 = 650.91 revs per mile...

There are some instructions around here somewhere....hopefully somebody will post them.

 

When I look up my tire size on the tirerack site it says that the tires are 30 inches high?? WHen I messure them with a tape messure they are more like 32" high. Witch messurement do I use?? This will have an effect on my shift points right?? THe whole hunting for gears thing is starting to get old!!

 

Bump!!
I am going to have this done at 13:00 PST!!

 

My trans shifted a lot better after I got the recal done. The ABS works a lot better as well.

 

And it was just one process right?? THey didnt have to change the speedo then change the shift points??

 

Go with the static height that is actually measured by you - not what it says in a book......The shift points should be more appropriate once the PCM gets the proper vehicle speed signal.

 

The easier way to get revs/mile is off the spec sheet from the manufacturer. They test them and publish it.

It's interesting to see that different companies have different revs/mile for the SAME SIZE tire! Bt most 265 tires are around 655 revs/mile, most 285s are around 625 revs/mile, and most 315s are 599-602 revs/mile.

All the more reason not to mix and match brands.

 

Sidewall flexibility has a lot to do with that since the loaded radius is what ultimately determines revs/mile. A tire with a more flexible sidewall, all else being equal, will have a smaller loaded radius and more revs/mile.

Rusty

 

For sure. Perhaps this could be a good way to compare the expected rolling resistance (and thus, mpg) of one tire against another?

hmmmmm

 

Got a measuring tape? Make a mark and move the truck enough to move the tire one rev. Then measure the distance. This gives you feet per rev. Devide 5280' by the distance and this gives you REV per MILE. This will also take into account tire wear (if the tires are not new).

Randy

 

Well I went ahead and used the formula provided by CTD Nut and the height from the TireRack site, however the dealer wanted no part of my info. They wound up doing it the way they wanted (hey, they are the mechanics right) and made me sit in the waiting room. According to the GPS they used I was showing 7.5mph slower than what I was actually going and they now figure it is within .25mph of what it should be. Havent got to drive it a whole lot since it was done this afternoon but i will keep ya'll posted as to how I think things turned out.