A little help if anyone has any ideas.....

[CRUSH'EM]

Folks,

Am looking for a conversion factor or 'K' factor to some of you... . I am running 285 coopers on stock rims, factory was 265's, I know my MPH is off by ~1. 9-2. 1MPH versus actual speed. Thus I know my odometer and overhead are off on indicated mileage, however the dealer doesn't want to redo my pinion gear in the 'puter. So what I need is a 'k' factor to multiply my mileage by to get a more accurate actual mileage. Any ideas?? Thanks for your time, and keep the rubber side down!

 

[nickleinonen]

well, numarically,

a 265/70/17 tire works out to 31. 606" in diamater

a 285/70/17 tire works out to 32. 709" in diamater



the size difference is about 3. 5%. so at 60, you are doing 62. 1



now cooper lists their tire 285 @ 33. 00", and michelin lists their 265 @ 31. 4"



so the difference between those would be about 5. 1%, so at 60, you are doing 63. 1



and tire pressures can change the effective diameter of the tire too. and who knows how accurate the stock speedo settings are too...



i had the 265's stock, and now have 315's under my truck, and i have not used a gps to verify speeds, but i have driven through many of those radar traps mounted on trailers on the side of the road to make you double check your driving speeds, and it has been between 11% and 12% faster than what i actually am doing by the speedo, but numarically it works out to 34. 362" tire, while bfg lists their tire @ 34. 5" so it works out to between 9% and 10% depending on which numbers you use. .



i generally use 10% for speed calculating as it is easy to add 10% to your speed in your head [60=66, 100=110]

 

[LightmanE300]

Crush'em, get a different dealer or your dealer to reset the pinion factor based on revs per mile. . My local dealer wanted nothing to do with it, but the one 20 mins across town said they do them all the time, and that I didn't even need an appt. Expect a half hour labor charge for 5 minutes of work... but the speedo works now, and also the auto shift points are now correct. .

 

[BHolm]

It is sooooo easy for them to change. I am fighting my dealer now to get it done on my '06. They claim it isn't in the electronics... . BS.



I used the revs/mile of 634 for my 285/70-17 BFGs and it worked perfectly on my '05.

 

[CRUSH'EM]

Ok, finally got the dealer to flash my pinion factor....BUT

Finally got the dealer to flash my eprom for the pinion factor for the 285's... . now, my overhead is showing the crappiest mileage I have seen..... was averaging 16, now in town it is showin 9-11 and on hiway, its only reading 14-15... ... ... ... haven't had a chance to GPS the speed and see if he flashed the right code either..... any other ideas? Checked the position of my ROKKTECH CS and it appears to still be mounted correctly... am out of ideas gents... ..... lemme know what y'all think. Thanks