Need quick help about PCM recalibration for tire size increase!!!!

[Challenger-II]

HELP!! My '99 truck is at the dealer today to have the PCM recalibrated to correct for increase in tire size so my speedo is correct. I was told that I could change the tire size in the PCM from a 245 tire to a 265 tire since the 265's came stock on the sport models. My truck was stock with the 245's. Dealer says they hooked up the DRB tool and there is nowhere to change this that they could see?? Anyone have any experience with this or no where I can direct them??



Thanks all!!

 

[SS]

Don't know for sure it was an available option in 99 software, my guess is that it should be - I know it's there in 01. Have them check under something like Chassis or Interior menus. Should be there and if it is 265/75R16 is one of the choices. If you can't get it changed, just remember that indicated speed is about 4% higher than actual speed. 50 mph indicated = 52 mph actual.



Hope this helps.

 

[work2muchplay2little]

If your dealer can't fix it, this will if you have the rear wheel anti-lock brakes and not the 4-wheel anti-locks. https://www.turbodieselregister.com/forums/showthread.php?s=&threadid=22253&highlight=abbott

 

[RonaldLarson]

Nope

Your '99 cannot be reclibrated for tire size difference. Just for your future use, the difference between a 245 and a 265 is 6. 7%. Calculating fuel mileage then means simply multiplying your indicated miles times 6. 7%+ and you will have the correct distance driven. For road speed, add 4. 9 mph to the indicated reading. I have varified these figures with a GPS.

Ron

 

[SS]

Nope

I beg to differ on "factory" tire specs. May be (count on it) different for aftermarket tires of the same nominal size. Inflation pressure, wear, and even date of manufacture of tires will likely change indicated speed.



According to revs/mile from a Dodge spec book, the difference is 4. 27% from 245/75R16 to 265/75R16 (at least for '01). I'm pretty sure that my '99 had the same 265 tires as '01, possible that the 245 *could* be different, but I don't think so. Still best to check the tires you have by GPS or at least by milemarker method.



Sounds like Ron has checked into reprogramming.

 

[RonaldLarson]

Yup

I tried to recalibrate with our DRB, and it cannot be done on the '99's. You are correct on the 01, it can be done, but only one tire size. 245 t0 265 only.

I was not clear on the percent difference, was I? My actual tire size difference was 6. 7 %. What I should have said was to measure the unloaded tire diameter, as I did, subtract the difference, divide the difference by the larger diameter, and this gives you the percent difference. The GPS did varify my figures. My 265's are aggressive treaded mud/snows and may be taller than most.

Ron

 

[Challenger-II]

Re: Yup

Originally posted by ol ron

I tried to recalibrate with our DRB, and it cannot be done on the '99's. You are correct on the 01, it can be done, but only one tire size. 245 t0 265 only.



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I guess I'm screwed trying to have the dealer calibrate my '99??? The '99 service manual lists the revs/mile for the tires??

 

[FF1381]

what about 285

ok i saw the 265... and i kinda understand all of the math ya'll threw out there, but what about the 286/75 is there a formula for that and can the dealer recalculate my odometer?



i have a 2001

 

[SS]

Not the service manual

Revs/mile for 2001 are shown in a Product Guide I saw that your dealer may or may not have. Obviously only shows factory available tires.



I wasn't clear on percent difference either. What I should have said is that all 265/75R16 tires are not created equal. They can vary greatly from manufacturer to manufacturer. Best to measure and test with new tires as Ron did. If you have to guess, let's say it's somewhere around a 5-6% difference.

 

[SS]

Dealer can only reprogram for stock tires as Ron said.



To find approximate difference (based on nominal size):



For 285/75R16:

Divide 285 by 25. 4 to convert from mm to inches (tire width)

Multiply result by 0. 75 (% of sidewall height to tire width)

Add 8 (radius of 16 inch wheel in inches)

((16. 42 inches))



Do the same for 245/75R16:

((15. 23 inches))



For percent difference:

Divide 1. 19 by 15. 23 (the radius difference of the two tires (16. 42-15. 23) divided by the original tire radius (15. 23))

((7. 8%))



This totally disregards measured sizes, amount of tire deflection with load, and a bunch of other stuff, but it gives you an idea.

 

[FF1381]

Man i totaly got lost in all that, but i do understand the 7. 8%... and i guess that applies for mph as well as mpg... ?

 

[SS]

Seemed less complicated when I wrote it

Sorry about that - basically you are finding the difference in radius of the two tires you are comparing.



Too many variables to say the exact effects of tire size on mileage. While a bigger tire does lower engine RPM for the same vehicle speed, the weight, the rolling resistance, driving habits, etc. may be different. You may find that one tire is generally better than another for fuel economy, but to base it purely on size would be difficult. That being said, I think my 305/85R16 (36. 4") tires may give slightly better mileage than the stock 265/75R16 (31. 8") with 4. 10 gears.

 

[joel]

my dealer did it for my 98.5

the tech hooked up his computer, got to the tire page and found that there was a pretty narrow limit to what he could change. The tool only had the stock size in it but you could put in a manual revolutions per mile figure if you knew it. Problem was, as far as I can tell, it was still a little too hi for my 265s. Strange.



And DRW - you're finding the radius difference, but really you need the circumference difference - for rotations per mile. Of course, since circumference is pi x radius squared... .

 

[SS]

Ummmm....

Circumference is actually 2*pi*r (you probably remember that now) so comparing circumference or radius give the same percentage difference.



I've seen them reprogram for stock size tires, but I never heard that you could enter the revs/mile? Sounds like it would still only be within the stock tire sizes.



You said 98. 5, right? Did they use a sensor on the transmission then for vehicle speed? I think 99 might have been the first year to use rear ABS signal for speedometer. It's getting too late for me to think about this one.