I'm thinking about going up in tire size to 32 inch tall tires from the the factory 31 inch size and was wondering what problems it may cause. Could any one comment with past experience? Thanks, Tpepper

 

Your speedometer will be a little slow, but not much. You can either live with it and be carfull not to get a ticket or pay to have it re-calibrated to the new tire size.

I just lived with it when I went up in tire size.

 

I read somewhere on here that it affected the computer causing it to run bad. Is this true? Tpepper

 

The difference between 31 and 32 is not great enought to throw off the CPU. There are a few websites that you could GOOGLE and enter the tire information and rearend ratio and it will give you the RPM, speedometer, and tire rotation differences if you care to have all the information.

 

You can have the computer reflashed at the dealer to compensate for the larger tires, should run less than $100. Another option is a tuner, but not every tuner can adjust for tire size.

My dad drove around for a year or so with his 33s before the dealer reflashed it, with no ill effects other than the speedo being off a little. I have 33's on my truck and I adjusted everything with my SCT tuner.

 

Thanks for the info. Tpepper

 

I have run 285/75/R16s (33" x 11.5") for 2.5 years without getting it flashed. I have no problems what so ever. I had a cop hit me with his radar gun while I was going 30mph and his radar said that I was going exactly 30mph. You might be a little off at highway speeds. I just drive like around 5mph slower on the highway to compensate.

 

You will be just fine, as said a 33" tall tire will make your speedo dead on. The speedo is a little on the fast side from the factory. So a 32" tall tire should put you right on or about 1 MPH slower than your speedo is reading.

 

Does the PCM ever know how fast the truck is going...physically?

If so, how?

Test tomorrow...

 

No the PCM does not have any other source about vehicle speed----except the data from the ABS computer where it gets the RAW speed signal pulses

The raw speed pulses are sent first from the differential tone ring to the ABS computer where they used to calibrate the actions of the Anti-lock brake system and the Seat Belt retractors and firing of the air bags

From the ABS computer the signal is sent "unconditioned" to the PCM

The speed signal is one of the most important sensors on these trucks

The PCM uses this signal for power train management and also distributes it "conditioned" to the dash cluster for use in the Speedometer indicator and also send it to the GEM for the speed adjusted volume controls of the radio (if equipped)

If you have an auto transmission the signal has a impact on shift points and line pressure strategy

The speed signal also has an impact on the emissions loop, ignition timing, and injector pulse durations

If the vehicle is a 05 and up with the Integrated Trailer Brake Controller (ITBC) the signal has a impact on the voltage sent to the trailer brakes

The factory can reset the REVs per mile within a certain limited range
Some chips/programmers can set to a wider range
A TrueSpeed module can set to larger range of revs per mile and hold two calibrations (say one for summer and one for winter tires)

I recommend having the Revs per Mile properly set in the PCM so all these sub systems can do their computations with a accurate set of data...just my 0.02

This is much more complicated then just having your Speedo read a little high or low

 

I went to 285/75R16's over the summer, and found that the speedo is almost dead-on now - 61MPH at real-speed of 60.

But the odometer, it's WAY off now, around 10% too low.

I haven't recalibrated it ... yet. No issues...

 

I have always assumed that the odometer and speedometer were working hand and hand. I just put 285's on my 07 and using 60 miles of turnpike mile markers I calculated a 3.25 percent error in the odometer.

The reflash for 59.99 at the dealer asked for both revs per mile and tire size. Now the odometer seems spot on, but the mph....?

What is a good way to check other than have a cop use radar? A stop watch seems like a hard way to get very accurate.

BTW, my 99 was slightly fast from the factory and the 285's made it right on the money. But again, that was only an odometer check.

 

hand-held GPS works for me.