tire sie and speedo calibration

[Mike M.]

friend came by last year and gave me 4 new yokahamo tires he'd ordered for his c-5 vette(then traded it off before using the tires). fronts are 255-40 x 18" while rears are 285-45 x 18". The OD of both front and rears, when mounted, are 26 1/2" +/- 1/4". got some 18" american racing wheels , mounted tires and installed them on my 2005 full sie chevy 4x4 pickup. with the yokahams, when the speedo registers 96 mph(when the computor shuts off the fuel), the truck was actually only running 80 mph. i could have lived with the speedo inaccuracy but passing on interstate and having fuel shut down at 80 mph sucked. i went to the local GM dealer this am to have the speedo re-calibrated for the smaller od tires. they told me that the fronts and rears being different size would tear up the transfer case. Since the od of both fronts and rears is practically identical, i find this hard to believe. Opinions? thanks, mike

[Patrick H.]

Re: tire size and speedo calibration

Mike,

Find a new Chevy dealer.

Think about it: how often are you even running in 4WD down the highway anyway? And the little difference in size will not make a difference.

You can purchase programs or calibrators to adjust your speedometer reading to match the correct speed. I'm sure the hot rod sites can give you references or you can Google it.

I'm surprised the fuel shut down. I think I've gone faster than that in my 2003 GMC before. Maybe I'll have to go out and try it this afternoon.

Patrick

[Mike M.]

Re: tire size and speedo calibration

pat: re 96 mph and fuel shuttin off---our new 97 s10, new 2001 s10, new 2003 full size 4.8 and the newest 2005 4.8 all shut off at 96 when they were new. regards, mike

[John H.]

Re: tire size and speedo calibration

The fuel shutoff speed is set electronically in the PCM at the factory during the roll-test sequence by the machine's emission test computer based on the speed rating of the installed tires; the speedo pinion calibration is also set electronically at the same time based on the true rpm of the rollers in the machine (no speedo gears).

[Verle R.]

Re: tire size and speedo calibration

Mike,

There are several aftermarket services that can change computer programs to match rear end ratio and tire size.

I don't know that they can reprogram the Tahoe computer, just have to call and ask.

Verle

[Paul O.]

Re: tire size and speedo calibration

Mike here is a good web page to help Paul

http://http://www.idavette.net/tech/ratioc.htm