Speedometer not accutate....

[Jim2029]

1995 ECSB 5.0L 4L60e 2wd. I just bought this truck. It was an unmolested truck from Florida. Still has the factory exhaust system that is quite...

The speedometer is off by 7 MPH. When it says 60 my GPS says 53. Any ideas where to start? Has the correct size tires (235/75R15). Techs at work say the rear end has been swapped out. Unable to check currently...

Truck shifts fine and I'm doing about 1600-1800rpm doing 60pmh on gps.

 

[redfishsc]

You can check gear ratio by pulling the diff cover off and read the engraved letters on the ring gear. If you havent pulled the diff for inspection lately and swapped the fluid, might as well do it now.

Usually the ratio is in the form of tooth count:tooth count


Compare your findings with your dashboard RPO code.

I suspected mine was swapped for a 373 because I have the same problem on
a 99 K1500 that originally had a 3.42, and i confirmed, still does.

Apparenty (maybe) a previous owner had calibrated the PCM for larger tires because when I got rid of the crappy undersized tires it had for 31" tires, the speedo went to "nearly accurate", only off by about 4mph at 60

 

[GL1500]

You should be able to find the sticker with the RPO codes in the glove box. Once you have that, you can lift the rear, spin the driveshaft by hand (in neutral) and count how many times the drive shaft spins for one complete revolution of the tire. 3.42 will spin nearly three and a half times, 3.73 three and three quarter times, 4.11 just over 4 time, etc. Compare your findings to the RPO code and see what you get. I believe GM specifies the speed should be within 7% of actual vehicle speed but my rememberer is down with the flu. Let us know what you find.

 

[Nathaniel2g]

4mph at 60mph is factory allowance. It's 7kmph at 100kmph. I believe you have to replace the cluster if that is what's going on.

 

[RichLo]

I switched clusters on my 93 from a no-tach to a tachometer cluster and my guage was off by about 9mph at all speeds (going 10mph read 19mph, going 70mph read 79mph) if thats your case you can take the needle off and set the cruise control via GPS on your phone to 25 or 30mph and push the needle back on and it'll be spot on