speedometer calibration.

[philo_beddoe]

My speedometer reads exactly 10 mph under, why?  Pretty sure its factory.

[philo_beddoe]

Oh, and my wheels are 235 instead of 225. But that is hardly a difference.   Right?

[Captkaos]

What is the exact size of the tires?  if the aspect ratio is different that factory it will be off also.  Several years have passed since it was new, so unless you have always owned it and keep it the same anything could throw it off. What are you measuring against to determine the 10mph off?

[philo_beddoe]

The spedo cable sure looks like it's from 1977. Anyway, factory tires calls for 225/75/r15, I have 235/75/r15. One the way to work there is a police trailer with radar which shows everybody's speed. I know it's true because my wife's care is doing what it says. 

[Captain Swampy]

Check this sight first for speedometer gears. I havent seen them yet, but they might sell them. Otherwise transmissioncenter.net sells speedometer gears.

[zieg85]

Check your odometer the next time you are on the highway, mile marker to mile marker to determine how much you are off.  My blue 84 C10 is off in the winter and fine in the summer.  The speedometer spring is probably fatigued on mine.  If the transmission was ever changed out and not the original rebuilt that could be the reason.  There are several combinations of the drive and driven plastic gears on the transmission depending on the tire size and rear end gearing along with add on reducers to get the speedometer correct.

[Captkaos]

Check this article on speedo calibration.  You can get the gears from GM...

http://73-87.com/7387garage/drivetrain/speedo.htm

[philo_beddoe]

Im picking the orange one because i like the color.

[philo_beddoe]

Too much math involved, i'll have mechanic figure out.