Please help with my mph reading

[dpok69]

My digital dash reads a slower speed than the actual. ITs fine from 0-20mph. But above that it starts to slow down. If it reads 50mph then i am going close to like 80mph. Scanmaster reads the same values as the dash.

It has done this ever since i have bought the car.
I had a rebuilt tranny put in from a well known GN tranny shop 3 months ago, and was assured i have the right speedo gear. So according to the shop the culprit has to be the rear end. And from a test drive i was told i had 2.73 gears from a monte carlo most likely (becuase of my cars past with a monte carlo owner).

Today i jacked the rear end up. Spun the rear tire 1 revolution and had a helper watch the amount of turns the drive axle made. The axle turned almost 3.5 revs aproximetely. This sounds like the original factory 3.42 gears.

So whats left? Vss sensor or speed buffer? Any help would be appreciated.

 

[mayosmechanic]

Sounds like the VSS.

 

[Lee_Burough]

TCI automotive may be able to help you troubleshoot your system: http://www.tciauto.com/tech_info/speedo_gears.htm

Good luck

 

[turbofish38]

Your problem isn't with the dash or the VSS. Its with your drive and driven gears in your trans. I bet when they rebuilt it they put the wrong gears in. Its easy to check if you want to drop the pan. The drive gear which is also the govenor gear should have 11 teeth on the widest band of teeth. The driven gear on the side(you dont have to drop the pan to check) should be blue in color and has 30 teeth.

Eric Fisher

 

[dpok69]

Thanx alot , i will check for that blue gear outside the trans. Hopefully thats the problem.

 

[ijames]

I can never remember which way it goes, but if you have an open rear (no posi) and have one tire on the ground and spin the other, you either get twice or half as many driveshaft revolutions as you expect. With a posi you need both tires in the air and you get the expected number of turns. Try turning the tire 10 times while counting shaft revs; you will get a much more accurate answer than just one time. Put a chalk mark on the driveshaft so it's easier to see the turns, and you can estimate the last fraction, too.