I purchased my cutlass a month ago and i finaly got on the hiway and I notced my speedometer is way off I was doing about 90 to 100 and traffic was passing me easy at 70 mph speed limit....car is all stock 350 th350 21470R14 rallys could some have change gears in past for bigger rims or what can do to fix this any help would be greatly appreciated..thanks guys.

 

A good transmission shop can calibrate your speedo to correct changes made in the rear end or wheels. Most newer cars (60 and later) they just change a plastic gear where the speedo meets the transmission, earlier years a adapter is added to the side of the transmission to accomplish the same thing......Tedd

 

If you just bought the car, ask the previous owner if he changed the rear gear ratio but didn't change the speedo gear. That may have something to do with your problem.

The other alternative is your speedometer head or cable is whacked.

I don't know if your car is MT or AT. Most were AT so if you look at page 4B-31 of this link, it will tell you what speedo gear to use with different gear sets. http://wildaboutcarsonline.com/membe...Section_4B.pdf

Do you know how to index the drive shaft and wheel to determine what gear ratio you have, or do you know it off hand? What Tedd says ^^ is a very good suggestion.

 

Sounds like a gear change was made without changing the speedo gear. Not a hard one to change. try here for help and a new gear: http://speedometersolutions.com/

 

Ill get with my local tranny place thank you

 

I am not sure of the original tire size but maybe that might be your problem right there shorter tires etc.

 

A tire diameter of 1" will change the speedo reading by 5%, so at 100 mph, an error of 5 mph, 50 mph, 2 1/2 mph, it would have to be a pretty big change of diameter/tire size to affect it dramatically.