How fast am I going?

Got a 53 Mercury with a 390 Cadillac engine, GM 350 auto transmission,
a limited slip Granada rear end, and a stock 53 Mercury speedometer. I
know that the speedometer reads higher than the speed I am going: but,
how much is it off? For Christmas I got a GPS that shows the posted speed and your true driving speed. I checked it out at different speeds and it averaged 10 percent off. Just subtract the 10 percent -- you can
do that in your head. That solved my problem. I can now easily know my
true speed.
TwoChops

 

do you mean like a Tom-Tom or something like that ?

 

Yes but mine is a GARMIN

 

I never thought of that. i haven't hooked up my speedo yet so last year I just went with the flow of traffic and by ear depending on what gear I was in. sad thing is I could've had a working speedo anytime. Just need to stop being lazy and hook up the pulse generator I have for my new guage, lol that would have taken up too mych drive time

 

My cutlas is way off too. Had to follow my wife and she wrote down her speed and mine. Im running 3.73 gears with a turbo 350 tranny. When im doing 20 speedo says 30. When im doing 55 speedo says 80.

 

I'd pull the gear out of the side of the trans, and see if I could find one with 10% more teeth....then the speedo (and odometer) would read correctly.

 

I use my GPS for a speedo in my Metro.