Speedometer is way off

[live4theking]

Okay on the trip to and from Carlisle I kind of determined that my speedometer was off, significantly. How did I do this everyone was passing me. So, I timed from one mile mark to the next.

I confirmed this with my father-in-laws GPS yesterday. The inaccuracy gets progressively worse the faster you go.

Indicated vs. Actual
50 = 45
60 = 53
70 = 60
80 = 68
90 = 75

This is on my 68 NYer running 255/60R15. I know that they are about an 1" shorter than stock. Are the tires alone causing this variable inaccuracy? I've not pulled the differential so I don't know for a fact what gearing I've got.

I saw a table somewhere that showed what speedo gear to use for different height tires. Would changing the gear correct this problem?

Thanks guys.

 

[detmatt]

Yes but you'll need to know what gear you have. I've used this site recently as I just swapped the rear in my vert and had to change the speedo gear.

Speedometer Gear Calculator and Charts - TCI® Auto

 

[stubs300]

I confirmed this with my father-in-laws GPS yesterday. The inaccuracy gets progressively worse the faster you go.
I've not pulled the differential so I don't know for a fact what gearing I've got.

Why do all that work, don't you know how to check what ratio you have? Guess not by your statement? Jack up the car in the rear, spin the tire 1 complete revolution and count the number of turns the drive shaft spins. It's not Rocket science.

 

[Mr C]

Did you 68 originally come with 14's?

 

[jct]

i sort of fixed mine with another gauge cluster, i still have the wrong speedo pinon off by one tooth now... which is about 5mph now so i'm ok wih that for the time being

 

[Turboomni]

Mine is off also and I haven't gotten around to it so if I need to know I plug in my old gps and use that

 

[live4theking]

Yes but you'll need to know what gear you have. I've used this site recently as I just swapped the rear in my vert and had to change the speedo gear.

Speedometer Gear Calculator and Charts - TCI® Auto

Thanks Matt.

Why do all that work, don't you know how to check what ratio you have? Guess not by your statement? Jack up the car in the rear, spin the tire 1 complete revolution and count the number of turns the drive shaft spins. It's not Rocket science.

Okay, that's simpler than pulling it out.

Did you 68 originally come with 14's?

Yes, it came with 14's.