How to Correct Speedo Error?

[SFM6S087]

Asking for a friend.

1966 GT350 with T10, 3.50 rear gear and 14 wheels & tires.
At true 30mph his speedo reads 35.
At true 60mph his speedo reads 70.
In round figures that calculates to a 17% error at both speeds.
I think the gear on the end of the speedo cable that goes into the trans has 17 teeth.

Does he need a speedo gear with more teeth or fewer teeth? Anyone know exactly how many teeth would correct this error?

Thanks,
Steve

[Coralsnake]

Increasing the number of teeth on the gear will slow your speedometer reading. Decreasing the number of teeth on the gear will make your speedometer read faster.

I am not sure, but I would probably try to a 19 tooth

[texas swede]

Use this program.

www.bgsoflex.com/auto.html
Texas Swede

[texas swede]

Using this program you must know the tire diameter and most important, does your
T-10 drive gear have 7 or 6 teeth. My 65 has 7 teeth but I have heard other cars (66's)
have 6 teeth.

Texas Swede

[KR Convertible]

The gears are pretty cheap.  To lower your speedometer reading by 17%, you need to raise the number of teeth on the driven gear 17%.  Which would be 19.89.  20 teeth will be the closest.  There are smart phone apps that will tell you your speed according to GPS.

[SFM6S087]

ALL great information. THANKS, GUYS!

Steve

[SFM6S087]

Just thought I'd update this thread. My friend replaced his 17 tooth gear with a 19 tooth and his speedo is now perfectly accurate from 0 to 65mph. He hasn't been any faster than that yet.

THANKS again to everyone who shared their knowledge and experience on this topic. Your help is greatly appreciated and was instrumental in bringing this to a happy ending.

Steve

[Coralsnake]

I love it when things get fixed

[Don Johnston]

For those who dropped in a T5 and have 14 inch rims and 3.50 or 3.89 rear, what trans gear are you using?  Thanks.

[sfm5s081]

don't forget to keep your receipt! So when you get pulled over you can tell the cops that you just had it fixed and it's not quite there yet