Speedo reads fast

[68 Coronet RT]

My speedometer is reading 4/5 miles per hour faster than my GPS. According to every chart I have looked at with my set up of 3:91 gears with 26.6 inch rear tires I should use a 40T gear. So do I go to a 42 or 43 tooth gear ? Does anyone know the ratio of what one tooth makes in MPH ?

 

[diymirage]

missing one key ingredient...4/5 miles fast at what speed?

 

[pishta]

have you seen this one? Says use a 38!

question: How are you measuring your rear tire diameter? Dont take the size as the end all-be all tire height, use the distance from the ground to the spindle center and mult by 2. perhaps your rolling tire size is 26 even measuring the radial bulge.

 

[68 Coronet RT]

From zero till around 50/55 4 mph from there up 5 mph

 

[jtkaaa07]

I would try a 39 tooth, your a bit high on the 40 tooth, bet your right in there with a 39 tooth. I run a 3.91 with a 28.5 drag radial and a 35 tooth gear I'm within 1 mph

 

[68 Coronet RT]

I would try a 39 tooth, your a bit high on the 40 tooth, bet your right in there with a 39 tooth. I run a 3.91 with a 28.5 drag radial and a 35 tooth gear I'm within 1 mph

I decided to go with a 38 tooth gear, hooked up the GPS and took it for a ride. I'm no longer 5 mph fast on the speedometer, now I'm 10 mph fast. WTF gives ?

 

[mopardude62]

If it’s reading fast you need it to turn slower, so you need a gear with MORE teeth not less.

 

[68 Coronet RT]

Yup for sure already ordered a 42 tooth, anyone looking for a 33, 38 or 40 tooth speedo gear ?