Gear ratio?

[Loulang]

I marked my drive shaft and spun my tire and my driveshaft almost spins 4 complete turns Is that a 3.91? The reason I ask is because my speedometer is off by almost 10 mph. At 65 I’m doing about 55. My tire is 26in tall. So I’m thinking a 40 tooth would do. Any help Is very much appreciated Thanks in advance

 

[moparedtn]

Here's all the speedo gears available:
Brewer's Performance - Mopar A833 4-Speed Transmission and Component Specialists

 

[ruffcut]

You are correct going by the chart @moparedtn provided. A 40 tooth should fix you up! ruffcut

 

[BSB67]

You spin the driveshaft one complete revolution, then count the number of revolutions the wheel completes

I think you have it backwards

 

[69Bee]

With the wheels on the ground (in case you have an open carrier, or a loose Sure-Grip), you roll forward or backward one tire revolution and count how many times the driveshaft spins. Ratio of 3.91:1 is Input:Output, driveshaft will turn 3.91 times for one tire revolution. Yes, it will work the other way also, but it is easier to count the DS revs for 1 wheel rev, at least in my opinion...

 

[moparedtn]

I think you have it backwards

Oops, you're correct! Apologies...