Speedometer error

[dswancutt]

My speedometer on my 78 Chevy K-10 reads about 7 mph fast (i.e., 70 mph indicated is actually 63 mph). Here are the original specs on my truck.

400 small block
350 turbo hydomatic with NP203
3.73 gear ratio
10 x 15 tires.

I am now running LT235-15 tires and have changed the speedometer gear in the transfer case to what the factory parts manual calls for. It also read fast when I was running 10.5 x 31.0

I am on my 3rd speedometer and all read fast by the same amount.

It has a new speedometer cable and I have checked the speed with and without the adapter, and it is still 7 mph fast.

The only thing I haven’t checked is the driven gear, but checking the parts manual, they all are 7 tooth gears for NP203.

This leads me to believe the gear ratio has been changed. My question is, is the gear ratio now higher or lower than original if it has been changed? Is there another theory that any one has as to why my speedometer is off?

I hope this makes sense.

One last question, what metric size do 10 x 15 tires convert to.

Doug

[jimfulco]

Count the teeth on your driven gear (the one hooked up to the speedo cable). If your speedo is reading 10% (%, not mph) faster than your actual speed, get one with 10% more teeth. That's about as close as you can get without using an adapter with gears inside it.