Speedometer calibration

[69heavychevy]

I ordered a 22 tooth driven gear for my speedometer cable and according to my gps it reading about 10 mph too fast. I ordered the gear based on the online calculators I found.
The drive gear is 8 teeth, 3.73 gears, and 26.6" tall tires.
So would I order a gear with less teeth to try and get it more accurate? I figured less teeth would mean slower turning.

[stanski]

If its reading too fast you need a driven gear with more teeth. All the calculators are fine but usually it comes down to having a handful of driven gears and just keep trying till you get as close as you can.

[Dead Parrot]

Yep. If the drive gear has 8 teeth, it will move the driven gear 8 teeth for each rotation of the drive gear. More driven teeth, slower rotations.

[69heavychevy]

It looks like it jumps from 22 to 34-45. I assume I'll have to change the drive gear as well if I go with the 34-45 gears. It appears I'll have to try one of the 17+ drive gears. If I go that route, I read that I need a different speedo bullet for the driven gear.
Man, its too hot in Texas right now to be dealing with all this mess! HAHA!!

[hatzie]

Download the 79-84 parts books. I scanned all of the parts books for the 73-93 Squares and the 88-93 T400 chassis.
No need to guess... There are tables in each set of books that tell exactly what drive and driven gears GM used based on model year, 1/2 3/4 & 1ton, tire size, axle-ratio, and what transmission/transfer-case is in it. The parts book tables also tell if you need to run a ratio adapter and the GM part numbers for those too.
If you're running a transmission or transfer case that GM didn't use in 1980 just use the year that trans or TC was used as your model year.