Speedo Gear ?
 

I have a 71 C10 with a 350 / 350 combo . It currently has a 3:08 rear end gear and I am switching to 3:40 gears . What speedo gears do I need to read the correct MPH ? Tires are 255 70 15 .

Re: Speedo Gear ?
 

Is your driven gear housing marked 30-39 or 40-45? Hopefully it's 40-45 because that's what you will need for one of these gear sets:

driving driven
17 red 40 black - will read slightly fast
18 blue 43 purple - will read slightly slow

https://www.tciauto.com/speedometer-gear-calculator

Re: Speedo Gear ?
 

Thanks for the advice , I will have to check and see what housing I have .

Re: Speedo Gear ?
 

The early 350 had the bullet style speedo gears, didn't they?
9.876% difference between the two gear ratios.

The sidewall of the tire is 7" (179 mm), and the tire's circumference is 91.4" (2320.5 mm). This means a 255/70R15 tire will make make 693 revolutions per mile/

If you have the old style bullet gears it will take a 8T and a 19T gear.
The 17T and 40T is a ratio of 2.35:1 (new style gears)
8T and a 19T is a ratio of 2.38 (old style gears)
So both are with in the allowable speedometer error.

Re: Speedo Gear ?
 

you need to know how many teeth the drive gear is. My TH350 has an 8 tooth drive and the bullet style speedo gear housing.

This calculator is helpful.

http://www.stl-vettes.com/65Vette/GearCalc.htm