[asphaltgambler]

In my 1970 LS5 auto car it has a 3:31 gear with stock wheels and (similar size radial tires). Someone in the past has installed a speedo gear ratio adaptor on the trans. Does anyone know the correct speedometer drive gear (teeth and color) for my application? The MPH reading appears about right, but it's not original to the car.

Thanks :yes:

 

[shomejim]

Todd, if they put an adaptor on the trans, you may have to check the drive gear AND the driven gear. I think there are only 2 drive gears (not absolutely sure), but there are about 6 or so driven gears. The color is what tells the number of teeth on the driven gears. I can't remember the site but it decodes the color/teeth for you. I'd keep in mind though that there are different drive gears.

 

[Alan F]

The reducers weren't normally found with a 3.31 gear set. Is it possible that a prior owner had a 4.11:1 or numerically higher gear ratio and changed it? Or, they had the wrong gears in the trans and tried to adapt it. Personally I would ditch the adaptor. The textbook answer is below though.

Gear ratio X 20.2 (static factor) divided by the tire diameter equals the speedo ratio.
3.31:1 rear axle ratio and a 26.2" tire will give you 2.55. (**If you provide the tire size, or measured rollout we can be more precise.)

A 28 tooth driven gear and a 11 tooth drive gear = 2.55 or about as close as you are going to get. However, it's not the only close combination. These number combinations would also work: 9/23, 12/31, 14/36, 15/38, 16/41, 17/43, 18/46.

This all assumes that the speedo is calibrated accurately.

 

[asphaltgambler]

Thanks guys. I am very familiar with t400's but wanted an idea of what to expect before removing and then counting. I also don't know how much higher / lower the adaptor is. It's also something I need to look into and count to figure if it's reducing or increasing output to the speedo.

 

[Alan F]

There are two adapters that I'm aware of. The most common is 73.33%. The other is 94% IIRC.