speedo correction

[splinterddt]

Okay, here's the situation; I've put a big-block in my cars a few years ago. Speedo is off. Changed the rear end gears, speedo off some more. I had 14" wheels and changed to 15". I have changed the gear on the speedo cable and have the speedo reading about 10 mph fast. Being that I have a C-6 trans, the drive gear is part of the output shaft. I can get the speedo closer by putting taller tires on the back. To get the speedo the closest, I need to put on P235/75/15 tires. This size just doesn't look right and there is some clearance problems. Anybody have any suggestions on how to get my speedo to read correctly.

 

[LMan]

there are some conversion charts on the Net (dont have a addy right now, sorry) that convert speedometer-error (in %) to cable driven gears. Try looking for that, and buy the gear the chart recommens. I know for my 3.55 gears the 'black' plastic gear is the recommended one....

 

[splinterddt]

Unless there is an aftermarket gear that has a higher tooth count than a stock Ford, the chart is useless. I need about 2 more teeth on the cable gear, but I have the biggest available from Ford (21 tooth). I can change the speed by increasing tire size, as I stated before. I could chang the drive gear in the trans, but would have to tear douwn the trand and swap the whole output shaft.

 

[Max Power]

Yup, 21 is the max. I wouldn't change tires just to make the speedo accurate.

Put 4.11s in!

 

[70XR7ConvertCat]

Here is a formula. I've heard a 22 tooth Mopar gear is available (and works). Someone also said a 23 tooth gear can be had, but it doesn't last very long.

Steve C.

Driven Gear Teeth = Drive gear teeth * axle ratio * tire revolutions/mile / 1000