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Discussion Starter #1
My 68 standard has been retrofitted with a 9" rear, posi, with 4.10 or 4.11, whichever is the correct Ford ratio. C4 tranny, 3 speed, so I'm assuming 3rd is 1.00, if this isn't right, it's due to my ignorance. The car runs around 3000 or 3200 at highway speed, but the speedo is WAY out of whack, I'm sure due to the fact that the car was stock with a 3.00, 8" rear. Is there an intelligent way to go about figuring out what little plastic gear to use in the tranny to make things OK again?? Is this a painful ordeal to change out? I've done it years ago to a GM unit, but have not, as of yet, been as intimate with the C4. Ron.
 

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Contributing Sr Motorhead
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Speedo gearing

There are more factors involved with choosing the correct speedo gear.

First off, there are both a seven tooth and eight tooth drive gear available for the transmission tailshaft. If you didn't change trannies, that leaves that variable out of the equation.

Tire size. Taller or shorter diameter tires will affect your speedo calibration. If you are running the same tires before and after, that takes that out of the equation too.

Speedometer driven gear. This is the cheapest, easiest thing to change. First you need to know what gear you have on the end of your speedo cable now. From a 3.00 to 4.11 that is a 37% jump. 3.00 x 1.37 = 4.11 If your car has a 17 tooth driven gear now, you'll need one with 37% more teeth. 17 x 1.37 = 23.29 The closest you can get would be 23 teeth. You'll still have a little error.
 

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Discussion Starter #3
Algebra gone WAY bad!

Thanks for the info, I'm pretty sure the car has the original engine block, crank, and C4, but I wouldn't bet my next check on it. I'm thinking I'll just do the stopwatch thing on the interstate for 8 or 10 miles at 3000 rpm, calculate my speed, and use the info you gave to dial in the speedo gear. I've got P265-50-R14's on the back, which are new, which are definitely a different variable. I could go really pointy headed and do a 5 x 5 matrix and solve for the speedo gear, NOT! I'd probably end up with my underwear size.

Ron.
 
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