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Speedo reading incorrect. How to find the correct speedo driven gear

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Hi team

I have a 73 vert with a 351W and C4 transmission. 

Put the car together and discovered that the speedo is reading 75mph when I am doing 100km (62mph) using a gps speed app on my phone.

I discovered when adding an LSD to the 9 inch diff that the gearing wasnt the 2.79 as on the tag but 3.25 and my rims that came with the car are non standard 16 inch mercury grand marquis wheels. 

The tyres on them are 225/60/16s.

I have no idea which speedo driven gear is on the cable and am not sure which size I should get to bring the indicated speed down to reflect the real speed as my mileage is obviously going up quicker than necessary. 

Is there a formula from wheel size and diff ratio to get the correct speedo driven gear?



Edited by Ballaratcastle
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Thanks guys

I am still trying to work things out without knowing what the actual drive gear teeth number is. 

The trans guy who rebuilt my C4 thought it was an 8 tooth from a guess but the links you have given me tend to say for gear ratios up to 3.5 they are 7 tooth gears. As mine is a 3.25 (links say 3.27??) (Crown wheel 39 teeth and Pinion 12)

I did count the speedo gear tooth which is an 18 so am trying to work backwards with the indicated speed of 75mph being 62mph actual.

The tyres are 26.6inch so 758 revolutions per mile. 

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Unless you bought the car new and never changed anything on it you don't know what has been changed over the years. You know that it doesn't have the original rear end gearing nor the original engine, so likely the transmission has been changed at some point, too.

It is likely that you have 8 tooth drive gears, which means you'll probably need a 20 tooth driven gear. In any case, you need more teeth on the driven gear to slow down the revolutions of the input to the speedometer.



“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein


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  • 2 weeks later...

I installed a 3.50:1 rear gearing in my '71, with AC that came with a FMX, however, the factory did not make that combination available stock. The fix, was to go to my neighborhood Speedo/ instrument repair shop, where they set me up with a "ratio box", which is put inline with the speedo cable. The car was put on the rollers and the appropriate gear was put in the "box', to check indicated speed, as well as they shortened the speedo cable. They did it in an afternoon, really not a big deal thankfully.

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When I got my 1973 Mach 1 351 Q 4 speed I broke the rear end yoke in just a couple weeks. So they fixed it under warranty. Me and gal friend went to Disney in July and I just got the car in May and I broke another. After the third I told the dealer that regardless of how hard I was on the car the 9" rear should be holding up to the now weak 351. I told them something had to be wrong. So they brought a factory guy in and as soon as he walked under the car he knew what was wrong from color code on the driveshaft. They had put the small u-joint 9" rear in my car. So they ordered another driveshaft and the factory guy told the to put a 3.50 in and not a 3.25. So I have a Ford built Q code 4 speed with AC and a 3.50 rear. I wish I had got paperwork to support but never considered it back then. I never had the belt on AC anyway. 

When a man is in the woods and talks and no women are there is he still wrong??:P


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