Speedometer Gear help

Need some help choosing the right speedo gear, I checked my speed via GPS and my actual speed was 63 mph while the speedo showed 80. It's for my 62 Fairlane, C4 auto, 3.50:1 rear, 205/70/15 tires (26.3") best I can tell the speedo drive gear is a 7 tooth. I pulled the one thats in it currently and it's a 20 tooth.

All of the online calculators say to go to a 19 tooth but this doesn't seem to be a sufficient enough decrease given the large disparity in speed.

Any input?


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I would like the answer to this as well. I'm not sure mine is correct. It "feels" like I'm going the right speed, but I'd like to know for sure.

 

It would seem you need to slow it down by a factor of just less than 20 percent. More teeth on the driven gear will slow the cable, and make the speedo read slower. Twenty four teeth will make it run a touch slow, twenty three a touch fast. Both should be within a few mph at freeway speeds.

 

So you have a 17 mph span to correct. Been my experience that each tooth is worth about 2-3 mph.
You need to add teeth to slow your speedo down. So let's figure it at 3 mph a tooth which means you need to add 5 teeth to slow it down approx 15 mph which puts you at a 25 tooth gear. The biggest I have seen for the fords is a 23 tooth gear.
The other option is to have a speedo shop make you a reduction box to correct your speedo.

 

First off , your online calculator is going the wrong way . You need to slow down your speedo.

Your speedo is reading almost 27% too fast for actual speed

Get out your calculator and lets do the math.

Based on your figures 1 divided by 63 [actual MPH] then multiplied by 80 [speedo reading ] = 1.26984 too fast [ keep this number for reference ]

Now we work out your existing speedo ratio

20 tooth divided by 7 tooth = 2.85714 to 1

To correct speedo you need to slow down this ratio [ numerically higher ] by 1.26984

2.85714 multiplied by 1.26984 = 3.62811 to 1 [ this is the ratio you need ]

Because you have a fixed gear on the output shaft you need to multiply it by this ratio!

7 tooth multiplied by 3.62811 = 25.39 tooth so a 25 tooth will be close enough

 

Get a gear that will give the correct speed as close as possible, pull the speedo and I bet there is a lever somewhere around the needle, probably in the back, to calibrate the speedo. Speedos are built by the millions, for multiple years & models. There has to be a provision to calibrate them. My '41 Ford can and has been adjusted with this method.