I have done a lot of reading on how to calculate the correct speedometer gear but it still sounds like gibberish to me so I thought I would post all the info that I have and see if someone could help me out.

I have a 76' ford with a C6 tranny, 27 inch tall tires, 4.11 gear ratio, 16 tooth driven gear, unknown drive gear.

I have used a GPS and we have several speed check signs where I live so I do know that my speedometer is over by 30mph. Here is how my speedometer reads to actual speed.

speedo 90 / actual speed 60
speedo 85 / actual speed 55
speedo 78ish / actual speed 50
speedo 65 / actual speed 35
speedo 55 / actual speed 25

My readings could be off by a couple MPH because my speedo needle bounces a little but should be fairly close. Hope this is enough info to help me get the correct gear. THANKS!

 

So you need to slow the speedo down, that would mean a larger driven gear.

Info:

https://www.ford-trucks.com/forums/1...eter-gear.html

How to figure once you read this--short version--7 to 10 mph per tooth:

https://www.ford-trucks.com/forums/8...er-gear-q.html

So I'd try a 19 tooth.

You must be geared powerful short there--I have 4.10s, 32 inch tires and OD and it's still short for me.

 

I think you might have something wrong other than the gear. The wrong gear would make the speedometer off by a percentage, not the same offset throughout the speed range. Do you have any mile markers on the interstate you can use to check the odometer? Or drive at 60 mph based on your GPS and see if the odometer indicates a mile.

 

I was looking at my speedo yesterday and saw that my cable was not locked in to the speedometer head. Which is the reason why my needle was erratic. I went back and did some more speed tests and this is what I came up with.

For every one mile my odometer turns approx. 1.5 miles. I could not do the time test with my speed set to 60 because traffic was to heavy and I was moving to slow. But I did use the GPS on side road and got more accurate results this time I think.

speedometer 20 / GPS 12
speedometer 30 / GPS 18
speedometer 40 / GPS 24
speedometer 50 / GPS 31
speedometer 60 / GPS 38

If this is correct I believe that would make my speedo about 66 or 67% off. From what was stated from other threads adding one tooth to the driven gear will will add 7 - 10 mph to my speedo. But from what I see at 20mph on speedo I only have an 8mph gap which would give me a 17 tooth needed. But at 60 I have a 22mph gap which would give me a 18 or 19 tooth gear So do I just split the difference and go for a 18 tooth or do I need to use one speed over the other

Or is it better to use some formula with the 1.5 mile odometer to 1 mile marker difference.

 

I don't recall the "7-10" mph difference be listed an any specific mph.

Given that these are so cheap, you can try a couple without too much pain....