Ford 8" Rearend Swap - RPM Calculations

One reason I decided to swap my stock rearend for the 8" was to get a gear ratio that would be more fuel efficient on long road trips. I ran a wire from the tachometer terminal on my MSD module into the interior and hooked up a tach to take some RPM readings out on the highway. Here is what I found from a before and after test drive:

MPH 7.25"(3.5) 8"(2.79) % decrease in RPM
50 2900 2350 19%
55 3250 2500 23%
60 3450 2700 22%
65 3750 2900 23%
70 3950 3100 22%
75 4200 3350 20%

So the bottom line is an average 21.5% decrease. I'm now getting the same RPMs at 65 mph that I was getting at 50 mph. After the swap I changed my driven gear that runs the speedometer from a 20-tooth to a 16-tooth. This got me pretty close to a correct speedometer reading. I took a test drive and determined I'm now getting 10.2 miles on the odometer for every 10 miles I'm driving. This was the same range of error I was getting with the 7.25" rearend.

A website with calculators for RPMs, driven gears and more is available here if you want to get some figures without going for a drive.

Main Rearend Page