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.