'69 Fury 383/ 727 w/2.76 Diff. 275/60-15" rubber.
Using Randy's ring & pinion calculator I get a RPM of 1870 @ 55mph. http://www.ringpinion.com/Calc_RPM.aspx
I just put a tach on today and went for a ride. It reads 2250 @ 55. Checked the RPMs with my tach/dwell meter and my multi function timeng light with digital RPM readout. Seems accurate. Does the stock converter slip that much? If so, which converter would you replace it with?
Oh- I'll be putting in a 451 soon, so please keep that in mind.
I'm keeping the 2.76 gears and will freshen the 727.
451 is being built for torque and highway cruising.

https://board.moparts.org/ubbthreads/show...e=0#Post5071931

about 5-10 percent.

Speedometer accuracy varies with age drive pinion, a better gauge of accuracy is the odometer. Find a stretch of road with a known distance and compare it to the odometer, if the odometer is way off so will the speedo. Errors occur because of tire size change or incorrect speedo pinion. A stopwatch over the same distance/time will verify the speedo accuracy.

That was the first thing I did. One mile 60 mph = 61 seconds.