What RPM at 60 MPH with 1973 D20/W 318-3

[The_Handier_Man1]

From: thebluesliner  (Original Message)
Sent: 8/31/2006 7:07 AM

Hi Folks,

I was wandering what the RPM would be at 60 MPH  with a 1973 D20 / 318-3 as mine seems to rev aprox. 3500 RPM and not many people seem to be passing me at this speed. I would assume it's the original rearend gearing as well. I have six new Bridgestone Radials M775 SWP 8R17.5 10/E.
If the RPM seems high, what does your rig rev at the 60 MPH mark?

Greg Siddall
thebluesliner@hotmail.com



From: Coachmen1972
Sent: 8/31/2006 7:17 AM

A good question, which I have asked myself. My M300 chassis has a 456 rear, and seems to be revving up there at 60. I have an aftermarket RPM gauge sitting in a box waiting to be installed next time the tools come out, then I will know the answer. But 3500 seems close to my guess......

Oscar.



From: denisondc
Sent: 8/31/2006 3:34 PM

My tires stand about 33 inches high, so have a circumference of 8.639 feet, and turn 611 times in a mile. Since the rear drive ratio is 4.56 on mine, the driveshaft is turning 2786 times per mile. At 60 mph thats 2786 rpm. Allowing about 200-300 for slip in the torque converter, the engine would be turning about 3000 - 3100 rpm at 60 mph.
Considering you are asking the motor to pull a 5 ton vehicle with a flat-front through the air - I think 3100 rpm is good.
I dont plan to try it, but I imagine you could drive it indefiitely at 60 in 2nd gear (1.45 to 1) and the engine wouldnt care as long as the engine bearings were within tolerance. I figure it would be turning about 4100-4200 rpm, as the torque converter slip at that rpm would be much smaller.



From: thebluesliner
Sent: 9/1/2006 2:45 AM

So this may be why there are not many vehicles passing me when driving at 3500 RPM and the speedometer is showing 60 MPH, My radial tires must be a little taller than the old bias ply's that were on her therefore throwing out the accuracy of the speedometer. I'll have to get someone with a CB radio to follow me to find out if it is showing the wrong speed and hopefully it won't be the local Police car, that would be quite embarrasing  getting a speeding ticket with "The Bluesliner",  especially with my trucking buddys.

Greg Siddall



From: Coachmen1972
Sent: 9/1/2006 6:06 PM

Calibration is easy. Grab a stopwatch, then drive an exact mile....(there are little green signs posted along every inch of Interstate just for this purpouse). If you're doing 60 it should take one minute.......if it takes more, or less, you can calculate what you're doing.

Oscar.



From: jbmhotmail
Sent: 9/1/2006 9:49 PM

One easy way to figure out you differential is:block wheels and jack up one rear wheel, make a mark on the tire, make a mark on your driveshaft and on the differential, turn the tire two revolutions and at the same time counting the driveshaft revolutions. If the driveshaft rotates 4 and ½ revolutions your differential ratio should be about 4.56:1. Also check out this website for a lot of MoPar info: http://www.mymopar.com/tools.htm