M939 speed mystery SOLVED.

[wehring]

I have been racking my calculator and brain to figure out HOW my M925 runs 60 mph at 1850 RPM.

When I calculated 1850 RPM / 49" dia / 6.44 ratio I got 40.87 mph. The GPS says this is not true.

I got to digging...

The axle ratio is 6.14 so that is an easy fix but the numbers still do not add up.

THE TRANSFER CASE HIGH RANGE IS A .732:1 RATIO!!!!!! NOT 1:1!!!!!!!!!!

Now the calculations are right on with the GPS reading. Calculation is 58.57 when the GPS reads 60 mph.

This t-case (air shift) will be an excellent conversion to a bobbed deuce for extra road speed.

The list of things I need to do when I get home is turning into a scroll...

Justin Wehring
979 997 3112

 

[NDT]

Is this the 925 with the 250 or the C12 powerplant?

 

[wehring]

250

This is the 250 which covers all of the M939 series trucks. I may be wrong about the 6.14 ratio as it may be 6.44. The calculation deltas are narrow.

The ESP truck's t-case is not divorced and I it is a 1:1 ratio. I could not figure out why the two trucks ran close to the same speed/rpm when the ESP truck has a double o/d in the 7 spd allison and the 925 has a 1:1 final in the MT654CR.

In short. All M939 series trucks have the o/d t-case.

With this said, the M931A2 I bought from you is off the market. That drive-line is definitely going in a bobber. It combined with the new 4.90:1 axle gears will give me.... ready for this...

65 MPH at 1703 RPM!

Justin Wehring
979 997 3112