That is the question...
I'm hoping the smart people here can help shed some light on something I've been thinking about. My project is a 1953 GMC 2-Ton that has a 1350 2-Speed rear axle. The high speed ratio is 5.833, and the low speed is 8.11. This thing is really beefy, and I'm sure it's conservatively rated at a total weight over the rear axle of 13,000 lbs. Plus, the low speed is super handy to get going with a heavy load on a steep slope. I would love to keep the 2-Speed 1350 in this truck.
I also have a need to take this truck on some long trips. 2,500+ miles one-way kind of trips. I've already planned for more modern bigger disc brakes on all four corners, and a Hydro-boost for a comfortable pedal. I will also be running tubeless 9R22.5 balanced radials, and of course, a more powerful engine. I would really like it if I could cruise on the highway at a speed of at least 75 miles per hour.
So I thought to myself, "I'll just use an overdrive Brownie!" That should keep my engine RPM's down low enough to get better gas mileage on the open road. But then I realized something that might be a problem...
With a 5.833 high speed ratio in the 1350, and a 38" diameter tire, the pinion shaft on the 1350 would be spinning 3,866 RPM at 75 MPH! Something tells me this is not safe. At 65 miles per hour, the pinion would be spinning 3,352 RPM. Even this seems high to me for long stretches of driving. At 55 MPH, the pinion would be spinning at 2,837 RPM. This I'm guessing is much safer. I can imagine 55 MPH was probably the original safe top speed for this truck to begin with. Way too slow for our modern interstate highway system for sure.
With a pinion speed of 3,866 RPM, I imagine the gear oil would cavitate at that speed, and that would lead to lack of lubrication.
I understand the overdrive Brownie is used to help lower engine RPM on the highway at higher speeds. But at what cost to a rear axle with a high gear ratio?
I would appreciate any thoughts you guys and gals might have. Maybe my imagination is not correct, and these types of pinion speeds are normal? Maybe I need to be satisfied with a 65 MPH top speed?
Thanks in advance for your time on this.
David
