1Ton_Tommy and Hotrod Lincoln,
Great ideas on managing rear axle gear oil temps. EdPrus also suggested running a synthetic gear oil. If a person where to run synthetic gear oil, monitor temps and actively cool the oil, it all sounds like a great way to make the rear diff last a lot longer to me!
So now I've got good news, and bad news...
Good News first!
I think my worry about the pinion speed being to high is not as bad as I thought. This is what I've learned:
I decided to compare the Eaton 1350 2-Speed in my 53 GMC to a more modern comparable rear axle (in ring and pinion size that is). The Dana S110 and S135 commonly found in the Chevy/GMC 4500, and 5500 trucks, and the Ford F450 and F550 trucks (both respectively class 4 and 5 vehicles) have a ring gear of 12.5" in diameter. My 1350 has a ring gear of approximately 12.25" in diameter (I know, the 1350 is ancient, so maybe a poor comparison).
So how fast does the S110/135 pinion spin at 75 miles per hour? For my example, I chose a 2002 F550 with the V10 gasoline engine. The factory rear gear ratio is 4.88 to 1, and the stock tire size is a 225/70R-19.5. This tire is 31.9 inches in diameter. The truck is built to cruise at 75 miles per hour all day long. With a 4.88 ratio S135 and a 31.9" diameter tire, traveling at 75 miles per hour, the pinion is spinning at 3,855 RPM. My Eaton 1350 with a 5.83 ratio and a 38" diameter tire, traveling at 75 MPH, spins the pinion at 3,866 RPM. Only 11 RPM difference. I consider that good news for sure.
And now the bad news!
I drained the gear oil out of my 1350 this afternoon and immediately suspected I had a problem. The first half gallon of liquid that came out was water! I then pulled the rear diff cover off, and after careful inspection, the ring gear is in really bad shape. Because the main ring gear in my truck is heavily worn, I predict the pinion will be in much worse condition. The pinion gets an additional amount of wear, multiplied by the ratio of the two gears. So in my case, my pinion will be 5.83 times more worn out then the ring gear!
Aren't these old trucks so much fun!!! And we roll with the punches... LOL!!!
But seriously, there are very few things in life more satisfying then solving problems.
Take care everyone!
David
