The Stovebolt Site Forums The Shop Area General Truck Talk Bigger tires or diff. ratio?

Hi; I discovered a set of differential gears to reduce the ratio from 3.9 to 3.7 in my half ton. The idea being to go faster at less RPM. Then it occured to me to just use bigger tires and wheels if necessary. My current tires are 27.5 inches tall. If my math is correct, I can accomplish the same thing as replacing the diff. gears by going to a 29 inch tall tire/wheel. My question is: Is there a reason not to use the taller tires on the rear? Handling? Part two of this is that I calculate that with my current tires and gears, 3000 RPM is 62 miles per hour. That does not seem like excessive RPM, but it sure sounds like it's running much faster even at 50 mph. Any idea of the rpm range (limit) for a 235?
Thanks.
Rich

Rich I cant find a high idle figure in my 54 operators manual. The 235 216 261 engines are all low rpm torque building engines if you will figure your rpms at 40Mph youre probably getting closer to the rpm you would feel good about. The old 235 will pull well with those gears if your not trying to pull big loads......Ronnie

I would think that around 4200 would be tops for a 235 and if this is true, would not run it long at that............Fred

On my 3/4 ton, I have switched from 4.57 to 4.10 gears. I also went from a 29" to 31" tire. This has dropped my rpms from 3,000 to just over 2,500 at 60 mph. The only problem with running the taller tires is the truck sets higher, which some people don't like. I'm running 7.00X15 front and 7.50X16 rear. The fronts are the stock size for the 3/4 ton.

Running an unbalanced 235 at high rpm for extended stretches is not recommended. Over time, the rods will fail. While I have turned 4,500 rpm on a few occaisions, I recommend 2,500 rpm to be the top for cruising. A fully balanced engine has a red line over 6,000 rpm.