Tires sizes vs. 3.07 gearing question
 

Does anyone here have any input on how well it would work if I put 33" or 35" x12.50 tires on my "89 Cummins truck that has the 727 transmission and 3.07 gearing?

I am not new to diesels, and I know that the motor has the torque to pull off running almost any size tires, but 3.07's seem a little tall for much play room as far as putting bigger/heavier tires on the truck.

The main use of the truck is supposed to be a daily driver, but I live in a very hilly part of California on the west coast. I don't plan on towing much, but if I do, the heaviest thing the truck should see is about a 5500lb Mastercraft wakeboard boat and trailer.

I know a lot of guys prefer the manual transmission trucks and they have a little shorter gearing in the axles, but I am a truck driver and I like the fact that I have an auto out of work. I would like to kind of build the truck to what I want it, but have things still be comfortable with the truck the way it is now, too. I plan on lifting it a little bit, but I won't want to spend the money and time on changing the gears in the front and rear ends just so I can run taller tires.

I've seen some posts about this topic over the years, and consensus seems to be don't do it. Something about the 727 not liking the higher load at lower rpms. Pump probably doesn't generate enough pressure at lower rpm to hold the clutches. Thats why the manual fares much better in this situation.

If it were me, I'd try some cheap/used 33's or 35's... and make sure I had a trans temp gauge and maybe even a pressure gauge to monitor the 727 line pressure. Do some trials with an empty truck and see how it does.:c:

Thanks. I appreciate the reply.

I have a buddy that has a second gen 12-valve with 33's on it. I'll have to talk to him and see if he would be nice enough to loan the wheels and tires off his truck to me for half a day or so.

I also have an 06 2500 cummins that has 35's on it, but it still has stock 3rd gen wheels on it. I heard there was some kind of fitment issues with 3rd gen wheels on these first gen trucks. I guess I'll have to do some research on that.

I ran the 3rd gen wheels and 33's on the rear with the stock axle for a while. Looked a bit odd, but it cleared fine. Now, with all 4 tires/wheels I run 2" spacers on the front. None on rear as the Dana 80 is 5" wider already.

Ya may wanna think about re-grearing if ya wanna run those size tires. Your 727 will love you for it.

Are there actually companies that sell gearing for these axles? I've also heard that one of these axles is a wierd one and is specific to the trucks with the 3.07 gearing.

I wasnt aware of that, but to make certain, do some research on that issue. Sounds a bit far fetched to me, but then again, Im not well rehearsed on those older trucks. Is it 2wd or 4x4? If whatcha say is true and ya cant find suitable gears, ya might wanna consider a rear axle swap for a later model one. IMO, doing your own research overrides "word of mouth." for sure.