Gearing

[rednexx]

I'm doing a 4x4 conversion on a Motorhome and wondering what gears to go with. It's a 24', 7.3 IDI turbo now, zf 5 speed standard. Originally with the 460 c6, it has 3.73 in the rear. The front diff I put in has 4.10. It weighs 12000 on it's own and I tow 3000-6000 on occasion. I've got 245/75/16 which work out to be 30.5".
3.73 seem to be good in a superduty with a GCVW of 20000 that has about the same power, (I've towed this weigth).
4.10 seems a little 'buzzy' for regular driving to me.
Thanks for the input in advance!

[TWeatherford]

First of all, I don't have a lot of experience with this. I'll chime in with what I know, so take it for what its worth.

In a pickup I prefer 3.73 because the vast majority of the driving I do is unloaded, and when I do tow it is less than about 6000 lbs. I'm actually looking for an IDI right now, and considered going for 4.10s, but decided that the extra help towing wouldn't be worth it for the lower weights, and small amount of weight I tow. However, a 12,000 vehicle that is even less aerodynamic than our trucks (if thats possible), going up to 15-18,000 seems like 4.10 would definately be worth it. And if its already in there, you could try it out and see how you like it at least.

[SmokeyWren]

3.73 is about perfect for grossing 12k with a stock 7.3L SuperDuty engine. For grossing 15k to 18k, you need either a hot-rodded engine with about 300 horses, or 4.10 gears. So for your less-powerful IDI engine, I'd go with 4.10 gears.

The 3.73 might be okay when you stay in the flats and no trailer, but when you get into the hills or mountains, or tie on a trailer, or both you'll wish you had shorter legs of the 4.10 ratio.

Without a hot-rod tune, you can make the engine run a lot better by adding an intercooler (charge air cooler) and improving the intake and exhaust.