Two Vehicle Transmission

You're correct in thinking that at a quarter million miles the trans could be on borrowed time however the engine, t case, axles could be as well. Although, interesting fact that the 4.8 and 5.3s are some of the most bulletproof engines out there. They actually hold up to serious hp with stock internals.
If you don't have any issues currently personally I'd save the $ for now and run it and maintain it as you have been. Couple thousand would go a long way towards a newer truck and at the miles you're at, not worth putting thousands into it IMO unless absolutely necessary.
Ditch the 6 speed idea. Not feasible.

I have never heard of anyway swapping in a 6-spd trans into your generation truck. The design of the vehicles have electronic modules that communicate with each other, in this case transmission and engine as well as stability control. I do not know if they would work together.

At 230k miles, any number of things can go wrong. Heck, at 1000 miles anything can go wrong while traveling/towing. Prepare accordingly. My advice, save your trans fund money. Carry cash or a credit card for when problems arise. Hotel, towing, service, etc.

Thank you very much for your input! The main reason I am looking to replace/rebuild the tranny is it has the 236,000 miles on it and I don't want to end up in the boonies when it goes out. I just replaced the radiator 6 months ago and the rear seal on the engine 3 months ago. New heavy duty shocks and good rubber and the truck is paid for. I guess I'm having a hard time shelling out $30 - 50K for a new truck or $25 -30K for a used (someones else problem).

I totally appreciate the comments though and knowing they come from people that actually live the situation - that is priceless.

Mr.Beebo wrote:
To the OP, a 6 speed is nice, but I think you are wanting one for the wrong reasons. You need to keep your engine in its powerband when you are towing. Lowering rpms isn't good for the engine or the transmission under load, and it won't save you any fuel if the engine is working even harder to hold speed.

This is a good response. We went from the 4 spd in our '05 Suburban to the 6 spd in our '10. The 6 spd tows very nice, and is a considerable improvement. Mostly because it is more likely to have the right gear for the job.

But we don't really get any better towing mpg with it. In fact, it only shifts into 6th when towing down hill, and most of the time is between gears 3-5.

It does get quite a bit better unloaded mpg on the freeway than our '05 did (the '05 had 4.10 gears). But towing, you're gonna get 10 mpg with a 4 or 6 spd, I'm afraid.

And I can't imagine what it would take to swap in a 6 spd ... certainly would not be economically feasible, if it could even be done at all.

Lower engine speed does not mean less fuel used. 9-10mpg towing at 60-65mph is as good as it gets for any gas engine towing any high walled RV. A very few % of people are consistently getting double digits when towing, even say a 2011+ Ecoboost that can tow in 6th gear at 1800rpm. Save your pennies for when you need to replace the whole truck.

I towed about 15,000 miles with my '01 half-ton Burb. It now has 191,000 miles and still has the original engine and transmission.

Everywhere I towed, I was in third gear, and the engine was spinning around 3100-3200 RPM. The truck had the 4.10 axles. I used that truck to tow to Florida, Washington DC, Yellowstone, and Colorado.

Your mileage is going to be terrible. Period. Rebuild the transmission, have fun, and don't worry about your revs.

263,000 miles can be meaningless if the drivetrain has been well maintained. I don't even use the odometer on my truck anymore. I use the trans temp and the tripeter gauges more than anything.
To the OP, a 6 speed is nice, but I think you are wanting one for the wrong reasons. You need to keep your engine in its powerband when you are towing. Lowering rpms isn't good for the engine or the transmission under load, and it won't save you any fuel if the engine is working even harder to hold speed.

It would be cheaper to regear the differential than swap the transmission. Still have the same problem with tranny swap. Will the swap ever pay for itself? If the tranny is new, all other systems still have 263k miles. That's a lot for a gasser. Set aside money every month for a newer truck.

The only way to lower your rpms is to have more power/torque to be able to hold the higher gears of the transmission.

I towed my #5000 TT with my 5.4 F150 with a 4 speed trans and it was in second gear pulling around 3500 rpms to go 50-55 mph.

Same TT with a Ecoboost F150 with the 6 speed trans and it's going up the same grades in 4th pulling 2500 rpms going 60 mph..

The difference is the 5.4 only had 230 hp/330 tq while the Eco has 365 hp/420 tq...

Simply adding the 6 speed would not help my 5.4.. It would still need the higher rpms to pull the load, since it only had so much power..

Same deal for you even if you could find an ECU to control the 6 speed trans for your combo.

Mitch

Swapping from your 4speed to a 6 speed will cost more than trading the truck off. The transmissions are electronically controlled and it will require a bunch of work and computer swapping to make it work..Just not a good idea to spend thousands on an old somewhat worn out truck...Save your money and buy a replacement truck..If you're getting 9-10 mpg it won't get any better...JMHO

B.O.