71' F250 Tranny swap options

[martinarcher]

Hello all!

I own a 71' F250 with a 390 FE and the stock NP435 4 speed with granny tranny and a Dana 60 4.10 rear end (44 front).

The truck is a garage queen and was a transplant from CA. It was restored here in PA and has never seen salt and rarely sees rain. It's sole purpose in our household is to tow our Malibu ski boat. We typically use it twice a week in the summer to tow the boat 20 miles each way to the lake. We do however tow up to 4-5 hours to get to vacation spots with the boat. The truck currently gets about 6 miles a gallon which isn't the greatest and with the 3 road gears really stuggles up hills on the highway. Waiting for the truck to get down to 35 mph to shift into 3rd means cresting hills at an underwhelming speed. :-banghead

I have kicked around the idea of converting to a diesel, but to really make the conversion effort and cost worth it I would have to use the truck more than i do. My other option is to swap trannies and hope for a modest improvement in fuel economy and a better hill climbing performance with the extra gear.

What trannies are a good option to mate with my 390 and my divorced transfer case?

I'd love to throw a ZF S6-650 in there, but after some digging it's not sounding very doable.

NV4500 - sounds like the most common and easiest swap. It would get me another road gear.

ZF 5 Speed - another 5 speed option.

Others?

Thanks guys....

Here's a pic of the rig....

Pulling the old boat....




Launch the new boat...

[fordman]

good gas mileage is only solved by the rear end ratio. since it is a 4wd you have to change out but front and back to match so they dont over run each other.

[martinarcher]

Thanks for the reply. I thought about a rear end and front end swap, but don't you think an extra gear in the tranny would be beneficial for towing when it comes to hill climbing? If I simply changed the diff ratios I think I would struggle to tow the boat even more than I do now.

[fordman]

a 3.73 would give a little better gas mileage than a 4.10. a wider ratio trans may be better for climbing hills you may be right. but i have no idea what to use.

[Caseys71]

As for tranny swaps I'm not of much help. The NP-435 is good for towing but not good for highway speeds or cresting hills at highway speeds like you said and neither is my T-18. As far as which tranny to go with I'm not totally sure what your best option would be that would give you good mileage but still leave you some towing capability. We had a truck with the ZF 5 speed, it was a great truck got fairly decent mileage and was pretty capable of just about anything it wouldn't be a bad candidate for your swap at all IMO just not sure how easy it would be to do the swap. Another good option is to go with a T-19, it will give you 4 street-able gears and dropping back to 3rd won't be as much of a decrease in speed as it would be with your NP-435 or with my T-18. However swapping trannys doesn't do a whole lot for mileage, a 5 speed would be good for mileage because it gives you 5 different gears so that you can run in the most efficient gear for your situation rather than being stuck with your current tranny and not really being able to find the perfect gear. However I have to agree with fordman the transmission doesn't do a whole lot for gas mileage, most of it is in the pumpkins. The lower the gear (higher numerically) the more acceleration you have, less top end you have and worse gas-mileage you have however the higher the gear you have (lower numerically) will give you more top end speed, slower acceleration, but better gas-mileage. However once you start getting into really high gears that give you good mileage, you loose the acceleration that you get out of a lower gear which does better with towing. What gears are you running in your axles now? Something around 3:60ish-3:30ish would give you okay gas mileage and still give you enough acceleration to be able to tow a boat behind your truck and not take 3 weeks to get back up to highway speed 3:30 would probably be pushing the acceleration factor though and would probably start making your truck feel slow off the line. Where are you located? I noticed you guys had a wake-tower on both of your boats loaded down with wake-boards, I'm really into wake-boarding as well .

[martinarcher]

Thanks for the reply!

Lots of options there. I keep finding info on the Ranger Torque Splitter overdrive (27%). Sounds pretty good and in 4th gear with the splitter in OD, I think it would be a bit better than the 3.30's, but I still have the option of shifting down and taking advantage of my 4.10 gears.

I'm located in western PA. We wakeboard a lot. Our home lake in Shenango, PA. Have you tried wakesurfing? It is a blast too. I saw your in NC. You near Lake Norman?

[Caseys71]

The Ranger Torque Splitter sounds like it may be the best option for you then . Here's a write-up you may be interested if you do decide to go that route:http://www.fordification.com/tech/torque-splitter.htm Only other thing you need to consider is shortening your rear and lengthening your front drive-shafts although I'm sure you already knew that. Western PA, that's pretty close to Erie . And yea Lake Norman is 4 hours away, a good drive but we make the trip at least 5-6 times a summer a lot of fun to be had there. Usually in the summer I head up to Coble Ski Camp for a couple of weeks straight, it's a private lake owned by the camp and you get to hang out and wake-board pretty much all day, and there's only 1 boat per lake so you don't have to worry about getting in anyone's way. I've only wake-surfed a few times it's tons of fun though and I am a little better at it than I am at surfing, still a good bit of wiping out though .

[70_F100]

Since you have a divorced transfer case, one other option comes to mind that would be a fairly simple swap.

Replace the NP435 with a T19.

It will give you 4 useable gears, instead of the 3 you currently have. On those long hills, just shift down to third, and it won't give you the screaming RPM's that the 435 does when you downshift one gear.

Couple that with a ratio change to 3.73 or 3.50, and you've accomplished a lot. Not a great deal of expense (T19 should be a direct bolt-in), better gas mileage, more useable gears, and your 390 has plenty of power to deal with the swap.

[70_F100]

The T19 is almost identical to the T18 externally, and therefore should be the same length as an NP435.

The driveshaft P/N is the same for both a T18 and NP435, so there shouldn't be any issues.

When I did my swap from a C6 to a T19 in my 70 LWB, I bought a used driveshaft from a junkyard that came out of a 72 LWB with a T18, and it fit perfectly.

[martinarcher]

Cool. Great info. Thanks!