How bad is my gearing?

[88trailblazer]

I'm currently running a mild 350, built th350, np208, 10 & 12 bolts with 3.07's and 35's. Lack of gearing hasen't really bothered me to much because of the engine and i really don't do much crawling.

Now i'm getting ready to install a d44/OX Locker/4.88's and a 12-bolt/full Detroit/4.88's. These are both ready to bolt in with the crossover steering setup and 2wd steering box.

I can't afford a tow rig and i do plan on eventually moving up to 37-38's, no bigger. The truck is driven in South Florida trails and mud, but i want to eventually take it on the road to try out the rocks up north.

Will my highway RPMs be too high or will i be okay with this setup? Would it be worth trading my tranny for a 700r4 / np208?

 

[xtrmjoe]

I would not go deeper than 4.56 in a d44 the gears just get to small. 4.88's and 37-38" tires should put you in a good rpm range for the street for power not so much for economy.
The 700r4 will get you a lower first gear and OD which is the best of both worlds. Yet the swap will cost a good bit of coin and that will take a while to catch up to the gas savings of the OD.

 

[88trailblazer]

The 4.88's wouldn't have been my first choice, but the were already in the axles with the lockers. I'm thinking a used 700r4 would still be cheaper than re-gearing the axles.

Would a 700r4 from a early 80's (no computer) tranny bolt on with any additional equipment or wiring?

 

[Stomis]

Solution is to get bigger tires

4.88's are way to steep for 35's. I'm running 3.73's with 36's and think its a little steep. Then again I drive a stick but still.

 

[Pookster]

run the calculator. Given the way gas prices are, you might be inclined to drive at 55... in which case, might be the perfect RPM.

 

[88trailblazer]

Which calculator are you using? And what would you consider a good RPM range for cruising?

 

[Stomis]

Depends on how the motor is built but generally 2200-2700 rpms for ideal power/mileage.

 

[Blue85]

That combination takes 2580RPM to drive 55mph, 3280RPM for 70mph. I wouldn't want to cruise down the freeway all day like that. The numbers are worse if you don't have a lock-up torque converter. By switching to a 700R-4, the rpms drop to 1806 and 2296, which are pretty comfortable.

To stay with the 350, you probably want a 4.10 gear (2170rpm @55mph & 2756 rpm @ 70mph). A 3.73 gear would get you down to 2510rpm for 70mph, but wouldn't offer very good performance.

 

[88trailblazer]

Well, now I have a line on two people that want to trade my th350/np208 for their 700r4/np208's.

One is a 700r4 off a car'b truck and the other is off a TBI truck (less than 500 miles on rebuild). The TBI one is getting ready to run 52 inch tractor tires and he knows that he'll blow his tranny.

What would you guys recomend?

 

[USSkoval]

When it comes to the 700r4, the newer the model year, the better. It certainly doesn't hurt that it's a fresh rebuild.

 

[88trailblazer]

Well, i finaly got the axles installed. 4.88's with 35's and a t350 is definately a bad combo for the highway. The fastest i could take it was a screeming 60mph. Driving on regular roads isn't too bad as long as i cruise around 40mph.

I'm planning on testing the OX and Detroit in the mud this weekend. Lets see how that goes.

The guy with the '87 700r4 backed out, but the guy with carb'd 700r4 may still be interested and i found out his has also been rebuilt within the last two months.