Newbie needing help!

[S.R.copper]

Hey yall, hope all is good. Call me Smokey. A friend turn me onto this site so figured I'd join and see what's up. I have a 81' 3/4 4x4 long wheel base camper special. Just pulled the old 350 and put a rebuilt 350 in. She runs good, quite a bit of after market stuff Goin on...but I'm lookin for something to make her run a little more efficient.
Note: 4 inch suspension lift and 35-12.50s.
The transmission in the truck is my topic of concern. Ppl call them different names, but mine is a granny low 3 speed. L,1,2,&3. I'm looking for one with granny low, and gears 1-4. That extra gear on the end would stretch my highway speed out while bringing my RPMs way down. I don't have a tach installed, I think they're tacky. Never the less, running 55mph, I'm Goin to assume I'm somewhere around 2.5-3k rpm. I shouldn't be complaining. Im averageing 15mpg. With 35s, engine bored 40 over with flat top pistons, small RV cam, 1.6 intake rockers and 1.5 on exhaust with poly locks, smaller harmonic ballancer, 15lbs shaved from factory flywheel, hooker headers. Fairly hot MSD distributor, wires and plugs. But a janky old elderbrok carb....taking that into consideration, it gets great gas mileage. Not to mention we put exhaust on it so my foots always in it some to hear the pipes. The boy that helped build it says she's damn near a street stock dirt track truck, if there were such a creature.
So my situation is, I want to keep my lift and tires. As well as my hotrod motor. This truck is my daily driver. And with that being said, granny low is wonderful in town at stoplights and on hills. So i dont want to lose low gear. Downside, I'm late for everything. I'll be late to my own funeral. And being late, I drive balls to the wall. Hammered down. The truck with run upwards around75-85. I generally stay way slower than that, but sometimes I dont. Running 60-65 is the normal. Even though she pulls all the way to the 80 mark on my speed-o-meter, if I had L,1,2,3,&4 instead of just L-3, 4th gear would allow faster hwy speeds as well as improved fuel economy since the motor wouldn't be wound up so tight. So, my underlying questions are; 1. What's the model of the transmission do I currently have, L,1,2,&3? 2. Whats the model of the transmission I am seeking, L,1,2,3,&4? 3. Would it be sensible to swap trannys? Or would it be useless, per say that 4th gear doesn't stretch much farther to make a different. Like if the transmissions performed the same, one not better or more efficient, one just had an extra forward gear with roughly the same top speed. And ? number 4, if the swap was a good idea and I decided to proceed foward with it, will my transfer case fit the new transmission, or would I need a different one? Sorry for the book guys, but that's what I'm looking for. Any and all help is greatly appreciated!

 

[blazinzuk]

You have an SM465

You want an NV4500

 

[blazinzuk]

Note your SM465 is a 4 speed transmission. It has 4 forward gears.

The NV4500 is a 5 speed transmission. It has 5 forward gears. 5th being an overdrive

 

[ktmoutfront]

What he said.

 

[rampage]

Are you sure about the 15 mpg? Were the gears in the axles changed to compensate for the larger tires so the speedometer and odometer read correctly?

 

[dyeager535]

If you were turning 2500RPM, with 35's, going 55MPH, you'd have 4.73 gears. I'm pretty certain that is not what you have.

With exhaust modifications/louder muffler, what I've found is that 2500RPM SOUNDS much higher than it is.

With your given information (55MPH, 35" tires) you should be turning 2165RPM, if you have 4.10 gears. 4.56's would put you at 2408RPM, but with those gears, and those size tires, you'd have no problem pulling a hill or stoplight in "first", not needing granny low.

Verify your MPH by GPS. As already alluded to, it's pretty unlikely your speedometer is correct. Which is going to throw the MPG calculation off.

Except if you have some crazy gears in there, I doubt the NV4500 will help you much. It's a fair bit of work, and a lot of expense to swap to an NV4500, comparatively. These sort of swaps generally need to be considered for things like reducing engine noise, or being able to run even steeper gears on the freeway, but using MPG increase to justify the swap is generally foolhardy...these trucks never get "good" mileage (even the OD ones in stock trim are at best 18MPG trucks) unless they are 6.2 diesels. If your gears are ridiculous? Sure, it might help. But I doubt you've got anything "better" than 4.10's, unless you left out that they've been swapped.

 

[Blue85]

If you're getting 15MPG for real (you know, real miles divided by real gallons, not an internet best tank ever number) that's about as good as it will get. What you haven't told us is what gears are in the axles. If you have 35's but stock 3.08 gears, you won't even be able to hold overdrive. That "granny" low will be like a regular 1st gear and the 1:1 top gear will be like an overdrive. With an overdrive transmission and 35" tires, you will want 4.56 or 4.88 gears.