Just started s project on a 75 460 highboy. It's got the granny gear transmission and I'm slightly worried about the Rpms at high speeds. Does the granny gear affect the trucks top speed or Rpms as far as freeway driving goes? Would it be better to get one that isn't grannied? It doesn't have a gauge so I can't tell yet but judging by the sound I doubt it's a good idea to hit 65 mph?

 

Assuming its a 4 speed your final drive ratio is 1 to 1 which depending on your rear end gears and tire size is going to effect your rpms at speed. 1st gear has nothing to do with 4th gear.

 

Yes it's a 4 speed. But wouldn't having a regular transmission give it an extra gear? Because I start in second. I'm just worried it won't be able to hit the freeway when it's all done.

 

Nope. Your tranny (in final gear - 4th) is EXACTLY the same as the 3.03 (3 on the tree) or a C6 Automatic. All the old trannies were a 1:1 final drive. Now, if you want an overdrive, you can swap to a ZF-5 tranny. Worse case scenario (assuming 4.10 rear gears and stock-ish (29") diameter tires) you'll be turning 3000 RPM at 65 MPH. Not great, but it's not going to blow anything up.

 

That granny 1st only matters if you try to put it in gear and you are moving (it is none synchronized). Your rear end gearing and tire size is what affects highway speed/eng rpm.

Low gears in the rear, add little tires, you get high RPM. Low gear in the rear and some taller tires (like 285's to 33's) will at least lower the RPM's. But you are not going to have a race truck and get great MPG.

NP-435 is what it is, only option to maybe get a better gear out of the trans, is to swap to a T-18. But do not bet the bank on that statement.

So bottom line, having a low 1st does not affect hwy driving, but is great for pulling chebbies out of the mud. lol

What gear ratio do you have in the rear axle?