1978 Chevy C10 w/ IL6 4.1 tranny questions

[filmsmurf]

I got a super base model, (Custom Deluxe) 1978 truck with a long bed and a 4.1 straight 6. At some point, someone changed out the column shift for a floor shift 4-speed (its really 3 with a granny gear) and cruising speed is about 60 mph. 65, if I push the rpm a bit but at 65 the engine sounds a bit strained. I don't know if the speed limitation is from the tranny or the rear differential. I really like these old trucks but this transmission just seems a bit clunky and limiting. I'm not looking to go 90, but I wouldn't mind a comfortable 70. The trans works fine, it just seems slow and clunky.
I've read that often these old trucks are hampered by the rear end ratio (I don't understand much beyond how a bicycle shifts gears). Whatever I do, I know it's going to be expensive, but I would put some money into a better rear end, if that would help, or even a rear end/trans combo, but that would be extreme. I am not a mechanic, but I did quite a bit of work on my old 64 Bellaire with the same engine back in the day. Nothing like changing out a transmission tho. I think what I would like to find is a transmission that has a highway gear and a little smaller shift pattern. I'm not looking to race or speed shift, but the gears on this tranny are quite far apart.
I'm just wanting it to go a little faster on the highway and not feel so clunky.
Ideas?

 

[Rickf]

If it was me, I'd look into a Gear Vendors OD unit, not cheap, less work than a gear swap.

 

[bucket]

The current trans (SM-465) has the same 1:1 top gear ratio as the original 3-speed transmission.

Swapping in a later model NV3500 (5-speed including overdrive) from a mid-late 90's GM truck might be a good option. It would not be a direct swap, but perfectly doable and could be budget friendly with careful parts shopping. It would be more than capable of handling the 6 popper power.

 

[Matt69olds]

Before doing anything, find out what gear ratio you have.

As mentioned, high gear is 1:1. To lower the cruising rpm, you either need to regear the differential, or add a overdrive. Taller tires will help to a lesser extent.

 

[filmsmurf]

Thanks everyone. After some research, I decided to hire a Dif' guy with a great local reputation to switch me from a 3.75 to a 3.08 plus posi-track so both wheels are engaged. Not a cheap option by any means, but cheaper than a new transmission and the rear end needed help anyway. It had water in it. This will get that axle healthy, go faster and do better in snow and mud. I'll post an after-report with pics.

I appreciate everyone's input. I'll be bringing you all along as I restore this bad boy one step at a time.

 

[WFO]

Thanks everyone. After some research, I decided to hire a Dif' guy with a great local reputation to switch me from a 3.75 to a 3.08 plus posi-track so both wheels are engaged. Not a cheap option by any means, but cheaper than a new transmission and the rear end needed help anyway. It had water in it. This will get that axle healthy, go faster and do better in snow and mud. I'll post an after-report with pics.

I appreciate everyone's input. I'll be bringing you all along as I restore this bad boy one step at a time.

That's what I was thinkin too. Especially if that rear end was already pretty worn out.