Mildred, my '41 pickup, is fitted with a 283 chevy, tuned to 270 HP at the flywheel, an S-10 five speed, and a Salisbury rear end from a 1951 Studebaker. Definitely an old school rod build.
The rear end needs bearings and I am considering replacing it.
The S-10 five speed has an overdrive ratio that drops the engine speed to 1500 RPM at 75 MPH. While the motor seems happy, any time I am driving down a country road at, say 60 MPH, the trans stays in fourth gear and the RPM sits at 2500. Overdrive just about cuts the RPM by 40%.
Question - is driving around at 1200-1500 RPM is a properly tuned, warmed up motor a bad thing? I don't mean going up a hill, just cruising on a flat road. Once I get the Salisbury diff apart I can do some calculations given the current rear end ratio. if bearings are available, I may just keep it as is. Or change it out for a 9" Ford with a lower gear ratio.

 

Great looking truck, and I am really glad to see that is a manual! I really doubt you are doing any damage with that RPM. If you want higher RPM, just drive faster.

 

I also doubt if you are causing damage but may not be running at peak efficiency. And most of those T5 are only about 20-25% overdrive, not 40 so maybe your tach is goofy. A couple personal recommendations, run a vacuum gauge to make sure you aren't lugging it. And even with a mild cam, I'd think you want the normal cruise rpm in top gear to be closer to 2000.

 

When you are traveling along at 1500 RPM and you push down on the accelerator just a little bit, does the vehicle respond easily? If not that would show that you may be lugging the engine. So, if the engine is lugging you can push down on the accelerator and not much happens. Whenever the engine responds easily it is running in its power band, it is on the pipe so to speak.
Hey, I like your pickup a lot!

 

1500rpm at 65-70 doesn't seem so bad to me either.

 

1500 RPM at 75 MPH.
2000 RPM is near 85..... a little faster than I want to drive on a regular basis.
Tach is correct, as is the speedo. I believe the previous owner built the S-10 to have this kind of reduction in 5th. The first 4 gears are very close ratio, so the truck really gets up and goes.
Hadn't thought of a vacuum gauge.... that's a good idea.
Thanks to all for the suggestions and comments. If I can, I'll upload a short video of the truck at speed.

 

You will have to post the video on Youtube. Then upload the link here.
Are you saying that the OD gear has been modified to a higher gear than stock?

 

There are 0.63:1 fifth gear ratio gears for T-5 transmissions. Sounds like you have one.

 

That sounds right, so I guess it's a stock box. Can't imagine why the OEM would have that kind of reduction.
Bearing kits for the rear diff are not too easy to locate, so may change it out.