I'm trying to figure out if I keep this up, I'll be looking at blowby after a while? I've been driving my truck to work and it's a lot of freeway driving. 20 miles one way. I get up to 70, sometimes 75. When I first got this truck I didn't want to go over 55 but I've been doing it more and more. I found out it had the original governor spring in it which I believe regulates to like 2800 RPM or so, so I guess I'm going less than that? I may even be able to hit 80 or more if I tried. But I don't want to wear my engine out.

It's a 1990 3 speed, no overdrive. Wondering if I should keep it down at 65 or below.

 

I cant speak from a whole lot of experience but i have been driving about 65-70mph back and forth to work (50 miles round trip) for almost a year. Is your stock speedo reading 75-80 mph? Id be surprised if that is accurate without having an overdrive. Try to check it with a gps if you can. If not then my truck must have something up with it.

 

Well I checked with a GPS on my phone and it was about 5-6 off. I got it up to 80 on my speedo, and my phone said I was going 74. I tried putting the accelerator down more, but it was about maxed out. So 75 is about max for this truck.

1990 Cummins stock drivetrain, stock pump, 104k original miles. Just don't want blowby so will keep it below 70 MPH on the speedo, meaning i'm actually doing 65.

 

you wont hurt anything.

 

You can have one of these engines running at redline all day long and not hurt a thing, it's a Cummins, they're meant to be worked.