speedo revolutions per mile

[olnick]

I'm replacing a worn out (only 42 years of use) 1978 100 MPH speedometer with my only find an 85 MPH unit. Do both of these units calculate speed using the "1000 turn per mile" sae 678 standard or does the 85 MPH speedo use a different standard? I want to make all changes at once so I am NOT chasing around for 6 months trying to get the compatible parts?

Thanks

Olnick

 

[1987 GMC Jimmy]

Not to deter from your question but whatever the issue is on your 100 mph unit, I’d send it off to be fixed. I hate the 85 mph units, they are totally ubiquitous where the 100 mphs aren’t, and there are mail order places all over the continent that’d do a good job for you. That is if your speedo cable is good.

To play to your question a little bit, the head on the 100 mph unit has more resistance when you compare it to the 85 unit. I don’t know the mathematics of it, however, but it’d be plug and play.

 

[Turbo4whl]

I agree with Jesse, you may miss having 100 MPH gauge. Yes the 85 MPH gauge will work fine.

Before I swapped out my 85 MPH gauge for the 100 someone might ask how fast were you going? I would say 2 or 3 seconds. "What?" That is how long it would take for the needle to drop off the pin on the 85 MPH gauge when I let off the throttle. Got to love a torque built big block with highway gears.

 

[olnick]

I thought they might be compatible! The 100 MPH unit works fine up to 60 mPH then it jumps up to 80-90 MPH and the GPS shows 64-65 MPH the miles traveled are still correct?? If I slow to 60 or less the needle goes back to the correct speed. I think i'll stick the 85 MPH unit in and get the original repaired.
Thanks for the input.

Olnick

 

[1987 GMC Jimmy]

A bad or even a dry cable can do that. Any length of good cable and a drill will tell you if your speedo head has a problem or if it’s the cable. If you wanna try to work with the cable you have, I believe it’s best to pull the cable out of the sheath if you can, clean both parts up with mineral spirits, reassemble, and dump it to the brim with a mixture of powdered graphite and denatured alcohol, just enough to liquify the graphite (not a lot). Do all that while spinning the cable with a drill. You can also use that liquid wrench dry lube stuff. It works okay, and that graphite is very messy.

 

[SquareRoot]

My 85 K20 has 4:56 gears and a turbo 400.

Anything over 65moh is a pipe dream. However, the Dakota Digital dash allows infinite and quick calibration of any drivetrain combo.

Bittersweet reply I guess.

 

[olnick]

I'll check the cable, however back in 2015 I did remove both the upper and lower cables cleaned them lubed (white lube) and reinstalled them. I have not put on 5000 miles since then! But knowing Murphy and his law I'll check it out..

Olnick

 

[Snoots]

I'd go with repairing the 100 MPH unit too.