todd estal

what rpm do you see at 70 mph ? i believe my tach is out of whack seems to work right at idle but at highway speeds not right at 75 its showing 4200 the car is all stock 1975 350 4 speed 20k original miles . thanks

62corvette

The answer has a lot to do with your rear end ratio, but 4200 at 75 is high. Not for my 62 with factory 4.56 however.

todd estal

yes i understand that . but it is all stock i was looking to see what some other stock 75 vette owners are seeing for rpm at highway speeds i have been told they came with 323 gears but not sure

C3Hawk

To calculate expected rpm, you need
1) your wheel-tire combination and
2) your differential ratio.
To keep it simple, ignore possible slippage in the drive train.

Go to a tire calculator like this one
http://tire-size-conversion.com/tire-size-calculator/
For my '75 coupe I have 235/60R15 tires so I get 773 revs/mile.

Look at your differential casing to get the ratio code (assuming the original ratio is still in there.) I have a 3.08.

So, at 60 mph, I get (773)(3.08)= around 2400 rpm.
At 70 mph, I get (70/60)(773)(3.08)= around 2800 rpm.

I have yet to fix my tach, but this fits with how my '75 sounds. If your car spins way faster, putting the car on stands and marking the drive shaft while turning the rear wheels will verify if the rear end was changed or not.

Procrastination Racing

Try this.

http://www.angelfire.com/fl/procrastination/rear.html

With a 3.08 rear, you will get (rule of thumb) roughly 1000 for 25 mph. So 4000 rpm is roughly 100 mph.


Speedo at 70


Speedo on 75

Mike Ward

Ratios available for '75 w/4 speed were 3.08 or 3.36 with L48 engine.
L82 had 3.55

There's something wrong with your speedo or tach or both. Calibrate the speedo first using a GPS.

todd estal

speedo is dead on with GPS. . Yes i know my tach is not reading right i was just wording how far off it was . thanks for the pics i frigged it was at least of by 1000 rpm . now how do i fix it . thanks

LS4 PILOT

75 s used a HEI distributor , so your malfunction is most likely an electronic issue of the gauge itself. Not sure if they can be user calibrated .
Maybe removed and sent off traded out .