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The easiest way to check your tach is with a digital timing light, like an Innova 5568.
The easiest way to check your rear gear ratio is to put the rear of the car on jackstands, mark the driveshaft with tape, then count the driveshaft revolutions while someone turns your rear wheel one full revolution.
And the easiest way to get good answers to questions on this Forum is to post more photos!
The problem with our cars and doing the jack stand driveshaft tape method is when the rear of our cars are jacked up with the wheels off the ground the half-shafts will be at such an angle that they will bind up if you try to turn the wheels. Both trailing arms need to be raised up until the u-joints won't bind.
BE VERY CAREFUL WHEN YOU RAISE THE TRIALING ARMS UP AS TO NOT TAKE THE WEIGHT OF THE CAR OFF THE JACK STANDS.
The problem with our cars and doing the jack stand driveshaft tape method is when the rear of our cars are jacked up with the wheels off the ground the half-shafts will be at such an angle that they will bind up if you try to turn the wheels. Both trailing arms need to be raised up until the u-joints won't bind.
BE VERY CAREFUL WHEN YOU RAISE THE TRIALING ARMS UP AS TO NOT TAKE THE WEIGHT OF THE CAR OFF THE JACK STANDS.
Had the rear on jackstands on my ‘70 The universals are in good condition, I can turn the wheels with no binding whatsoever.
Okay - thanks for all the advice. Wow that driveshaft is tucked up in there. I hate getting under cars that are jacked up, even on stands.
So car in neutral, jacked up rear end, rear tires spin easily with some effort, no binding. Driveshaft and tire marked with chalk.
I did it about 6 times. It was almost exactly 3 drive shaft rotations for every full rotation of the rear tire. There is about 5 degrees-ish slop, so I made sure to take any slack out before I marked them.
Stock rear end on these is 2.88 I think. Darn it. What the heck? The car is running such high revs at 55mph (I have had Chevy v8s for 20+ years) it's hard for me to believe this, but I did it over and over. It's either tach (unlikely) or maybe some wacky transmission? Stumped.
Okay - thanks for all the advice. Wow that driveshaft is tucked up in there. I hate getting under cars that are jacked up, even on stands.
So car in neutral, jacked up rear end, rear tires spin easily with some effort, no binding. Driveshaft and tire marked with chalk.
I did it about 6 times. It was almost exactly 3 drive shaft rotations for every full rotation of the rear tire. There is about 5 degrees-ish slop, so I made sure to take any slack out before I marked them.
Stock rear end on these is 2.88 I think. Darn it. What the heck? The car is running such high revs at 55mph (I have had Chevy v8s for 20+ years) it's hard for me to believe this, but I did it over and over. It's either tach (unlikely) or maybe some wacky transmission? Stumped.
- Jack
nothing mysterious, it's your tach. Others have already suggested checking the accuracy by using either a hand held tach or timing light with an rpm gauge. 2.72 is rear gear for manual.
If you are going to spend money, get the timing light with the tach readout, unless you already have one. The next thing everyone on the Forum will help you do is to time your engine correctly.
Stock rear gear for an 81 4-speed is 2.72:1. 2.87:1 was for automatics. (2.88 was the first gear ratio for your 4-speed, not your differential). http://corvettec3.ca/axle.htm
Perhaps it's a measuring error. Or perhaps your car was originally an automatic. Your transmission looks correct for a B&W Super T-10 4-speed transmission. The number in the blue paint appears to be your VIN derivative. On your engine stamp pad (below the AC compressor, on the block in front of the head) you should find that VIN derivative, and another code with a suffix. That three letter suffix will tell you how your car was delivered.
I've never seen an engine stamp like that. But the VIN matches the other photo (transmission?), even the light "7" that looks like a slanted "1". If that matches your car's VIN, I wouldn't worry about it.
Please let us know what the tach diagnosis tells you!
Thanks for the VIN/code verification for the transmission.
Through the magic of Amazon, I ordered a digital RPM reader and got it 2 hours later. I marked the harmonic balancer with white tape. My wife held the pedal at the below RPM the best she could (she's a keeper...)
Gauge idle of ~750, RPM meter reading 650.
Gauge 2,000 RPM = meter 1,320 RPM
Gauge 3,000 RPM = meter 1,925 RPM
Gauge 3,500 RPM = meter 2,250 RPM
Y'all have been a great help, thank you! Looks like my attention needs to be on the gauge. So this issue was same with both HEI units, old and new.
Thanks for the VIN/code verification for the transmission.
Through the magic of Amazon, I ordered a digital RPM reader and got it 2 hours later. I marked the harmonic balancer with white tape. My wife held the pedal at the below RPM the best she could (she's a keeper...)
Gauge idle of ~750, RPM meter reading 650.
Gauge 2,000 RPM = meter 1,320 RPM
Gauge 3,000 RPM = meter 1,925 RPM
Gauge 3,500 RPM = meter 2,250 RPM
Y'all have been a great help, thank you! Looks like my attention needs to be on the gauge. So this issue was same with both HEI units, old and new.
- Jack
try removing the radio suppression capacitor near the distributor to see if that effects the tach
I have a completely stock (so the old owner said) 1981 C3 with a 4 speed manual. Since day 1 of owning this car (1.5 yrs) it has always been high strung on the freeway. I replaced the carb and manifold with simple Edelbrock parts and installed an Accel ignition - the car runs and shifts nicely.
However it runs at 3500rpm at 55 mph on the freeway. Yeah. It's not slipping, accelerates just fine. It has new 255/60- 15 rear tires. As far as I know it has the stock T10 transmission. The car hasn't been touched with stock motor, etc. Hard to imagine the 2 previous owners would have changed just the rear end. This would be a helluva rear end ratio. I checked speedometer with GPS and it's accurate within a MPH too.
What the heck? I will jack the car up this week and try to figure this out. 4th gear should be 1:1, right?
Thanks for the VIN/code verification for the transmission.
Through the magic of Amazon, I ordered a digital RPM reader and got it 2 hours later. I marked the harmonic balancer with white tape. My wife held the pedal at the below RPM the best she could (she's a keeper...)
Gauge idle of ~750, RPM meter reading 650.
Gauge 2,000 RPM = meter 1,320 RPM
Gauge 3,000 RPM = meter 1,925 RPM
Gauge 3,500 RPM = meter 2,250 RPM
Y'all have been a great help, thank you! Looks like my attention needs to be on the gauge. So this issue was same with both HEI units, old and new.
- Jack
Time for a new tach board or whole new tach!
I had a similar "reading high" issue like you have. I tried taking the tach filter out of the circuit and it didn't help. I tried a new tach circuit board and it didn't help either. I bought a new tach assembly from Zip and that solved the issue. If the filter doesn't change anything for you, then its the board or the tach's movement. I'd buy a new whole tach assembly. You could take the board off the new tach and put it on the original and see if that helps, if not, put the new board back on the new tach and just install the whole thing. Tach's are ridiculously easy to change out on 78-82 cars.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
I gave up on my mechanical tach...after the tach couldnt keep up with the revs climbing and dropping and the chinese tach cables snapping every month it wasnt worth it any more. I bought a converted tach and run it off the MSD. I like it
