Corvettelvr73

How can I tell how much my tach and speedo are off by? About how much are you guys off? I am trying to figure out my rear end gear ratio. Are they usually off by alot? :confused: :flag

vettfixr

You can measure how far off your speedometer is by going out on the highway and finding a milemarker. Read the mileage on the odometer and then drive 10 miles by using the mile markers. When you get to the tenth mile stop and read the odometer again. Any difference from 10 miles will be how far you are off. Example - odometer reads 5100 miles at the beginning of the test. When you reach the mile marker for 10 miles the odometer reads 5109.5 miles. Your speedometer is 5% too low. If your trip meter works you can use it by setting it to 0 when you start and reading it when you hit 10 miles. Speedometers are often off because of non-stock height tires or missing adapter gears on the tranny. Can't help you with the tach. Good luck.

mrvette

IF you have an old mechanical tach, by now it's most likely not as accurate as it possibly could be..due to age and dry oil/lube.....much less needing repair...typical problems....
IF you have a modern enough...meaning late shark electronic tach, and have a properly functioning tach filter, and unit....it's VERY accurate....dead nuts on...mine is, and so I see no reason others wouldn't be either....
aftermarket tachs are pretty good too, but the cheap junk can be off by a few hundered rpm's....
I calibrate a tach with an Oscilloscope, and use say a Techtronics timebase, it's dead nuts on.....just do the math then....

GENE

Koz

You have several questions here: What are you trying to figure out.. Rear end Ratio? or IF your speeo/tach are off? There are several ways to check the rear end ratio and you could always use another vehicle for speedo comparison test.. (DON'T ask the wife or girlfriend to help here.. trust me..)
If you need work done on your guages Contact Redline: http://www.redlineg.com/ This guy was really helpful over the phone. When I get the back end of my car put back together my speedo/tach will be next. My speedo is off by 5 to 8mph.. tach is way off.. It moves but can't be right..

no_radio

My speedo is off because of tire size changes, I knew that going into it.
The tach...well, with the new heads, intake, and roller rockers the engine is bouncing off the MSD rev limiter (6000 RPM) while the tach shows about 4500 RPM.
Jeff