My 1985 El Camino SS used to show 2400 RPM at 60 MPH. Now it shows 4000. Is there possibly a problem with the tachometer, or the transmission or could it be related to my problem with my speed sensor?

Milton

The transmission and the VSS have nothing to do with the tachometer. The tachometer gets it's signal directly from the distributor. You probably have a problem with your tachometer.

Here is an example of how to determine your RPM at any given speed.

For our calculations,
we can use the following formula: mph x ring-and-pinion ratio x transmission - final drive ratio x 336/tire diameter = rpm. For example, 60 (mph) x 4.10:1 (differential gear ratio) x 0.70 (overdrive ratio) x 336 (constant)/26 (tire diameter) = 2,225.4 (rpm). Applying the same formula for 4.56:1 gears gives us 2,475 rpm at 60 mph.

Hope this helps.

Thanks for the replies. I have determined the problem is with the tachometer already. Is there a schematic of the tach available anywhere so I can remove the faulty part instead of replacing the whole tach?

I've never seen a schematic for it in any of the manuals. It's so much easier to just plug in another one.

I know this is late to ask a question of one of the responders but here goes. Ernie you said it would be easier to just plug a new tach instead of looking or trying to fix the tach. Question: where would you find a tach that plugs in to fit the 5th gen dash?

www.opgi.com
www.dixiemontecarlodepot.com
www.mikesmontes.com

Thanks a lot, Dustin

where would you find a tach that plugs in to fit the 5th gen dash?
It's GM part number is 25059057 and it costs about $100. The last time I checked with my friend who works at a GM dealership parts department they were still available. Check with a GM dealer near you and give them that part number.

Be careful if you buy a used tachometer. There is one for a V6 and another one for a V8. They are not interchangeable

Ernie, Thanks. I called my Chevy dealer and the parts guy said the they were no longer available. I'll try some other vintage sites using the part # and see what happens.