Eastflorida

It doesn't seem right that my tach shows 3,500 RPM at 50 MPH. It is shifting to third, so I've kinda eliminated that as a problem.

Where do I start to figure this out? I read an old car magazine test that said I should be cruising at about 70 MPH at 3,500 RPM... I'm showing about 4,500 RPM at 60!

Had a friend pace me and my speedo is off about 3-4 mph at 50 (he indicated I was running 54). Can the tach be off too? Seems like a lot of R's for a BB..

Thanks for any suggestions...

oldcutlass

It would not be the first time a tach is off, there are a multiple rpm formulas on the net that you can plug in your trans type, rear end gear, and tire size to come up with what your rpm should be at different speeds.

Eastflorida

Can the tach be corrected if it is off?

oldcutlass

It may not be the tach if your running aftermarket ignition. Others may chime in on who they used to repair gauges.

Eastflorida

Thanks, still stock with points! lol

BangScreech4-4-2

In that case, why don't you hook up a dial-back timing light and confirm your RPM that way?

Eastflorida

Dude, I got rid of that stuff decades ago!!! LOL

Thoughts on switching to HEI?

Fun71

It won't change anything other than no more routine points maintenance.

What kind of tach do you have? Aftermarket ones have a switch for 4-6-8 cylinder, so if the switch is not set correctly the readings will be way off.

tru-blue 442

Do you know what rear gear you have under it?

BangScreech4-4-2

[QUOTE=Eastflorida;1026890]Dude, I got rid of that stuff decades ago!!! LOL/QUOTE]

Well, if you're gonna drive an old car it comes in mighty handy.

HEI won't solve your problem.

oldcutlass

Tools needed for old cars that are a must, tach/dwell meter, timing light (preferably adjustable), vacuum gauge, and a test light. You will never get a car to run perfectly without these.

Rocketbrian

Is it the factory Tic Toc Tac, as they are notorious for getting out of kilter?

30706005

I'm having similar problems. I have a 67 Cutlass with a 400 engine, freshly rebuilt TH400 tranny and a 3:42 posi rear end. My tack is showing 5500 rpm at 55-60 mph. The tranny shifts twice, so starts in 1 shifts to 2, shifts to 3 however it shifts to 3rd very quickly. Can't figure it out.

RetroRanger

307 your tach is wrong unless you have tiny rear tires.

Florida my car is 3200 ish rpms at 70 mph w 3:42s an 255 60 15s that are about 27" tall

30706005

I have 235-60-14, 25" diameter