Help me figure out my rpm's

[Mr. B5]

I've got a '70 Roadrunner with tic-toc-tac, but tach is not accurate.
I've got a 440 6 barrel with 3.54 gears and a 4 speed, with 15" wheels.
When I'm on the highway traveling at a steady 60 mph, my tach is reading 4k rpm's.
I'm thinking it's about 500 too high?

 

[Propwash]

Need to know the tire height, not just the rim size Mr. B5

Either way, your tach is reading way high unless you're running radio flyer wheels in the back
Example.......with a 27.5" tall tire, 3.54 gear's, at 60 MPH your RPM should be right around 2595 RPM

 

[1970GTX]

Sounds high. I'm around 3000 @ 55 mph, 3.55 rear gear, 275/60R15

 

[Propwash]

Sounds high. I'm around 3000 @ 55 mph, 3.55 rear gear, 275/60R15

That sounds high as well.......I run the same tires, but run 3.91's and my RPM at 55 is around 2700

Here's a Gear/RPM calculator for ya:

http://www.ringpinion.com/Calculators/Calc_RPM.aspx

 

[Mr. B5]

My rears are 255/60x15.
The chart above shows I should be around 2,650 rpm.
Looks like my tic-toc-tac is good for show but not go.

 

[65 Coronet]

Rears 255/60/15 with 3.91 gears. At 55, RPM is about 2700.

My rears are 255/60x15.
The chart above shows I should be around 2,650 rpm.
Looks like my tic-toc-tac is good for show but not go.

 

[440+6]

My 70 GTX tac has always read high, at 55 it says a little over 3000 but I know it's around 2500, tires are 235/60-15, hemi 4 speed and 3.54 rear.
You can have them calibrated if you want, personally I never did pay much attention to the tach in any manual car, I just let it sing to me.

 

[Meep-Meep]

Those factory tachs never work properly!

 

[kiwigtx]

Those factory tachs never work properly!

Correct. And for the most part, the 'Toc' part worked for about another 5 minutes after you drove it off the Dealer's lot from new. :grin:

 

[Rocketman]

Need to know the tire height, not just the rim size Mr. B5

Either way, your tach is reading way high unless you're running radio flyer wheels in the back
Example.......with a 27.5" tall tire, 3.54 gear's, at 60 MPH your RPM should be right around 2595 RPM

That is interesting, I have a good tach and good reading at 3000 rpm I show 60 miles per hour. I thought I had a 3.54 after spinning the shaft and watching the tire. A guy at a local cruise in thinks I have a 3.73 based on what you stated above.

 

[kiwigtx]

If it helps any....my Tic-Toc-Tac has been calibrated to very close to actual RPM's - running 4-speed and 3.55:1 rears.

:hello2:

....Happiness is a Full tank of gas on an open road.

 

[67 Mopar]

Here's a nice calculator...

http://vexer.com/automotive-tools/speed-rpm-calculator