Tach calibration - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**pjmoreland** · 05-02-2005, 04:08 PM

If you know your speedometer is accurate (mine is), then you can do a pretty simple test of your tachometer as follows.

My factory tachometer reads about 2000 RPM at 65 MPH. I have a 700R4, so with the torque converter clutch locked, the transmission has a 0.70 overdrive. My rear end is 3.73. I have 255-70R15 tires, which have a diameter of approximately 29 inches (15 inches plus two times the sidewall height, which is 70/100 x 255 /25.4). So...

2000RPM x 1/0.70 x 1/3.73 x 29in x 3.14159265 x 1ft/12in x 1mile/5280ft x 60min/1hr = 66.08MPH

That's pretty close in my opinion.

05-02-2005, 04:08 PM	#13
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 6,066	If you know your speedometer is accurate (mine is), then you can do a pretty simple test of your tachometer as follows. My factory tachometer reads about 2000 RPM at 65 MPH. I have a 700R4, so with the torque converter clutch locked, the transmission has a 0.70 overdrive. My rear end is 3.73. I have 255-70R15 tires, which have a diameter of approximately 29 inches (15 inches plus two times the sidewall height, which is 70/100 x 255 /25.4). So... 2000RPM x 1/0.70 x 1/3.73 x 29in x 3.14159265 x 1ft/12in x 1mile/5280ft x 60min/1hr = 66.08MPH That's pretty close in my opinion. Last edited by pjmoreland; 05-02-2005 at 04:08 PM.



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