O.K. guys don't laugh,

I've got a '72' 402,TH400,4.10,26.5" tires. Since I bought this car (about 2 months ago) I thought the tach was inaccurate, but tonight I found out for sure. I was testing speedo to see how far off it was, trust me it is off. My wife drove next to me in her car with cruise set at 50mph. My car was showing 74mph. Based on this I know I have some speedo gears to deal with. Now, on the way home I'm cruising at a cool 50mph (chevelle time) and I'm turning 3700 rpm's. I know the speedo and tach have no relationship together, but when a tach simply hooks up to the coil for reference, how can the rpm's be high, and I don't mean double. I've always thought this was the case, but I guess I was in denial. By the why, ignition is MSD connected to tach terminal on back of printed circuit board.Based on published formula for rear end ratio,tire height,true mph, I should be at 2619 rpm's at a real 50mph.

HELP!!!!

Joe
Check out my web site for help with speedo recal.
A thought on the tach. Is it original? If aftermarket, could be wired for a 6 cyl. That would make it read 33% high.

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Fred Aldrich
Web Site: www.GeoCities.com/~69_chevelle (http://www.GeoCities.com/~69_chevelle)

Fred,

Tach is original factory installation. That's what concerns me. Also, shouldn't the dash wiring harness supply a tach signal? There is a single wire (white) from the MSD box that attaches to the terminal marked TACH at the back of the printed circuit board.

Tach wire is separate from dash wiring harness. MSD = Multi-Spark-Discharge and your factory tach may not be compatible. MSD sells a tach adapter that should solve your problem.

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Steve Strasemeier (70SS)