Interesting comment.
BT's tach also reads WAY too high in his turbo conversion. I'm not sure if it did before or not.
One way is to know your tires, gears, and trans ratios and use a calculator to figure what engine RPMs should be in a given gear at a given comfortable cruising speed for that gear.
Even if you have an auto box, cruising on level ground, not trying to accel or decel, TC slip should be minimal.
Also know your ACTUAL speed via GPS, dont trust the speedo.
Other than that, the only thing I culd think of would be to try and sample the pulse off the crank sensor at the diagnostic socket.
I'm curious Steve, how is your tach wired? When I got my 617A motor I took the diagnostic plug, cap, and wiring from the car. In those cars the diagnostic plug socket is actually the tach signal conditioner. The crank sensor, +, and _ battery voltage go IN to the diagnostic plug cap, and a tach signal comes out. In the cars the tach is dead if the socket cap is off.
I've been hoping that I can just wire the tach output line from the diagnostic plug to my tach and have it work right....but so far it's only hoping. If you tell me yours is wired as I plan to do, and it doesn't work, then I'll be figuring on having the alternator wired for a tach that senses that way.
How isyour tach connected?
-Dave G.