Since no one else spoke up... I think the tach works off the frequency of the AC produced by the alternator. I doubt it would be suitable for your purpose. I'm thinking you're going to have to tape some magnets to the crank pulley or something. (Wasn't there an RPM signal generator that ran off the old vacuum pump drive?)
The Constitution needs to be re-read, not re-written!
If you can't handle Dr. Seuss, how will you handle real life?
Current oil burners: MB GLK250 BlueTEC, John Deere X758
New ride: MB GLS450 - most stately
Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510