1972 Factory tach accuracy compared to aftermarket

Pretty close

It's actually pretty close on the reading. I used all the Mach gauges, but ran a volt, oil pressure and temp gauge into the glove box. You might look at putting the shift light down by the headlight switch. The factory had a red "brake" light in a hole next to the switch. You could also wire the high beam Mustang light if you wanted it higher on the dash. I don't think I'd try to get into the tach to install a light.

Pretty close

woops

I would test it in the engine compartment while having someone check it inside. I remember mid-80's testing on factory tachs being off by as much as 500 r.p.m. in the upper ranges. At 7500, I would not skimp and have that be 8,000 and blow the engine.

I tested the factory tach in my 67 GT350 vs. a very expensive Autometer tach. It is the same all the way to 5000 rpm (haven't tested it higher than that) but the factory tach "lags" a little on its way there.

Example: if I hold the engine at a steady 3000 rpm, they both say 3000 rpm. If I rev it to 4000, the aftermarket tach gets there first.

There are several people who can calibrate a factory tach. You can have it done so that it uses the factory wiring harness OR so that it hooks up like an aftermarket tach.

most aftermarket tachs use a a stepper motor and the factory tachs use a coil winding type system which is the factory tachs react a little slower and aren't quite as accurate at higher rpm. a lot of places can actually convert your factory tach over to better aftermarket tach guts while keeping the factory tach face essentialy giving you the best of both worlds.

red line gauges is one such company i can think of right now, also i think the Tachman can do this as well. he can also recalibrate a factory 6k tach over to an 8k tach too.

I was making suggestions to keep it looking "factory". You could use you high beam indicator or the a light in the dash next to your light switch.

make sure you one of the more reputable shops when you do this and also make sure the original tachs guts back in case you ever want to put it back stock later down the road. it's kinda funny i was checking out the classic cougar forum the other day and there were guys that were downright pissed that some shop replaced the stock guts with aftermarket stuff. i don't know what brand the shop used but these guys were upset because they replaced the original factory stuff with aftermarket. i was like ummm...ok? maybe these guys don't realize that on most of the factory tachs when they die the car won't run because the tach is an integral part of the ignition system and usually even a cheapie aftermaket tach is more accurate than the factory stuff or at least not any less accurate. maybe it's just becuase they didn't know that's what was going to happen and they weren't warned about it first, or maybe because the original stuff is so expensive and the shop (s) kept the original guts, which i'm assuming weren't working in the first place otherwise why would they need to be sent out for repair.

anyway, you may want to make sure to get the original mechanisms back just in case.

personally, im fine with the stock stuff since my cars are usually fairly stock or not too far from it but if i was ever to build a really high dollar car or even just an engine then the aftermarket stuff would be nice for piece of mind. i also build all my own engines for piece of mind but the stock gauges have never let me down and i never rev the engine to more than 6k anyway, so even if it's off by 500 rpm it'll be fine since i build the engines to handle that much.