Tach accuracy? Or do I have 3.9 diff?

[Guest HALLOWEEN Z]

I was just wandering about the accuracy of the autozone sunpro tachs. I put a 350 in my 75 280z which was all original. I've had friends drive at my speed to see what rpm its running at different speeds and it seemed a little high. And at the drag strip the tach says 5000rpm exact which when I enter my tire size and everything it should be 3.90 gears. Its not a big deal to me or anything but I was thinking about putting 3.90's in it and if I've already got them I don't see a reason to. Also the reason I've got an autozone tach is because the original was way off.

[blueovalz]

I'd verify the differential gearing first by counting the revolutions of the propeller shaft (or drive shaft) verses the half shafts. Propeller/Half shaft is the ratio that you want. If only one half shaft is turning when you turn the driveshaft, then this will cause the rotating half shaft to spin twice as many times (single track diff) as it would if both half shafts are spinning (LSD), so your 3.9/1 would look like a 7.8/1 if you don't correct it, but this would be obvious when you see it. Then you'll know the final drive ratio and be able to verify the acuracy of the tach when you plug in tire rotations per mile, and test it in say, a 1:1 gear to make it easier (4th gear on most manual transmissions), at a specific speed. Most speed verses rpm tables will give you the information you'll need for this.