I'm trying to figure out why at 75mph, with a 2:56 rear end and a TH400 (speedometer gear has apparently been changed? But unclear on that) the Tach reads 4000rpms. According to Summit Racings calculator, I should be closer to 2500 rpm. Am I missing something? Tires are 225.70R14
Thanks for the help.

 

Yes, you are missing the skepticism you need towards factory instrumentation.

A large v8 at 4 grand lets the world know power is happening. You would get the feeling that the thing is about to go into orbit, whereas 2.5 grand is nice, low powerband cruising. You should verify your tach is correct, and verify your speedometer is correct. Tachs are checked with another tach, like an aftermarket or an old engine analyzer, and speedos can be checked with GPS's, or holding 60 and timing a mile.

 

What type of tach do you have? OEM or aftermarket? I wonder if the tach is set to the wrong engine family (as in set for 6 cylinder instead of 8 cylinder)?

 

Well my speedometer is off by 5mph (so the gauges reads 70 but GPS reads 75) I will have to get another Tach and check it. But it seems right at idle (700-750)

 

OEM, with an inop clock 😁

 

Tachometers are off proportionally per rev. My H/O's tach looks right at idle, but is at 4k when I'm at 2.5k on the road, and I will adjust it.

 

Either 3rd gear is missing, or the tach is way off. I’m betting the tach is wrong. Factory tachs are designed to look good, but aren’t very accurate even when new. And if the engine really is spinning 4K, you would know it.

 

OK, I did verify that I have all three gears working properly. I'm going to hookup my meter tonight when I get home and verify RPM. I would have to guess that the Tach is off as well.