FL442

I was having an issue with my 1971 442 455 overheating. One concern that came up was it appeared the rear might have be swapped for something like a 4.10 based on the speed vs RPM in third gear. At 70mph I am over 4000 RPM. However, with the car on the lift counting axle spins to tire rotations, we got something like a 3.08 rear ratio. I am definitely in 3rd gear at this point so I am not sure what could be causing it. Could this be a problem with the transmission? Gears swapped?

gs72

To be 100% sure of the ratio you need to pull the diff cover and count the number of teeth on the ring and pinion and do the math. Then you will know for sure. 4k sounds a little high for a 3.08 ratio

lshlsh2

Something else to check. Is your tach reading accurately.

Rallye469

Somethings definitely up. I have 3.90’s and I’m around 3k at 70.
Do the RPM’s feel that high?

FL442

I will check the tach. It would have to be off by over 1k RPM to get that difference in rpm vs speed.

The speedo is off by about 5 mph at highway speeds based on speed GPS apps. So if the speedo shows 70, I am actually closer to 65. I am sure the tach might be off a little, but it certainly doesn't feel like it could be off by that much.

We counted the rotations several times on the lift. Just a little past 3 each time. It wasn't near another quarter rotation which is why we figured it must be a 3.08.

Fun71

My bet would be the tach is off. Try temporarily connecting a different tach to verify the RPM at highway speeds.

edzolz

Is the rear a POSI limited slip? If not, it is really hard to get an accurate reading on the number of rotations. I tried this on 67 442. The rotations run out to a 3:91 rear but it was actually a 3:42 when I pulled the cover and read the gears.

Fun71

If the rear is open diff (or if the posi is worn/slipping), then you need to block one wheel and multiply the turns x 2 to get the ratio.