3.42:1 gear ratio versus?

We have a 1965 chevy impala with a rebuilt 396ci engine, rebuilt TH 400 tranny and new rear end with 3.42:1 gear ratio. At 55 mph, the engine sounds like it’s working a little hard

1) seems like the tranny should have shifted to third to put less stress on the engine and

2) since it’s not shifting, is the gear ratio wrong for freeway driving or

3) is something not right with the tranny and what needs to be done to correct the problem of not shifting to the next gear?

Gear ratio is wrong for a 3 speed trans. The solution, an overdrive transmission so you don’t lose your take off speeds.( if you change rear end ratio it’ll be mud on take off )
re; my 79 chevy c10 pickup 350 eng, th350 trans, same exact thing. You can feel it shift through all 3 speeds and by about 45 or so your mind is looking for another shift, but it’s all done with the 3 that it has. I just don’t use it on highway trips but it’ll yank a stump and efforlessly haul a 27 foot loaded trailer. In fact, the transmission shifts SMOOTHER when pulling a load. On the rare occasion that I need to go to Albuquerque ( 140 m one way ) with a load I just ‘suck it up’ and stay in the slow lane about 55/60.

By my calculations, using the gear ratio calculator at F-body.com . And using a 1:1 3rd gear and guesstimating tire size at about 235/70/R15 (probably not exactly right, but probably in the ballpark as far as overall tire diameter goes. It looks like with a 3.42 rear end you should be turning around 2900 RPM @ 71MPH. This a little high for a big block, but not outrageous either. In 2nd gear @ 55 MPH you should be turning around 3400 RPM, but under cruising conditions the transmission should’ve shifted into 3rd by this point. I’m not a transmission guru, but you may have a slipping band that’s keeping the 2-3 up shift from happening, but that’s just speculation on my part.

For a weekend/fun car the 3.42 rear end ratio is fine, even a little conservative.

A tach reading would be nice for verifying true engine rpms versus “it seems like it’s working a little hard” and would also allow you to determine if the shift is actually happening. As an alternate test, you could manually shift the trans. Select 2nd gear and get the car up to around 40mph or so. Then manually move the gear selector to 3rd and see if the engine rpms go down accordingly. If not, it’s not shifting into third. Simple test that eliminates a number of potential issues. Try that and post back.