My 1985 Camaro iroc z28 has a 396 big block in it with a th475 transmission which I know is out of a motor home. And being it’s a 3 speed automatic with no overdrive would it be normal to run 2700 to almost 3000 rpm’s at 60 and would it hurt the motor any running it at faster speeds in higher rpm’s cuz at 65-70 she’s at about almost 3500 or so. Any help would greatly be appreciated.

 

What do you want help with?

In 2023, there's a transmission that's essentially a Turbo 400 w/ overdrive. It's been in the heavier-duty light trucks like the 2500 series since the mid 90s. Sounds like from the direction your post is pointing, that maybe that's kinda what you're looking for. Also sounds like either you have 4.xx gears, or you're getting those RPM "numbers" off of the way-pointer trim package thing in the dash, which is well and widely known to fail in the direction of reading WAY TOO HIGH.

 

Im pretty positive I have 3:73 gears in my car and yes i was going my the tachometer on the dash as to what rpm’s I was running at those speeds I mentioned. And my questions are with the trans that’s in my car now would it be good to hop on the highway and cruise or should I look into swapping the trans out for say a turbo 350 or 400 or even a 700r4.

 

The 475 IS a 400, for most practical purposes. About the only significant difference is that it has straight-cut gears rather than helical, for a little more strength (and noise). But AFAIK the clutches and whatnot are all the same.

A 700 or a 4L60E would probably not be a good idea behind a 400 CID motor unless it's one of the lower-option ones ("325 HP" ... maybe the one out of the same motor home or school bus or whatever, that the 475 came from?) and bone-stock. And I do mean BONE-STOCK: no other cam, no other heads, no other carb, still has the exhaust manifolds, etc. Anything that's been upgraded, or if it's one of the higher power ones, no.

People have been driving transmissions without overdrive on the highway since ... the beginning of transmissions and highways. There's no reason you "can't" use it that way. Yes it will wear the motor out faster than one WITH OD; every time it turns over, some amount of wear occurs, therefore the higher the cruise RPM is, the more it wears. Also tends to use more fuel. Other than that, it'll be fine.

Best option, in 2023, is probably a 4L80E. And check your trim package's accuracy against a true "instrument" just to be sure.