3.88 trans

are the 3.88 trans different ratios in the gears. or is it just the r&p. have new vintage and 388 is writen on top of case. and how much does this affect r.p.m. on highway.the 388 i about 17percent smaller # than 412 so is r.p.m. about 17 percent less. bill

That's likely the ring and pinion, more often seen as 3:88 to 1. Your engine doesn't have to work as hard at highway speeds with that ratio. You'll save on gas.

Larry, I still use that calculator of yours from time to time. Usually late at night, surrounded by car parts ... when nobody else is awake ...
Math is sexy, isn't it?

Assuming you have 195/65/15 tires, a .89 4th gear and a 3.88 Ring and Pinion, at 3,000 RPM's, your actual speed is 64 MPH

Same tires & same 4th gear but, a 4.12 Ring and pinion,your speed at 3,000 RPM's would be 60 MPH

I don't think anybody really answered his question about the intermediate gears, including 4th gear. I'm no expert, but IMHO there are dozens of intermediate combinations that can be fitted into a standard gearcase. If I went from a 4.12 to a 3.88 it suggests that I have made some engine mods and have a bit more than stock horsepower - maybe a lot more? If that be true, you may want to adjust all your intermediate gears to make the trip from 1st to 4th more satisfying.

This calculator may be helpful.
http://www.glenn-ring.com/tech/tranny_calc.xls