Here’s the latest reader question, along with my reply!
Peter asks: I remember reading about how you swapped out the three speed transmission in your 1976 Trans AM for a Six speed transmission which resulted in higher mpg. I have owned this 1988 Corvette for thirty years too and was wondering if a modern eight speed would be worth the cost in updating the electronics which would enable higher mpg? Over the original three speed transmission with a fourth gear overdrive. Look forward to your thoughts on this idea.
My reply: My ’76 TA did not originally come with an overdrive transmission; therefore, installing one (in my case, a 2004R four speed automatic, with 4th being the OD gear) dramatically cut cruise RPM and greatly improved mileage. At 70 MPH, my car’s engine now turns about 2,000 RPM. Previously, it turned closer to 3,500 RPM at the same road speed.
Your ’88 Corvette came with an overdrive transmission, so changing it out for an overdrive transmission won’t make that much difference and so is not worth doing, in my opinion. Your car’s engine probably turns about the same RPM at 70 as a new Corvette’s engine does at the same road speed. It might be a little less – but not nearly as much as the difference between my car “stock” and modified.
It is true that a current eight speed (or ten speed) may have a deeper OD ratio, but the difference won’t be significant once you’re in OD. As an example, my 2004R’s OD fourth ratio is .67 – that’s about as “deep” an OD ratio as you’ll find in any modern OD automatic. The big difference between a four speed overdrive like my 2004R and the modern OD automatics with multiple OD gears is that the transitions between the gears aren’t as steep.
My 2004R goes from 1:1 (third) to .67 (fourth/OD) whereas a modern eight-speed has two steps before it gets to top (eighth) overdrive. This make for a smoother drive, but – honestly – it’s not a big difference there, either.
My car drives very much like a new car despite being an almost 50 year-old car.
So, I’d stick with what you have; maybe consider getting it “built” for more positive shifts if it ever needs to be rebuilt.
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