[Mike C]

I did read your post Fitz and as usual you have good info. But I responded because I don't think going to any gear less than 3.73 would be a good idea with an OD auto. And you suggested he do OD and a lower numeric gear but didn't specify what that would be.

And it's probably not relevant to the OP but a stall converter can skew the RPM at speed as well. My brother's Camaro with 3.73 turns more R's on the highway than mine with 4.10's and that's because mines a manual and his has a 2800 rpm converter. And on top of that, the OP says the tranny slips at speed. And he has no tach.

Since he didn't say what gear he had we all made some assumptions. If he had 3.07 and added an overdrive that would be too low an RPM for the most part. And either 3.73 or 4.10 would work fine on the highway with an OD to your point. The only disagreement I had was doing the lower numeric gear (since I was assuming he has 3.73 which is highly likely) AND going to a lower numeric gear.