[86k10]

Well since I have a similar truck I can tell you my experience. I did a mileage check during 2 trips to Florida and back. And that was back during the 65 MPH speed limits. I ran an entire tank out more than once running in 4th and did the same running in 3rd and my difference was 16 in 4th to 15 in 3rd, not much difference. And that was when I was still running the original 305 and 3.08 gearing. As for what it gets now with the big Olds and 4.10's is about 14 in 4th and 10 in 3rd. So I think you would not be that much better with the 4 speed auto. But you can always save up and put one back in if you desire. Just my 2 cents.