Thanks, I don't think a differential swap is practical (financially) any time soon. I too keep records of my fuel economy on my receipts. My 300D runs 80mph at roughly 3800rpm. 75mph at roughly 3500rpm. So I'm guessing 70mph will run at 3250rpm and 65mph around 3000rpm.

Won't a drop in RPM result in a loss of power? Here goes the balance between power and fuel economy...

Thanks again