Fuel economy 325 E30's

I have seen a lot of posts for performance parts but little on fuel efficiency. Does anyone know if there is any modification to change the gear ratios on the final drive or manual transmission to enhance fuel economy? I have seen some posts claiming that some of the M3's have done modifications that have taken them into the 40mpg range. I would think a 6th gear overdrive with a smaller final drive ratio would allow a high top end speed with less rpm's. Yet, still would have the lower gear speed for winding alpine driving. With the price of gas rising I would like our 87 325 to get a little better. When tuned and everything new (timing belt, coil, plugs, wires, oxygen sensor and canister) we see as high as 32 mpg with just highway. This is with cruise set at about 70 (found out that at 70 on speedometer we were only doing about 67). Around town and highway, we get about 23 to 25. If we push the car to 80 or faster, we get about 21 to 24 on the highway. Curious to hear other comparisons and thoughts.