How did you manage 46-50 mpg with your '89 Civic Si? I could only manage 36 or so highway with mine. That was still better than what the EPA sticker said, however.
The 46-50+ MPG with the '89 Civic SI was at the 55-60MPH range. It dropped closer to 42 when I cruised at 75 and hit the high 30s at higher rates. It was a sweet running car and would cruise at 95 all day, if you could find a place to do it, like northern highway 65 in Alabama after dark. When I souped it up (cam/header/k&N air filter and a performance chip) it would really scoot, but the highway mileage started averaging in the mid to upper 30s. I got rid of it at 178,000 and I still trusted it at red-line and a tad above. The guy who bought it moved from S. MS to the northeast and put another 72,000 on it before selling it again. The '89s were one of the best, all-around Civic hatchbacks they made.