Very interesting! Iīm very curious for technical details, for example gear ratios, axle ratio, and further modifications. Canīt find a Chevy Caprice with manual. Are those datas in the big thread, you showed me?
40-45 mph? Oh, thatīs very slow (65-73 km/h). If anybody in germany would drive that slowly on a autobahn or even a country road, he would pull over by police soon.
Nevertheless, your fuel consumption is stunning! In the united states you can drive over such long distances that slowly without any problems? Are you driving behind trucks? Here in germany we usually drive not below 100 km/h (62 mph), usually even faster - way faster.
My car (BMW 523i, 1997) reaches 5,5l/100 km (about 43 mpg), when driving a average speed of 56 mph. Only modification: A rear axle with a ratio of 2,65 (stock: 3,15). Manual of course.
EDIT: Are those ratios correct?
1. gear: 3,46
2. gear: 2,14
3. gear: 1,38
4. gear: 1,00
5. gear: 0,73
final gear: 2,14