[hat_man]

Thank you Cripple Rooster.

My thoughts were this.....

Let's assume that my engine really likes the 2000-2200 rpm range verified by my temporary vacuum gauge hook up. 55 mph in 5th gear gives me this range with my current tire size (already 5% larger than stock) and rear gear (3:73). 15 miles of my drive every day is at 65-70 mph. Even in 5th gear it's nearly 3000 rpm and I'm sure not very fuel efficient. If I do some math I might find that swapping to say a 3:55 or 3:45 rear gear may give me the magical 2100 rpm range at 55 mph but in 4th gear rather than 5th, and the same 2100 rpm range at 65 mph in 5th gear. This would give me optimal FE for the whole drive to work except for the short in town bits. Maybe I'd get lucky and 3rd gear would be my <40 mph gear. I just need to find the optimal rpm range for each gear and speed and then get out the pencil and paper.