"When I hear claims of 20 plus mpg with a stovebolt, I have to laugh." Denny
"You can shake your head and laugh if you want. . . . i got 19 mpg" Bart

Bart,

There are a couple Tech Tips on gear swaps, resulting in different rear-ends/ratios.

What is your rear-end ratio?
What was your average speed?
How did you recalibrate your odometer/speedometer?

Denny's incredulity (regarding 20mpg) is most likely directed to a stock 47-55 stovebolt - referring to a 4.10 rear end.