With 3.50 gears, 31's and a bone stock 302 my '73 got nearly 14 mpg consistently then at about 16 years old I started screwing around with carb, headers, intake, etc....mileage went down but fun factor went up.
Gas was cheap then, so I didn't mind.
Fast forward about 4 years and I built a 351W for it. Mild head work, clean up of the casting for better flow, nothing crazy, roller rockers, Rhoades bleed down lifters, Edelbrock Performer intake, etc...
10 to 1 compression. Erson can with similar specs as the mild Edelbrock grinds we all love today, Duff headers, 390 Holley 4 barrel, etc...
Making approx 330 hp according to the package of parts I was using. I nearly exactly copied a magazine article from the early 80's, even though it was nearly 10 years later....it took me a long time to finish the engine build, haha....
Anyway that build netted me about 14 miles per gallon still.
Whut?
This much extra power, torque, sound, and I'm still getting the 14 mpg I started with, yeehaw....life is good.
I asked my friends at Duffs back then if this was common and they said yes for a multitude of reasons like the emissions era engines (my old 302) were often not more efficient just cleaner at the tail pipe, and that my 31's had my stock gearing just a little higher helping to likely lower the highway rpm, but most likely was due to the 351's additional low rpm torque, saying that it can simply do more work at a lower rpm then the 302 and that this often translated into more mpg.
Keep in mind the speed limit was only 55 mph too....so highway rpm is relative to the mph laws of the era.
Fast forward another 20 years or so...gulp...
Windsor is about the same small changes but not much. The carb was now a 670 TA. 33" tires, and a 1" bl with a 3.5" suspension lift. So I'm pushing a good bit more air out of the way and my 3.50 gears don't like the 33's quite as much as the 31's performance wise,.....and my mpg plummeted....about 9 mpg.
4.11 axle gears made a difference in town with mpg....it actually improved a bit. But not so much on the highway where I was now getting about 8 mpg....
Most recently I have bumped up to 35's. I may have gotten back to 10 mpg on the highway....prolly not though....
So .....from all of this you can ascertain that when your still down at 31's you can really get some decent mpg's but once you start going with bigger tires and more power it falls off precipitously.
One large factor though is how much faster I/we drive at highway speeds today.
When I was a kid I rarely drove the eb over 60 mph (speed limit was 55 then). Now I routinely drive 80+ mph just to stay with the flow of highway traffic, and usually at about 3400 rpm soooo its hard to compare today's highway mileage with yesteryears....at least for me.....today's ethanol blends don't help either....
