I've fooled around with cars for a long time. Experience is that sometimes an increase is seen, but there is no free lunch accompanying it. As above, most mods take a tremendous amount of miles to pay off.
Overall, stiffer rear gears do the most. The tradeoff is a lower cruising speed for the same economy.
My truck is geared with a 3.73 limited slip. Great balance of power, great rpm range in each gear (manual transmission truck; and that is a narrow range compared to a gas motor), great performance (power AND economy) when towing or solo . . . but only if I keep the solo cruising at about 66 mph, the top end of the rpm "sweet spot".
My "economy" (fuel mileage solo, only) would improve if I went to numerically lower rear gears. But I would lose the great blend I have now.
The reason I chose a 2003-4 Cummins powered truck is that the balance was about as perfect as it could get. Not many tow vehicles see 19 mpg around town.
1990 35' Silver Streak Sterling
; 9k GVWR.
2004 DODGE Cummins 305/555; 6-manual; 9k GVWR.
Hensley Arrow. 10-cpm solo, 18-cpm towing
Sold: Silver Streak Model 3411