Test case, 1987 Syncro Westy, NY to Florida road trip.

Last year, traveled an indicated 3,216 miles, round trip including some
touring. Average mileage was 17.4 miles per gallon. Tires were Goodyear
Wrangler Aquatreads, 215/70-14, (~26" diameter). Speedometer/odometer
error still over reading by ~ 3% when compared to the GPS. Corrected
mileage traveled was 3119, 16.82 MPG. Engine is the original, turned
200k during trip home.

This year, same van, same engine, tires are now 215/65-16, (~27.25"
diameter), Goodyear Wrangler HP. Engine mileage now at 215K. Traveled an
indicated 2812 miles averaging 16.08 MPG.
According to GPS Speedometer/odometer now reading ~2% lower than actual.
Corrected distance, 2868 miles averaging 16.41 MPG.

It should be noted that speed made a huge difference in mileage, local
touring along route 1 last year yielded  21.17 mpg on one tank. Worst
tank mileage was 14.53 (indicated). Engine appears to work harder taking
hills or head winds. Mileage was probably affected by actual higher
speeds being traveled due to speedometer correction.

Moral! Don't count on tire or gear changes to improve fuel economy. The
biggest improvement will come from driving slower and doing something
with the lack of aerodynamics.

Dennis