I am going to say you can't answer this question in the real world
with the current information, though there is possibly a mathematical
answer someone may be looking for if this is some kind of homework
Assuming you mean to go from 15mpg to 20mpg with a truck you actually
have, I think 33% bigger is the number to ask about (15 miles plus 5
miles, 33.33...% of 15, = 20 miles). 20% doesn't even add up as a
theory, though I don't know if you actually have room for a 33% larger
But even then, you can't tell for sure.
The input and output efficiency of an engine is not in perfect
proportion through the rpm range, and they will usually have a narrow
range in which they work best. Matching the mpg of truck #1 might be
more likely if both trucks have the same engine, just different
ratios. Normally we would assume this as a hypothetical, but if this
is real you very well may have different engines, or even the same
engines in different conditions.
You can't ever get completely "free" energy out of anything, so larger
tires only help if the current configuration is wasting energy for
some reason, like the top gear being too low for your intended use.
You are fixing a problem, not adding free mileage from nowhere. Here
are some "problems" you might be able to fix with a higher ratio:
Vehicle was made for more stopping and going than you actually do.
Vehicle was made for a lower top speed, or lower average speed, than
you are traveling at.
Vehicle was geared for lots of towing/hauling, but you drive it empty.
If you do a lot more traveling at 65 than the manufacturer assumed,
you might benefit from a ratio that does better at 65 than 55. If you
actually use the truck as the manufacturer expected, you might save
gas at 65 mph only to waste it at 5 mph, or you might actually lose
mileage on average. Make sure truck #2 doesn't already struggle at
1500 rpm in any gear.
It's possible it could help. Most passenger vehicles have a slightly
low top gear, and at high speeds are running a little high with rpm's.
But don't be certain you can get it to 20, you will be disappointed.
Also be prepared for nothing. I wouldn't spend my last $300 trying to