My 1997 6.5 diesel Chevy 3500 has 4.10 gears, great for towing but when not towing and in Tahoe I wanted larger tires better for snow etc. I'm wondering if larger tires mean that changes the gear ratio so I can get better fuel milage as well? Getting 14mpg now. I guess I'll also need a different chip to make speedometer correct or do I change the little gear in transmission that year?

it depends on how much bigger you are wanting to go with your tires

3.73 is 91% of 4.10

The tires on my truck are 245/75/16, or 31.4 inches. To get the same gain as going from 4.10 to 3.73, I would need 34.5 inch tires. The difference would be no more than 1-2 mpg. In other words, you need to make a significant change in tire size for a minor difference in mpg.

ApeOr
There is something wrong with that calculation. For larger tires you need to increase the gear ratio (higher numerical number) and not the opposite.
There is normally not an easy answer to your question "Dude".
Depends on a lot of factors, like driving style, speed etc. to keep the motor in the best operating range. If you lug the engine with too big tires you will actually loose mpg and not gain.
What is the standard tire size on that truck Dude? I assume with 33 inch tires you will be just fine with an 4.1 rear end.