ok, just be aware that you are in an extreme minority of people who can fill over 100 gallons in a single fuel-up. You'll probably have some frustrating experiences entering fuel-ups here but the site should still work for you.
When you say Fuelly "stops calculating", what do you mean, exactly? Where are you running into trouble? On your vehicle profile page? On entering a fuel-up?
Looks like the problem isn't the number of gallons--it's that you have some odometer readings out of sequence. Take a look at Your View All Fuel-ups page. Your odometer reading on March 19th is higher than your odometer reading on April 23rd. I bet if you fix those up you'll be set.
ok, now I'm seeing things. Your odometer readings are ok, I'm just not used to those long distances.
Behind the scenes we nix any fuel-up with total miles > 1500 because 99% of the time that's caused by a typo of some kind. We just put in an override for when people enter a greater value manually, but there's no override for behind-the-scenes odometer/mileage calculations. I just upped the limit to 1700, and we might need to revisit that at some point.
Most of our assumptions are based on a 50 gallon tank max.
Thank you. For you future revisit and FYI I'm and doing a lot of improvements to this truck and hopping for 2100-2300 per fill up. My next fill up I should see around 18 mpg which should put me around 2300 miles.