Instead of moving the upper limit, I added the confirmation step today. So this time when you enter 1703+ kilometers, you should get this warning message:
"Looks like the number of kilometers you entered is unrealistic for a single tank. If you really went 1703 kilometers for this tank, click Fuel-up! below to bypass this check."
Click Fuel-up! again, and Fuelly should add the fuel-up. It's a bit of a pain since you have to go through that extra step, but most people won't be getting that kind of distance from a single tank. So we have the check there with a way to get out of it.
Oh, and reviewing the code again, the hard limit is at 1700 miles or kilometers. It's based on the raw number not the units. That's why you were able to enter your previous fuel-ups, Ron. I was misremembering how that worked.