I think you'll find that it is between 5 and 10% before recalibration. I don't have a Navman, but a Standard Horizon - but I believe it uses the same pickup and essentially the same gauge, and therefore think they would be similar. Mine stted in the specs that accuracy should be within 5%, and it turned out to be within 5% right out of the box (1 gallon off in 20 burned) <br /><br />As for calibration, your method would work, the accuracy depending on how accurately you can measure actual fuel burned when you top off the tank. If you're in a "safe place" where it wouldn't be a problem to run the tank dry before the test, fill with a measured amount, and then run dry again, that's what I'd do. I've never had a built in boat fuel tank with which I could really accurately "top off". There would always seem to be a couple gallon variation and even a 1 gallon uncertainty in a 20 gallon test would most likely be as much or more than the factory calibration variance. If you run a 4 - 5 gallon test and have any uncertainty when you top off, I think your calibration is likely to be worse than from the factory. The more gas you use for calibration, the more accurate it will be and the less meaningful the uncertainty. I used 20 gal. I started by running my tank dry, filled with a measured 20 gal, and ran it dry again.Originally posted by prawntrap:<br /> I have a twin eng. set up, so I guess the way to calibrate the unit would be to fill up the tank and run one eng. for the req. time ie. 4 gals. of burn.Fill up and compare it with the fuel used on the 3100.re-cal. if req. and then do the same for the other eng.It is to much of a pain to use a portable tank as the manual suggests.Do you find that the senders are quite a bit out when you do the comparison with the actual burn before you recalibrate?<br />PT