I haven't got one but a quick read through the user manual seems to suggest it depends on the accuracy of the fuel sender?
There are better gauges on the market for petrol that use fuel flow rather than consumption calculated from the possible inaccurate fuel tank sender.
As for diesel, it would tell you roughly how much you've used but I can't see that it can be that accurate. With diesel you need to measure fuel flow to and from the engine to give accurate consumption...
Unless you've got posh common rail diesels, the only acurate (ish) way is the old fashioned method of topping up the tank, do some miles and then top off again to calculate fuel usage.
This of course depends on sea conditions and loading being the same every time...
The gauge is for a fuel injected petrol engine.
The only other gauge I have found is made by Northstar(Navman),but its not suitable for fuel injected engines.