The same things happens in a car, or at least it does in mine. The difference is that there is about 2 gallons left in a 19 gallon tank in the car, and in a 6 gallon tank on the bike. When the float hits the bottom of the tank there is no way that it can drop any lower. Therefore it can't measure the last bit accurately. The last bit is the same in both vehicles, but it is a larger percentage of the smaller bike tank and a smaller number of miles in the car which does not quite get the economy of the bike.