[2000neon]

The problem with only filling part way is that it makes it virtually impossible to get accurate fuel economy numbers when only filling the tank up half way. A lot of us, myself included use tank-to-tank averages to calculate fuel economy. By filling up to a relatively consistent point (a full tank) I can figure out exactly how much fuel was used. By only filling up to a half or quarter tank, I can't tell how much fuel it actually took to drive X number of miles.

For example, from a full tank, if I drive 200 miles and it takes 4 gallons to fill up to full again, that was 50 mpg. If I start near a half tank and drive 100 miles, I could put in 1 gallon and guess 100 mpg, or put in 5 gallons which shows only 20 mpg. One thing that just about everyone here consistently agrees on, is that keeping up to date, accurate fuel logs is one of the most important things you can do.