peggy,

I believe there are 20 bars for gas. To be safe, you
can do Jason's method of calculating according to
10 gallons (as opposed to 12 or 13 actual), so that's
a half-gallon per bar. So 4 bars would be 2 gallons,
conservative estimate.

That's why I haven't gone for a 600 gallon tank
yet. With Colorado's up and down temperatures
this time of year, I can average between 45 mpg
on a cold trip and up to maybe 55 mpg on a warm
day.

But with a 100 mile (round-trip) commute, when I see
under four bars after driving over 500 miles on the tank, I
go ahead and fill up. I'm limiting myself to the same gas
pump for the first couple of months to monitor changes
in mpg.

Hope this helps,
jinno