I have an '05 2005 800A with 18/39 sprockets. I usually go about 170 miles before I start looking for a gas station. I've actually run the bike dry before to know the true capabilities. I hit reserve at 178 miles (3.1 gallons to fill) and 209 miles when tank was dry. That means the 800 only has 3.6 gallons of usable fuel and not the 4.0 gallons as stated in the specifications. Mine is a California model so I don't know if that makes a difference.
I also keep close tabs on my fuel mileage and the bike has averaged 58 mpg over the last 10 tanks.
I don't know if the difference is mine is an 800 Classic & yours is a California model 800A (perhaps it's not the same tank), but the 800 Classic tank holds almost EXACTLY 4 Gallons.
As a carefully controlled test, I deliberately ran it bone dry (sputter, then stopped) as I rode around town, poured in 0.501 Gallons of gas (carefully measured, at my usual pump, into the container that I brought with me), rode 2.85 miles to the gas station, and put in 3.564 gallons. That's 4.065 Gallons. Subtract the approx. 0.060 gallons (conservatively calculated at slightly less than 50 MPG) used to ride the 2.85 miles to the same gas station and the net total came to 4.005 Gallons.
Usual station, usual pump, usual incline, usual "Full" level - everything usual. I've performed this test twice and got almost identical results. The only way I can think of to measure it any more accurately would be to run it dry within pushing distance of the target gas station.
Similar testing methodology shows the '03 800 Classic consumes approx. 2.9 Gallons before she starts to sputter and a switch from "Primary" to "Reserve" is required. This leaves approx. 1.1 Gallons in "Reserve".
How you ride and how your 800 Classic is modified will vary one's particular MPG's and therefore your individual miles to "Reserve" & "Empty", but you can take these capacity numbers to the bank.
But, for me, at 145 miles, get ready to reach for the reserve toggle. At 195 miles, get to a gas station, or get ready to coast, or push.