When the reserve indicator comes on, you've left with 2,5 liters of gas.....
A trip meter resetting effect is a battery replacement warning but before you do that check for loose connections
1) Battery's terminals
2) Gauge's plug (must remove windscreen if not the upper cowl)
3) Negative terminal bolted on the frame on your right knee height, under the rear cowl
4) Fuses and fusebox (it's on your left knee height under the left cowl above the rad's expansion tank
5) Bolt that holds the battery positive terminal's lead wire on the starter's relay and the 30A fuse too.
A good test to check your battery's health is to switch the key on, turn your high beam light on, count 8 seconds and hit the start button.
Usually if the battery is about to die it won't pass the test.
Personally I prefer to do a different test but you need time to do it.
Put the battery on the charger (MF) over a night. Take it off in the morning and let it rest for 24 hours. Then using a volt-meter check it's voltage. If you see less than 12.8V then you start looking for a new one