You are on the low side of average for an R65, in US units of measure, 17 km/ltr is about 40.25 mpg.
The average seems to be 42-45 mpg. Or about 210-225 miles (340-360 km) before reserve is reached.
45 mpg works out to be 19 km/ltr.
It depends on the fuel as well, after mandating 10 % ethanol in most fuel available here in the US, I saw my mileage drop 3-4 miles per gallon on my R65.
If I run the R65 on rural highways where the posted speedlimit is 75 mph (120 km/h), the mileage drops down to 35 mpg ( 14-15 km/ltr).
My 1991 Honda CRX cage does better than that at 75 mph !!!!
I can't remember what year R65 you have, but I'll go out on a limb here and say that the rubber parts and gaskets probably are in need of replacement if they are original in your carburetors, as your bike is at least 23-24 years old.
I don't know anything about the topography of your area, but if is mountainous, or steep grades where you need a lot more power than flat land riding it may be a contributing factor as well.
Factor in multiple owners, and who knows if the carburetors have had some 'tampering' with inside by some previous owner that had no idea what they were doing as well.
I did a complete rebuild on my carbs, and found that the left jet needle was set to the wrong position from the factory, as I am the original owner, and no-one has touched the bike except me since I took delivery of it in 1981, so just about anything is possible here !
If you ride at higher than sea level elevation, it could be a factor as well, the Bing carbs are set-up for sea level elevation, and temperatures in the 59 F. (15 C.) area , commonly referred to as 'standard day conditions', as far as main jet sizing.
Bing has a correction chart for non-standard elevation and temperature for main jet sizes.
It shows that for me, I should be using smaller jets for summer riding, due to high temps, and an elevation of 1200 feet ( 400 meters).
Just northeast of the Phoenix area you get into the southwestern corner of the Colorado Plateau, and mountainous grades are quite common, and the R65 isn't in it's element here as well as significant increase in altitude, it's really down on power.
Not knowing what kind of work has been done on the bike as well, check the torque on the nuts that hold the cylinder head on, then check the valve clearance for proper adjustment.
If possible do a compression check to see if it is in the area where it should be, which is in the 140 -145 psi range, which is 10 Bar, 10.2 kg/cm, or about 950 kpa .
