HI I've got T'birth 900
Accurate gas mileage is difficult to determine due to the constant variations in driving conditions and styles. Even the conversion mathematics can cause what appears to be low gas mileage. With a motorcycle higher and lower mileages can be made due to wind direction and if the motorcycle has a windshield, single rider or two. Hera are a few points to check, adjust and consider for better gas mileage.
First the tires make sure they are at the proper air pressure the manufacture recommends.
The tires are of the correct size for the motorcycle, oversize tires will through off the odometer and cause a faults reading.
Another is that the speedometers/odometers on motorcycle are notoriously inaccurate, compounding the problem with gas mileage.
Make sure your drive line is in good order, I believe your motorcycle is chain driven so make sure it is properly lubricated and the sprockets are in good serviceable order.
Things to do mechanically to improve gas mileage.
Have the engine serviced at regular intervals. All lubrication's are not created equal, switch to a motorcycle grade of synthetics using a new oil filter at each change. Make sure the ignition and valve timing are within specifications and the valves are adjusted properly. With a multiple cylinder engine the carburetors must be balanced and the recommended grade of gas used. As mentioned above make sure the tires are in good serviceable condition, properly balanced, aligned and at the correct pressure.
Now for the rider.
If you are flogging this thing at every stop light, seeing red line every day, burping the throttle on down shifts, spending time sitting in city traffic or carving canyon switch backs and hair pins on and off the throttle then there is no way you are going to get good gas mileage.
Riding the motorcycle as you would normally. Every bike I've had, has had an odometer and or trip odometer, so you can see what your driving style is actually doing for gas mileage. fill the gas tank set the trip odometer to zero and then ride it 100 miles (give a couple miles either way) then fill it again to the same level. I generally fill so the gas level is just at the bottom of the neck. Then write it down the cost and gallons (liters) the trip mileage and the odometer mileage. Do this for five 100 mile fill ups then take you gas mileage from that.
The next test.
Do the same test above but alter you driving habits, ride the motorcycle conservatively, no red line, no burping the throttle on down shifts, limit your time in traffic by leaving earlier or later to get around the traffic congestion. Keep your speed down to the limit or a touch over, limit the weight of the gear or two up riding. Do this and you will see a huge difference in you gas mileage and when I say huge I'd say taking your 27mpg and stuffing that up to 50/55mpg.
I'm in Canada so our five gallons is the US six gallon so this comparison will be off but the difference between the numbers will be seen.
My GL 1000 Goldwing average 40mpg in the city only 47mpg on the highway.
My GL 1500 Goldwing average 42mpg city and 58mpg on the highway with two up and 50 pounds of gear
My 750 Magna average 42mpg city and 55mpg on the highway single rider light on gear
Previous to the Honda's my 650, 750 and the "new" Bonneville all gave me 38mpg city and averaged 48 to 50mpg highway.
Hey I've written a small novel here but I think you can see that gas mileage and motorcycle has a lot to do with riding style.
Cheers Live to ride and Ride to Live