Fuel tank and ZDZ 210

I switched to ZDZ 210 that burns 1,25 gallons in an hour. I have a 20,8 litre main fuel tank behind the firewall and 2 auxiliary 20 liter fuel tanks in the wings ( if needed ). Giving total flying time of 3.8 + 6.5 hours ( 10 hr 20 minute ). I briefly calculated that speed is around 195 km/h. So plane should fly in facourable conditions 2000 km. 700 km with only main tank with reserve. This puts my plane target empty weight to 55 kilos. since 21 liters is 15 kilos. My plane can only weigh 70 kilos tops. With extra fuel tanks the weigh is near or above 100 kg. Then it needs to be certified if sold. Limbach SFC chart to it; http://www.limflug.de/files/pdf/DS-L275E.pdf If we assume 65% happens at 6200 rpm we get SFC 360 gr/HPh and thus 4680 gramms / hr at 13 hp ( wich is plenty for cruise on my motorglider type ). That would be 1,5 gallons an hour with 275 ccm engine. I'd be inclined to think ZDZ 210 burns less than 1,25 gallons at 65% power..hardly a gallon.