So is it 35 litres or 40 litres = 250km? Btw how is your driving condition, if its mostly city driving with stop, go conditions, I reckon clocking 250km with 35 litres of fuel is not unusual. If you're concerned should check your O2 sensor and also perhaps go for a dyno-tuning to see if you're running too rich.
I'm running a modded 93T with over 200whp and i get on average about 270-320km on 31-32litres of fuel. However I drive mostly on freeways and highways with not much stop, go traffic in between.