Hey guys, i recently purchased ford smax 2011 in London. 

the petrol gauge to dashboard showing symptom like this, 

for example if i am driving then i park it for a few hours it shows "50 miles left to empty" 

then i come back and start driving again it shows 10 miles less "40 miles to empty" and this continues to minus 10 until it is empty. 

 

once i didn't use the car for a week, and it still showed the same problem, it minus only 10 miles. 

 

can i ask what will be the problem? 

 

thanks 

need to fill tank

The trip MPG 'computer' on a car is no more than a toy, no accuracy at all. The only true test of MPG is to brim the tank, then run it down, then re-brim it again. Work out how much fuel it took you to cover the mileage used. Do it a few times to get a proper average. Then, thereafter, every time you full it up, 'zero' the odometer, then you'll have a proper running guide of how far you have left in the tank...