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No - forget trying to calculate what 'might' be left in the tank - it sounds like (unless you drive in heavy traffic or thrash the car all the time) that the gauge is faulty, so all you do (as others have previously said):
- Wait until the gauge 'reads' 1/8th left (if there's a graduation);
- Buy petrol, filling it up until the second click-off as described or fully fill it (you may need to wait a few seconds the first couple of times for the air to escape and pull the hose out a bit and the fuel level to drop a bit to properly 'brim' it - the first method is better if you're not sure how to do this safely);
- Before leving the filling station, note down the mileage (not the trip meter) and fuel bought (if not on the bill);
- Run the car until the fuel gauge reads approximately the same 1/8th again and repeat 2 & 3;
- Repeat 3 or 4 times (minimum) using the car approximately in the same way (journey type/load/passengers).
Actual mpg calculation:
(Mileage at Last fuel stop - mileage at 1st fuel stop)/(total fuel used [add up but on final bill] except that bought at the last stop) = average miles/litre
Multiply that figure by 4.546 to convert it into miles per gallon (mpg)
e.g. (similar to mine for example - four fill ups, the last being 49 ltr [not included on calculation [only the odometer mileage reading that day)
average mpl = ( [55,200 miles - 54,000 miles] / [48 ltr + 47 ltr + 50 ltr] )
= 1200 miles / 145 ltr = 8.28 miles per litre
Therefore ave. mpg = 8.28 miles/ltr x 4.546 ltr per gallon = 37.6 mpg
Assuming yours is the 1.4 manual and you don't thrash the car/drive in too low a gear or lug heavy loads all the time, you should be getting about 90-95% (according to HJ's 'Real mpg data) of the following mpg figures, depending upon usage type:
Urban: 34 mpg
Extra Urban (fast free-flowing roads like motorways at legal speeds - they probably calculate at 60mpg rather than 70): 54.3 mpg
'Combined' (some of both the above): 44.8 mpg
Note that your '29.1 mpg' is near to the lower end of the urban range if the vast majority of your journeys are of that type. How well maintained your car is will also be a factor, as is the quality of fuel used - the bog-standard fuels get less mpg than they used to because of the EU requiring them to include a small percentage of bio-fuel (it isn't really environmentally friendly, just a sop to the green lobby) which makes the engines a bit less fuel efficient than with 100% petrol.
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