1st Jul 2015, 08:17
Anything gas mileage that is within + or - 4% of the computer is within spec. The last 10% of the miles you put on the car before filling up can make a large difference in your results. Most people get gas in an urban setting, therefore the results can register lower than they would if the driver had not been in stop and go traffic.
And it sounds like you got bad gas. Anything done to the car that would result in that big of a gas mileage difference should feel noticeable while driving.
27th Feb 2015, 02:51
O2 sensors have shown to rob your car of a lot of MPG. Did you have a garage do the tune up? If it's someone who doesn't know what they're doing, they could have caused more harm, or you could have had a corrupt garage doing stuff to your car, so you go back for repairs?
Timing is important, did you use the right spark plugs? Did you get new wires, cap and rotor? Did you do the fuel filter? Dirty or clogged injectors could also be a problem, EGR valve, catalytic converter, fuel pump, transmission slipping, coil or coils. Did you see if they're pushing out the right amps?
Cheap gas is another one, and goes down to the tires you bought. So it could be a bunch of factors? Usually when I do a tune up, I replace everything, do a fuel filter, timing, clean the injectors there, change the oil and filter, and keep the same tires for that car. Tires also play an important role in MPG, not just the right air pressure.