I am seeing variations in my MPG each time I fill up my 05 3.0i. I can get 15+ mpg one time and less than 12 the next.

I know these figures are accurate because my gas gauge is broken and I must keep an eye on my mileage each time I fill up. (I have been in once to BMW for service with no results)

Is this just a consequence of winter gas.

(Tire pressures are OK.)

thanks

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05 X5 3.0 sterling gray/porsche boxster