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Before when I would fill up the gas tank it would read slightly above 300 miles till empty. As of lately when I fill up, it reads to only about 250 miles. Should I be concerned or is this common?
 

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My 981 does the same thing. It seems to estimate my full tank mileage based on the average it was seeing before I fill up. I get a higher number if I had been doing mostly highway driving with better fuel economy prior to filling up than I will if canyon carving at high RPM.

I haven't checked to see if resetting all of the current values stored in the system back to zero changes anything, but its not really that important to me as I dont pay much attention to those estimates anyway. Way too many variables the way I use my car from one fillup to the next
 
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Mine does this too and it is consistent in its observation of "recent" average MPG. You'll find if you fill up before a long road trip and it's showing ~300mls range then you hit the highway drive 50 or so miles, look again and it's showing 295mls range, despite already having covered 50...
I've seen as much as 407mls of estimated range after a fill up in my 06 BS.
 

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Estimated distance per tank is based on the most recent mpg average the car has calculated. That mpg averaging calculates every couple miles but also uses uncleared historical mpg data in the averages calculation. Until a reset happens. If you want the most recent/updated numbers, it's best to reset mpg at each fill up. Then to test it, if so inclined, drive very conservatively for a few miles and you'll see it's recalculated mpg averages, based on those few miles, without any additional historical data included in the calculation, and recalculated distance estimate, also based on those more recent mpg and remaining gallons readings. I don't bother with it much but have done what I'm recommending in order to get more accurate/current info if I'm taking a longer trip. Hope my seemingly convoluted explanation didn't make it more of a mystery.
 

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As a habit, every time I fill up the tank I reset the odometer after dividing the miles between the fuel used, this gives me the mpg and if for some reason this drops, then I would check for a culprit, like low air pressure, a dirty air (or fuel) filter, clogged injectors etc., I do this on all our cars, specially my wife's as she usually drives multiple short distances and her mpg sucks ..lol
 

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Yes, there are so many variables to this Heat wave driving with AC more, low tire pressures, maybe you are using Sport mode instead of normal etc. Other things that can impact mileage is a different fuel perhaps with ethanol or some other blend. I can have less energy per unit, so you give it more gas to compensate. Lower octane fuel will give you less power as well as the engine retards timing and riches mixture to prevent detonation also. A bad O2 sensor or omitting an O2 will cause the engine to go richer to protect itself, but this one unlikely as you would usually get an engine light. Different tires have different rolling resistance and can increase mileage. Thicker oil can reduce mileage a bit. bigger/heavier wheels can decrease mileage. Have any of these things changed perhaps?
 

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One good starting point is to hook up a durametric or similar and check your fuel trims to verify if the ECU is compensating for anything. This would give you data and more clarity if there's an issue or not :D
 
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