My A6Q is chugging along still with 200k on it. I have tried an expirement over the last year that has left me scratching my head, maybe someone can point me in the right direction.

I always buy premium gas from the same 76 station every time I fuel. I always get 21-22 mpg highway driving. I can go about 375 miles before the low fuel warning comes on. (18 gals used)

If I buy a can of injector cleaner (any brand) or octane boost (any brand) the mileage drops to 350 until the low fuel light comes on for that tank.

Once I refill the tank with premium again and no additives, this tank will go 525 miles on the same amount of fuel (about 18 gals).

Once this tank is run through and I refill again, I am back to 375 miles until the light comes on.

The O2 sensors are the originals and I know they are overdue for replacement, but would they cause this severe a swing in mileage, and why in this pattern?

Thank you in advance for any help/suggestions.

Curtis

oxygen sensors degrade over time and use. You can get Bosch original fit fronts for around $100 at autohausaz.com I used generic Bosch for around $50 each but you have to splice them in.
By the way, I get around 400miles on the highway before the light comes on so you're in range. I'm amazed you get over 500 sometimes, that would be around 28 or 29mpg for a Quattro, rather unusual.

I had a Chevy that started to get variations in miles I could drive per tank and then I found out my normal 33 mile commute to work would register anywhere between 17-40 miles! Seemed to be related to how full the gas tank was for some reason.

Thank you. I have checked the odometer against the highway check areas and it is less than 1/2 of 1/10 of a mile slow over 5 miles.

When we first bought the car it was getting 25-27 per tank and has dropped off over the years. I am sure some of it is normal wear and tear, old injectors, bad sensors etc. It is just the huge jump on the first tank after the additives that puzzles me.

On a seperate note, any ideas why I blow headlights every six months like clockwork? Stock lamps, no high wattage or weird colors, checked connections and they are tight and clean. Has been doing this for 5 years.

Thanks again.

Curtis

I have a 96 A6 quattor - 120,000 miles. Bought at 75,000. Consistenly gets 20 mpg no matter what I do (unless I really gas it). Never better than 22 mpg on highway (@70mph+)

I assumed this is normal? Is it not?