Gas mileage dropped by 25%


I drive a 2001 audi a6 2.7t. It has approx 164,000 miles on it.

Problem: The last 3 times I have filled up my car with a full tank of gas it has only gotten ~290 miles. It usually gets ~400 miles/tank. On the last tank the estimated range on the car’s control panel has been fluctuating greatly, going down in 5 mile increments ever 1-2 miles and then jumping back up to above where it should be (ie showing 350 miles left on 3/4 tank.)

Question: What gives?

Known Issues:

  1. The power steering is going. When I brought it in for new tires the guy drove it around and said that the noise I was hearing (sort of like a rubbing noise) was the power steering and recommended a flush and possible replacement of the power steering.

  2. Off and on leak of antifreeze. Some times it leaks out a bit, sometimes not.

Thanks for any help.

You should really keep track of gas mileage by actual measurement - number of miles driven divided by number of gallons required to fill the tank. Preferably this would be done over several tanks of gas. Gas gauges and computer estimates are notoriously unreliable and themselves subject to problems.

However, on the assumption that you have had an actual & verifiable drastic change in mileage the first suspects are normally the thermostat and coolant temperature sensor. Given that you have a coolant leak anyway a full check up & repair of the cooling system can go a long way.

Any check engine light?

A coolant leak could lead to air in the cooling system. An air bubble around the temp sensor would give a false reading, throwing the computer back into open loop and using extra gas. That would also account for the fluctuations in the control panel.

Fix the known fault first, the cooling system. Make sure it is bled completely when done.

You need to address the coolant leak problem as stated before. In the meantime make sure all your tires have the proper pressure. Not the pressure on the sidewall of your tires but the pressure listed in your owners manual or vehicle placard.

FIx the leak.
Change the thermostat, and every 5 years from now on if you drive this daily.

I find it interesting that you mention tires as part of what was going on, but not as a possble cause - and I would agree that 25% is way, way too large for tires to be the sole affect.

But the affect tires have on fuel economy can be measured - and perhaps you have several things going on.

About this time of year, I get a lot of calls about loss of fuel economy that people tie to a change in tires. I’ve always noted the volume of calls increases when the winter fuel mixture shows up - and while I don’t think those 2 things can account for 25%, if we add something else, we could easily be looking at that value.

I’ve had 02 sensors go bad and cause 25-35% drop in fuel efficiency. If it’s a bad 02 sensor it will be an upstream sensor. The downstream sensor has no control over fuel mixture. It’s only purpose is to check the efficiency of the catalytic converter. Is the CEL on? If it is start by having the codes pulled from the ECU, this will probably give you an indication of where to start with troubleshooting your problem.