Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Should I keep my gas tank full, or just fill it when it is empty?

I drive a 2005 VW Jetta TDI, which gets close to 50mpg on the highway, which is the only place that I drive it. It has a 15 gallon tank, so I get about 750 miles on a tank of gas (diesel). My usual mode of transportation to/from work is a bicycle, so a tank will last me for several months (I last filled up in July, and it is half full now). Is it better to top off the tank each time I drive, so that it is always close to full, to keep the tank closer to empty, or to just fill it when it gets low?

Fill it when it gets to about 1/4 full. Otherwise, ignore it. It’s not a good idea to run the tank close to empty.

Since it takes you so long to go through fuel, I’d suggest filling it when it hits the 1/2 way mark. That way you’ll be mixing in fresh fuel about every three months (time based on your statement).

Diesel is a far less chemically volatile fuel than gasoline so it doesn’t deteriorate on it’s own. Over the very long-term it can grow bacteria, but if it’s stored properly (like in a fuel tank) it can be good for literally decades. I’ve heard of people salvaging hundreds of gallons of 15-20 year old diesel out of boats and using it with no troubles at all, though this is probably because the diesel was treated with a biocide anticipating a long storage period. So there’s no need to worry about keeping fresh fuel in your car.

My one concern would be that if you’re somewhere that gets cold, I’d try to make sure that you do your autumn-time fill up with winter blend diesel, so if you’re still on the tank from July, I’d run that down as far as you can and then fill up with winter blend which they should be selling at this point.

Diesel is a far less chemically volatile fuel than gasoline so it doesn’t deteriorate on it’s own.

Well fuel gasoline and diesel tend to polymerize (little molecules join to form big ones) It is only indirectly related to volatility. Of course volatility is a factor if the fuel is not in an enclosed container.

Other than than that, I generally agree with everything written.

I would not let it get below 1/2 full or even 3/4. Mixing old and new is good but my reasoning is with the tank full there will be less condensation and since your cycle time for a load of fuel is prolonged there will be less water in the tank, therefore, fresher fuel and less chance of the fuel tank rusting.

I had thought about that too, but is that really true now that gas tanks are so well sealed? (There were some cars here that went completely under water (Katrina), but we were able to drain the gas out later and it had no water in it. We used it in generators.)

I believe there is still a chance since there is air in the tank and water in the air. Temp changes cause this so less air space means less condensation. Of course the problem occurs over time so that is why it is important to keep the tank full.

He has a diesel tank and not all of them are sealed as gasoline tanks are since diesel is not as volatile.

If fuel prices are falling, wait untill the tank is nearly empty, why buy today when it’s cheaper tomorrow. If fuel prices are rising, keep the tank nearly full, buy before the price goes even higher.
That’s how I do it anyway.