Do Cars Consume Less Gas When the Tank Is Full? The Full Story

I’ve also shunned the very idea of converting my daily driver to run on CNG, ignoring all the gospel in the name of ‘economy’ and ‘efficiency’.

However, May 29th, 2023, is a day that many motorists will never forget in a hurry in Nigeria.

Gasoline, hitherto retailing for the equivalent of $0.17 a liter, fired up to the equivalent of about $0.42 (a 142% increase) immediately after the new president took the reins of power.

It didn’t matter that Nigeria has one of the poorest citizens in the world; it also was of no consequence that there was full budgetary allocation for subsidy payment up at least up to a full month later.

Nothing mattered except that the subsidy payment for gasoline should go. Gone, one thing was clear: for those who work hard for their money, the history of their gasoline consumption was about to change forever.

Fortunately (or unfortunately), this is where I queued up. And continue to queue up. Gasoline prices, on their part, keep going up every week or at least, on a fortnightly basis.

The First Fill Up After

The first fill-up after the hike, I focused my attention on the ‘Naira’ part of the pump as the attendant dispensed gasoline into my vehicle.

When my regular fill-up range passed and the total liters I was interested in buying hadn’t yet clocked a quarter of the overall total, I knew I was in for it.

I started sweating…never has a fill-up taken that long. It also appeared that I had taken gasoline for granted my entire life.

Curiously, the vehicle’s tank was still its regular 70 liters, the 70-liter tank capacity it has always had.

The First Ride After the Gas Hike

When I paid for the gas (just under $40 from less than $10 prior), I watched the gauge with the vision of a hawk and the observation of a GSD on guard duty.

The first 10 miles, nothing. The second 10 — still nothing. The third? Still nothing!

I still had a full tank — even after covering about 50 miles! It was after about the first 50 miles that the fuel gauge moved, just barely. 

I was impressed…this V6 was returning fuel efficiency better than a 2-cylinder hybrid!

Or, so I naively believed. 

Then, something bizarre happened…

The Second Half of The Tank

The 3.3-liter V6 was getting ‘amazing MPGs’ for the first half of the gas tank (or so I was thinking).

Unfortunately, when the fuel gauge indicated that the gas in the tank was at the half mark, things took a drastic turn: it was a fast one, downhill!

It was as if the gasoline in the tank had started vaporizing, instead of being burned normally, in the cylinders. 

What this meant was simple  and straight off: I noticed that for every 10 miles or so, the gauge moved, and it did this race till the gas nearly ran out — and then, something strange happened.

Is ‘Reserve’ Truly Reserve?

When the full orange reserve light came on, I noticed something: I still had about 10 liters of fuel in the tank (it took only 60 liters on the subsequent refuel).

This was eye-opening and truly made me think long and hard about the psychology that goes into the design of the modern gas gauge.

Do Cars Consume Less Gas When the Tank Is Full? Wrap Up and Making Sense of the Entire Story

Since the hike in the fuel price in Nigeria, I’ve come to the following conclusions regarding the design of the gas tank and fuel gauge on modern cars.

1. The fuel gauge is designed to only start reading (downwards) after about 50 miles after a refuel. This is deliberate, I believe, and is a psychological design to create the illusion that your vehicle is more fuel-efficient than it is.
2. When the fuel indicates on the cluster that it is at half tank, what has been consumed is more than half — and certainly, what is in the tank is well less than half (I got to this conclusion when I refueled at exactly the half mark and was surprised that the vehicle took about 45 liters (instead of 35). 
3. When the reserve light shows — and even remains on steadily, you still have about 10 liters in the tank! At an average of 20 mpg, that’s about a solid 52 miles before frustration, panic, and blood pressure set in! For the health of your fuel pump, running your fuel tank to this point isn’t advisable, however. 

So, do cars consume less gas when the fuel tank is full? The answer, as expected, is in the negative…

…as you can see, it’s all psychology, playing with your psyche and, importantly, making sure that you keep your vehicle in top condition, as far as it is dependent on gas!

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