Fuelling to the brim, how much extra does it cost?

I've always had an interest in fuel efficiency, particularly in cars as it's something I can easily relate to. Most of my experimenting has been in a 1.9TDi Seat Leon with a 54.5 litre tank, admittedly with mixed results. I once managed to travel 950 miles on a single tank, drafting behind lorries at 45 mph the majority of the time to average 79 miles to the gallon. I've also run out of fuel twice, once on a remote section of unlit Scottish motorway 10 miles from the nearest exit and not an experience I'd like to repeat.

Despite averaging nearly 80 mpg once I've found it an almost impossible task to repeat, mainly because it's extremely boring to never exceed 45 mph or 1500 rpm. I've found I'm more interested in working out efficient ways to drive as opposed to putting them into practice.

What I'd now like to work out is how much money you could save by carrying the minimum amount of fuel needed to complete a particular journey. It's generally accepted that an extra 45kg in your vehicle could reduce the average mpg by up to 2%, with Northern Ireland being pretty lumpy and the road surfaces quite heavy I'm going to use the full 2%.


Using a Seat Leon, 1.9TDi (54.5 litre tank) as an example:

Diesel weighs approximately 0.85kg per litre meaning a full tank will add 46.3kg to the car. This will reduce the average mpg by around 2.05%, compared to a car with a tank 2% full.

If the vehicle averaged 50 mpg with a tank 98-100% full, the average mpg should increase to just over 51 mpg when the tank is 0-2% full. The only issue being it's almost impossible to drive anywhere with a tank just 2% full, a better comparison would be a daily commute.

Say you had a daily commute of 30 miles, averaging 50 mpg you would only need 2.75 litres of fuel or a tank filled 5% to complete that journey. If you did that journey with a full tank the average mpg would drop to 49, improving the average as fuel is brunt and the car gets lighter. The tank 5% full will obviously need topping up on a daily basis.

When carrying roughly 12 gallons of diesel (full) the average mpg will drop by 1 mpg, 0.92 mpg carrying 11 gallons, 0.83 mpg with 10 gallons and so on. Over the course of an entire tank the vehicle will have traveled 6.5 miles less than if you'd put just 2.75 litres in the tank every day, costing an additional 87 pence.


It's clear that fueling specifically for a particular journey just isn't worth it, not to mention the fact you'd only save money if nothing ever went wrong. I'd question the sanity of anyone that would spend an extra half an hour at the pumps every week to save less than a pound. The exact savings will obviously vary slightly from car to car, the bottom line however will stay the same.

You'd be far better off removing the spare tyre along with the passenger seats.