There’s talk in the press today of raising the speed limit here in the UK. While admittedly most people seem to drive at 80 anyway, increasing the limit will definitely affect most people’s pocketbooks. I also feel it will increase pressure on other people to drive faster. If the limit is raised, perhaps it should become an absolute limit, removing any margin of doubt.
This summer, we drove to Spain fully loaded with a roof box on top of the car with my wife and two children. A 2500 mile marathon!
On the journey down through France, we drove at just under the speed limit of 130 kph (~ 80 mph).
We don’t do a lot of driving long distance in the UK, but specifically bought our Kia diesel for long trips like this one, and carting my daughter’s sailboat around the country to various events, and we use the cruise control a lot.
The car is fairly efficient, having averaged 46 mpg with a boat on top. On the journey to Spain, we averaged only 44 mpg, with some legs in France getting as low as 38 mpg. Once in Spain at 120 kph and the mountains of the Pyrenees, the economy rose, bringing the average up.
You’d think that would be the end of the story, but on the way home we made another change. We bought wine; a lot of wine! About 60 litres in Spain, and then added another 30 in Calais.
When we stopped for dinner, I realized my tyres were looking a bit low, so I dug out the manual while crossing in the tunnel, and decided that they were probably pretty underinflated.
Don’t forget to keep your tyres inflated properly
I decided that despite the very late return home, we had better get some air, so stopped at the exit of the tunnel, and inflated them to the recommended level for the load.
What a difference. On the journey back through Spain and France, we managed to average only 39.5 mpg. That’s 11% down on the journey down, just because I didn’t inflate my tyres properly.
To complete the exercise, we drove home in the car in the same state, but this time stayed at 60 mph, as it was very late at night, and we were tired, and the car was very loaded.
Having filled up in Calais, I had a new baseline, and this time with the same full load and roof box, we managed 49 mpg.
You can trade time for money
It looks like driving 20 mph faster decreases fuel economy by 11% as well. And driving additionally with underinflated tyres decreases by a total of 20%! I’m lucky the fuel averaged only £1.10 a litre on the continent instead of the £1.40 pretty much balancing out the extra unnecessary expense.
Further mileage tests
I also had the occasion to do two other long trips recently. To London in the C1, where driving at 60-65 on the motorway increased mileage from the 47 mpg my wife got to my 55 mpg, again a 15% decrease for a 10 mph change.
Finally, I drove to Southampton with 4 people in the car and no roof box. We had a good journey from Cambridge, and again, drove at between 65 and 70 mph. This time, we managed to achieve 58 mpg, a record for the car, showing that adding weight and a roof box appears to decrease fuel economy by around 11%.