A couple of months ago a Steve Sutcliffe blog on this site went viral and popped up in the national press.

Sutcliffe quoted his mother-in-law, who had heard of a sure fire way of working out what side of the car the fuel filler resided.

Read Steve Sutcliffe's original blog on fuel filler caps

As it happened, her sure-fire rule (that the fuel gauge graphic gave the clue) was not quite as sure fire as it first appeared.

Well, I haven’t got a sure fire way knowing either, but I think I can explain why the filler cap is where it is on your car.

When a car is designed, it usually designed from either a left-hand or right-hand drive point of view. Although it must date back decades, the general rule seems to be that a car’s drivetrain is engineered so the exhaust exits on the opposite side to the pavement.

So a car designed in a LHD country, such as a Renault Megane, would normally be driven on the right, so the exhaust will exit on the left rear corner of the car. Decades ago exhaust gases were rather more pungent, so that layout makes sense.

I’d figured that out a while ago, while pootling along in traffic, wondering what the logic was behind exhausts being mounted differently on different cars.

However, a few weeks ago another penny dropped. If the hot exhaust pipe was on one side of the car, safety concerns must have once demanded that the fuel filler is on the opposite side.

In the days when exhausts were rather more exposed than today, you didn’t want the driver dribbling petrol down the side of the car and onto the super-hot exhaust.

If I’ve got the logic correct, that means on LHD-engineered car, the flap must always be on the right rear corner. For RHD-engineered cars, the flap will be on the left rear corner.

A quick check in the Autocar car park seemed to prove the point. The German (LHD) Golf has its filler flap on the right and the Japanese (RHD) Mazda CX-7 on the left.

However, it’s also probably true that this ‘rule’ is just a left-over of historic engineering practice. At the same time as checking the Golf and Mazda, I noticed that the Saab 9-5 2.0T XWD and Volvo S60 T6 have twin exhaust pipes, so one hot pipe is on the same side as the fuel filler.

However, I also noticed an old Rover 75, which is set up (filler on the right) as if it was engineered from a LHD point of view. ‘Oh’, I thought, ‘that probably means its platform was designed in Munich rather than the Midlands’.

It also means I need to stop thinking so much about filler flaps.