A quick one today – some tips I employ when filling the car with petrol, which I’ve since researched (a bit) and have (a bit) of science to back them up! Much of the original wording comes from a forum post on this site, the author though sadly is uncredited, it was coincidence I happened to be following some of his tips.

Only fill up your vehicle in the early morning when the ground temperature is still cold. All service stations have their storage tanks buried below ground. The colder the ground the more dense the petrol, when it gets warmer petrol expands, so if buying in the afternoon or in the evening the litre rwecorded by the meter is not a true litre of fuel.

In the petroleum business, the specific gravity and the temperature of the petrol, diesel and jet fuel, ethanol and other petroleum products plays an important role. A 1-degree rise in temperature is a big deal for this business. But the service stations do not have temperature compensation at the pumps.

When you’re filling up you might notice that the pump hose trigger has two or even three settings: low, middle, and high. You should be pumping on the lowest (and therefore slowest) mode, thereby minimizing the vapours that are created while you are pumping. All hoses at the pump have a vapour return. If you are pumping on the fast rate, a greater portion of the liquid that goes to your tank becomes vapour. Those vapours are actually being sucked up and back into the underground storage whilst you’re pumping, but the dial on the pump doesn’t compensate for that so you’re getting less fuel for your money.

One of the most important tips is to fill up when your petrol tank drops no lower than HALF FULL. The reason for this is the more petrol you have in your tank the less air occupying its empty space. Petrol evaporates faster than you can imagine. Petrol storage tanks have an internal floating roof. This roof serves as zero clearance between the Petrol and the atmosphere, so it minimizes the evaporation.

Your car doesn’t have such an arrangement. I’m not sure of the science of this bit to be honest, my guess is the vapour isn’t drawn through to the cylinders unlike the liquid;  but I can vouch for the practice from a practical point of view. By never letting your tank drop below half you always have options for when and where you fill up. We run this protocol (steady, Ed.) for both cars and found it got us through that nonsense a year ago when a tanker driver strike was talked about, and everybody panicked and emptied the forecourts of what fuel they did have. We had sufficient in reserve to be able to avoid all that madness.

Of course if you have a specific one-use only money-off voucher it makes sense to have as empty a tank as possible before you fill, but these should be the exception not the norm.

Another reminder, if there is a petrol truck pumping into the storage tanks when you stop to buy Petrol, DO NOT fill up; most likely the petrol is being stirred up as the Petrol is being delivered, and you might pick up some of the dirt that normally settles on the bottom. At best the fuel won’t burn efficiently enough in your cylinder heads, at worst the crap will start to block up and damage the bits of your engine it comes into contact with. Again if you run the ‘no-less-than-half’ protocol mentioned above, you can just drive on and find the next cheapest forecourt.

You can see how these tips work together, as if you go earlier in the morning on half-a-tank, you’re more likely to drive straight to the pump, there won’t be a tanker unloading at the same time and there will be less people waiting so you can top up with the densest fuel on the slowest setting.