In Germany, the third week of November every year is dominated, not by Beaujolais Nouveau or Christmas shopping, but by motor cars and wheels. It is the week when Wagens must be fitted with their winter tyres.
To be early is eccentric; to be late is seen as irresponsible and antisocial.
Similarly, just now the talk is not of St Patrick or the closing stages of the dying football season, but of the need to change back again to summer tyres on time.
Winter tyres are soft to maintain a grip in icy conditions, but this also causes them to wear quickly on hot summer roads, and their chunky treads are rather noisy. In fact, the car seems to perform much better on its summer tyres.
There are other things which seem to make a car run better. I always feel that mine drives much more smoothly after I have washed it; it also seems quieter when the petrol tank is full, than when the tank is nearly empty.
I know of no scientific explanation for these two phenomena, but there is a third which cannot be dismissed as merely psychological: have you noticed that your car performs better on a cold, damp day, than when the weather is sunny, dry and warm?
The internal combustion engine runs on a mixture of vaporised petrol and air inducted from the atmosphere. Now, cold air is denser than warm air, so in cold weather the number oxygen molecules entering the combustion chamber per unit volume of air is significantly greater than when the temperature is high.
More oxygen provides better combustion, and hence a noticeable increase in power.
Similarly, on a damp day air sucked into the carburettor contains a lot more water vapour than usual. Although this has the detrimental effect of reducing the proportion of "useful" constituents of the combustible mixture, it is compensated for by the fact that water vapour, when heated, has a high coefficient of expansion; the net effect is more power than if the air were very dry.
Other factors also play a part. High barometric pressure, for example, increases the density of the air, so your car should perform better, other things being equal, driving through an anticyclone than when a deep depression is passing overhead.
And it should also run better at the seaside than on a mountain pass, because at high altitude the air is rarefied, again providing less fuel-burning oxygen than at sea level.
The accepted rule, apparently, is that the power from an internal combustion engine drops off by some 3 per cent for every increase of 1,000 ft in altitude.