Doctor Diesel, Features

Variable Resistance (Not a Rheostat)

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Web04
Hello Doctor.
I’ve been keenly reading stuff about fuel economy and economical driving in Diesel Car. One of the things that seems particularly effective is fitting eco low rolling resistance tyres, and I see that there are now ratings A, B, C etc. for these in the new EC tyre labelling regulations. Are these ratings comparable between different tyre sizes that might be fitted to a car – like is a B-rated 195/65/15 as good or better than a B-rated 205/55/16, or 225/45/17, for instance? Surely the wider tyre must have a greater rolling resistance than the narrowest one?
Jeff Symonds

Much to your surprise, no doubt, and to mine also when I first established the facts, wider tyres consume no more energy than narrower ones, all other things being equal. They have differently shaped tyre/road contact areas – the narrower tyre having a long and narrow one, the fatter tyre a shorter but wider one – but the end result is near enough the same, or possibly even favours the wider tyre. I think this phenomenon is also related to the fact that narrower tyres generally give more steering feedback (from the longer contact patch), but inferior road grip – in the dry, anyway. You can compare the A, B, C ratings between sizes, as the bands are the same for all car tyres in terms of the rolling resistance. To give some idea of the differences, rolling resistance of A-rated tyres might be as much as 30 per cent less than that of a C-rated tyre, and the possible benefits in fuel consumption amount to the order of three to five per cent. But the gains are possibly greater in some circumstances, as the rating measurements are made for tyres rolling in a straight line, as on a motorway or straight road. The energy absorbed by any tyre increases significantly when the tyres are flexing internally when turning a corner, and in low speed manoeuvres like three-point turns, and also when parking, so the differences in such conditions between tyres can be significantly greater in terms of fuel usage, particularly perhaps if you live in a rural area with bendy roads, and even more so if you tend to corner enthusiastically.
Doc D

One Response

  1. Two points re FRR tyres,

    The German ADAC test results showed tyre width to be significant, they showed that the best LRR wide tyre was comparable to the worst tyre of a next profile down.
    In other words simply doing down in tyre width was as good ( or better) than all teh research the tyre buyer could undertake.

    The test company Emission Analytics have performed tests and concluded LRR is significant for fuel economy when the vehicle is driven at speeds such as motorway legal limits, at medium speeds 40 to 50 mph LRR is of modest benefit. at slow speeds 30 – 40 mph, no benefit in fuel consumption.

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