Winter tyres - improved fuel economy?

[JB.]

Okay, I'm only basing this on one week's experience, but it has really surprised me.

I changed the default 18'' alloys with Continental summer tyres to 16'' steel wheels with Dunlop Winter Sport 4D tyres 10 days ago. During that time, I am now achieving 63mpg instead of my normal ~52mpg (both taken from onboard computer), during my daily 70-mile commute. I had read on this forum that some users had experienced some fuel improvements with winter tyres, but the significance of the improvement (based on my past week's experience) surprised me!

I assume that some of the improvement is due to there being less tyre contact with the road (255mm vs 205mm, from memory), but are there also other reasons for the improvement?

My driving style is unchanged (as far as I can tell), and I am filling up at the same Shell garage as usual.

Thanks for your input!

Edited 12 December, 2011 by JB.

[BossFox]

Is the overall diameter of the smaller rims and tyres the same as the larger ones?

Because if they are a bit smaller the car thinks it's going further than it is, hence better economy.

Some people fit smaller winter tyres, not only thinner.

I have fitted identical size winter tyres to my Fabia and economy has dropped by about 5%.

I'm guessing down to more rolling ressistance and a more aggressive tread catching the air as I pass through it.

[dieselV6]

18in to 16in for the same tyre circumference will result in noticeable fuel economy saving (and also improved comfort). Yes, the reason is smaller contact patch, ie you would see that also with summer tyres.

Some winter tyres have larger rolling resitance, some lower than your summer tyres. Though if you fit modern "eco" low rolling resistance summer tyres (near useless in the snow and rain, and on ice, or in other words anywhere except on dry tarmac, by the way), it is a safe bet that winter tyres have more rolling drag than summer ones. However, 16in to 18in contact patch difference more than makes up for it, so you still end up with fuel savings when on 16in tyres.

Edited 12 December, 2011 by dieselV6

[St]

Not on a Superb but 17" Pirelli 7000 or Goodyear Efficient grip to 16" with near as dammit similar rolling radii Michelin X-Ice or Conti WinterContact TS830 has given me about better 4-6mpg on winters, despite the colder weather, greater use lights, heater, heated seats, wipers, heated screen etc etc. Measured brim to brim style as opposed to relying on computer.

That's on two different vehicles too, one manual, one auto.

The Michelins showed a bigger delta improvement cf the Contis.

[900000]

Also remember the the air is now cold (which is of course what your car breathes) and that cold air is also better for fuel consumption.

[andrehj]

Also remember the the air is now cold (which is of course what your car breathes) and that cold air is also better for fuel consumption.

The person who told you that fairytale did not know a lot about physics. The only positive thing about low temperatures is the fact that you do not need your AC. There are however two large disadvantages to cold weather:

1. The engine needs much more time to reach its optimal working temperature of around 90°C. This means much more energy losses in friction during the warm-up phase.

This is even mentioned in your car's manual, in a graph on page 187

2. Colder air is thicker, which increases air resistance or drag. The difference between 0°C and 27°C is about 10% (air considered an ideal gas).

The lowest fuel consumption is probably achieved at temperatures of around 20° in clowdy weather (no AC needed) in a low pressure area, at high altitudes.

Edited 13 December, 2011 by andrehj

[jrw]

The person who told you that fairytale did not know a lot about physics. The only positive thing about low temperatures is the fact that you do not need your AC. There are however two large disadvantages to cold weather:

1. The engine needs much more time to reach its optimal working temperature of around 90°C. This means much more energy losses in friction during the warm-up phase.

This is even mentioned in your car's manual, in a graph on page 187

2. Colder air is thicker, which increases air resistance or drag. The difference between 0°C and 27°C is about 10% (air considered an ideal gas).

The lowest fuel consumption is probably achieved at temperatures of around 20° in clowdy weather (no AC needed) in a low pressure area, at high altitudes.

+1 Diesels take an age to heat up using more fuel. I find that around 9-15C is optimum for the best MPG on mine.

Back to the OP, lower rolling resistance and about 6kg per corner unsprung weight saving...

[dieselV6]

All true and in real life you do use more diesel in winter, also because winterized diesel fuel has less energy content (is diluted with kerosene to prevent gelling).

However, if you had fully warmed up engine and the same fuel as in summer, and drove under load for a while, chances are you would use less diesel because cold air improves volumetric efficiency (VE). Put simply, cold air contains more oxygen per unit of volume, burns fuel more completely, and expands more when heated (or on a MAF driven engine like Audi V8TDI the turbos are not spinning as hard). And especially under load the intercooler performance is much better.

So cold air is better for burning fuel in the engine, it's just all other effects of cold air (thinned fuel, longer warmup time and higher friction losses) that spoil the fuel saving.

[900000]

All true and in real life you do use more diesel in winter, also because winterized diesel fuel has less energy content (is diluted with kerosene to prevent gelling).

However, if you had fully warmed up engine and the same fuel as in summer, and drove under load for a while, chances are you would use less diesel because cold air improves volumetric efficiency (VE). Put simply, cold air contains more oxygen per unit of volume, burns fuel more completely, and expands more when heated (or on a MAF driven engine like Audi V8TDI the turbos are not spinning as hard). And especially under load the intercooler performance is much better.

So cold air is better for burning fuel in the engine, it's just all other effects of cold air (thinned fuel, longer warmup time and higher friction losses) that spoil the fuel saving.

Ah....... so not exactly fairy tales what I said then is it?! Re:

The person who told you that fairytale did not know a lot about physics.