Hi All,

Just after some thoughts on this one as its a side effect I wasnt expecting after deciding to go with Winter tyres on the Yeti...

My fuel consumption figures have a taken a bit of a sudden climb overall but particularly noticeable on the daily commute. On the Dunlop Sports Id see 32mpg regular as clockwork and since running with the winter steels and tyre set Im now seeing over 35mpg each way.

The only change has been the wheels and the change was noticeable soon after BUT the wheels now have a (very) slightly smaller rolling radius so if anything I would have thought that they would offer slightly worse fuel consumption overall.

The one thing I hadnt taken into consideration previously was that the winter tyres are a 205 width as opposed to the 225 width of the summer tyres - could this difference in tyre width (and presumably lower friction equalling less fuel use?!) be the reason why Ive seen the mpg figure rise? If so, isnt a 3-4mpg improvement over a 10 mile journey quite surprising?!

Not surprising at all, as you have a smaller rolling circumference you are not travelling quite as far as your odometer is telling you, (clock a regular journey and you will see you are now travelling further).

Then you also have the fact that the tyre pressures are higher (if you have the recommended +0.2bar in them over all season tyres).

Then finally smaller wheels are also lighter,therefore require less effort out of the engine to get up to speed.

With a smaller radius / circumference, your car's computer is counting more revolutions of your wheels for every gallon of diesel used hence the false increased fuel efficiency figures.

Hi Focus,

got the same with my former Monster last winter but on the 37 mile commute to work I could get up to 7mpg more indicated than on the standard wheels :wonder:

I put it down to the over reading of the speedo etc. Also driving at and indicated 60mph would put my actual speed in the mid 50's which is probably the most economical speed to be at for mpg.

However I think as you say the 205 profile tyre will reduce friction and it's interesting to see SUK are stating the GreenLine has this profile tyre.

Regards,

TP

Not surprising at all, as you have a smaller rolling circumference you are not travelling quite as far as your odometer is telling you, (clock a regular journey and you will see you are now travelling further).

Then you also have the fact that the tyre pressures are higher (if you have the recommended +0.2bar in them over all season tyres).

Then finally smaller wheels are also lighter,therefore require less effort out of the engine to get up to speed.

....and a 10 mile trip is too little to make any judgements from.

Ahh, so the reading is higher due to the circumference of the wheel being smaller and the maxidot 'not knowing' this!

Thanks very much guys, I seemed to have missed the obvious!

I fitted winter tyres (of the same size as summer ones) and noticed a very slight decrease (now averaging 54 rather than 56 on short runs) but put this down to extra grip (drag?). However, the additional traction far outweighs this small penalty.

FocusZtec The difference in rolling resistance of all the Winter tyres tested by Autoexpress was less than 3%. The Summer tyre was just about in the middle of the range of figures. The very worst that your fuel consumption could have increased by switching to Winter tyres would be something just over 1%,a figure of less than 1 MPG. I think the main reason for your apparent big improvement in fuel consumption is the much smaller circumference of the wheels . you are driving at a slower speed therefore using less fuel. :yes:

Edited 21 November, 201015 yr by bilun777

For me, the same. My winter wheels are 5% smaller, so real distance you make is 5% less, then the APPARENT fuel consumption is 5% less. Of course if you consider less drag due to reduced contact surface, and less mass and +0,2 bar pressure, the fuel waste can REALLY be less. With winter I suddenly passed from -indicated- 15 km/l to 15.8 km/l.

Bye,

Ratz

For me, the same. My winter wheels are 5% smaller, so real distance you make is 5% less, then the APPARENT fuel consumption is 5% less. Of course if you consider less drag due to reduced contact surface, and less mass and +0,2 bar pressure, the fuel waste can REALLY be less. With winter I suddenly passed from -indicated- 15 km/l to 15.8 km/l.

Bye,

Ratz

How are you working that difference out?

I cannot see how one tyre can be 1/20th smaller than the other. Most of the size comparison sites show nothing more than a 2% difference.

Right.

The difference is 4% (if the calculation are right).

First column is the dia in mm.

656,8 225 50 17

631,9 205 55 16

The smart thing is that you see immediately the difference just looking at the car (not a good appearance, I have to say...)

Bye.

How are you working that difference out?

I cannot see how one tyre can be 1/20th smaller than the other. Most of the size comparison sites show nothing more than a 2% difference.

Right.

The difference is 4% (if the calculation are right).

First column is the dia in mm.

656,8 225 50 17

631,9 205 55 16

The smart thing is that you see immediately the difference just looking at the car (not a good appearance, I have to say...)

Bye.

Sorry, but you are measuring the diameter, not the circumference. (the complete distance around the tyre)

I also noticed quite a significant improvement of fuel consumption (around 3 extra mpg) when switching to winter tires (Sunlop SP01 -> Studded Nokian Hakkapeliitta 7). I have the exact same tire profile on both sets, and both are also on genuine Skoda Alloys so there's no difference coming from travelled distance. To my understanding the summer tires are optimised to have as much grip between the tire and ground as possible, which means that they try to "stick" to the ground, with increased rolling friction as a result. The winter and/or all weather tires have a different mind set - they are designed to get rid of any unnecessary stuff (such as snow, water etc) between the tire and the ground so they roll lighter thanks to their decreased surface area and more sharp edges that ensure that at least a small part of the tire is getting any grip.

Sorry, but you are measuring the diameter, not the circumference. (the complete distance around the tyre)

Yes, but the percentage does not change even for the circumference. So 4% of apparently more distance than real...

I also noticed quite a significant improvement of fuel consumption (around 3 extra mpg) when switching to winter tires (Sunlop SP01 -> Studded Nokian Hakkapeliitta 7). I have the exact same tire profile on both sets, and both are also on genuine Skoda Alloys so there's no difference coming from travelled distance. To my understanding the summer tires are optimised to have as much grip between the tire and ground as possible, which means that they try to "stick" to the ground, with increased rolling friction as a result. The winter and/or all weather tires have a different mind set - they are designed to get rid of any unnecessary stuff (such as snow, water etc) between the tire and the ground so they roll lighter thanks to their decreased surface area and more sharp edges that ensure that at least a small part of the tire is getting any grip.

I have to say, im still not 100% confident the reasoning here is right for my better fuel economy on the winters. May its more the fact that I cant get my head round what exactly is being said as the reason behind it but on the way in this morning all I could assume was that as the wheels are smaller then the overall gearing is lower, therefore more revolutions, therefore more fuel used?! How does running smaller diameter tyres on the daily commute equal better fuel economy? I simply sit with traffic all the way for 10 miles so its not a case of me doing a lower speed than I would have previously.

Can anyone speak 'idiot' please?

I have to say, im still not 100% confident the reasoning here is right for my better fuel economy on the winters. May its more the fact that I cant get my head round what exactly is being said as the reason behind it but on the way in this morning all I could assume was that as the wheels are smaller then the overall gearing is lower, therefore more revolutions, therefore more fuel used?! How does running smaller diameter tyres on the daily commute equal better fuel economy? I simply sit with traffic all the way for 10 miles so its not a case of me doing a lower speed than I would have previously.

Can anyone speak 'idiot' please?

I'll try - should be qualified

Since the wheels are smaller, the odometer thinks you are driveing further than you realy are, Hence the better mileage.

OK?

I'll try - should be qualified

Since the wheels are smaller, the odometer thinks you are driveing further than you realy are, Hence the better mileage.

OK?

Thanks for being the only one prepared to speak to Briskoda's village idiot...

So because the wheels are smaller, the settings and therefore fuel usage calculations within the maxidot is basically now out? So the figure of 36.4mpg I saw this morning isnt actually accurate?

Edited 22 November, 201015 yr by FocusZtec

I'll try - should be qualified

Since the wheels are smaller, the odometer thinks you are driveing further than you realy are, Hence the better mileage.

OK?

Needless to say, the betterment in mpg due to wheels' smaller circumference is false as it is related to the figure shown by the on-board computer (it does not know that wheels were changed...)

Needless to say, the betterment in mpg due to wheels' smaller circumference is false as it is related to the figure shown by the on-board computer (it does not know that wheels were changed...)

Ah, which is what I was getting at in my post above. Ok, thanks for the clarification guys.

Not sure if this helps but with 16 inch winter tyres, my Sat Nav says that my speedo is over-reading by about 10% when compared to the Sat Nav speed display.

I presume that the Sat Nav is accurate.

John

Not sure if this helps but with 16 inch winter tyres, my Sat Nav says that my speedo is over-reading by about 10% when compared to the Sat Nav speed display.

I presume that the Sat Nav is accurate.

John

Do you run 205/55 profile tyres on the 16" wheels?

I presume that the Sat Nav is accurate.

John

Only if you are going along a straight road which is flat.......

Mike

Edited 25 November, 201015 yr by rockhopper

Do you run 205/55 profile tyres on the 16" wheels?

215/55-16

215/55-16

Thats why its out by 10% then isnt it?

Thats why its out by 10% then isnt it?

Don't think so. Using Plumber's Tyre Size calculator link, the 215/55s are closer in circumference to the standard 17 inch tyres than the 205/55s.

John

Edited 26 November, 201015 yr by jst_at_home