Good Evening all.

As the title briefly explains ive recently changed the wheels on my vrs, changing from the standard 16" alloys to the 17" spiders. The tyres ive put on are 205/40/17.

My question is this, would puttin on these wheels and tyres affect the mpg? I struggle to keep my MPG above 40. Admittedly i do a fair bit of town driving, but my drive to work is along an A road for 12miles.

The only reason i think its an issue is because my MPG was a lot higher before the wheel change. I used to get around 450 miles from a full tank, now i get less than 400! Is it the different wheels and tyres or has something else happened at the same time and its a coincidence?

I havent had my wheels aligned since having the wheels done but surely this couldnt drop it that much if they were mis aligned.

Any ideas? suggestions? Help!

:(

My MPG has dropped past few months due to the colder weather and use of winter diesel, possibly happened at around the same time?

Bigger wheels are heavier and thus take more energy to rotate

Bigger wheels are heavier and thus take more energy to rotate

and if the tyres are wider the rolling resistance is higher.

Yup, combination of things

Have you had your speedometer re-calibrated? With a bigger wheel the car will travel more for each rotation, thinking that the car is going slower and traveling less distance than it actually is.

Have you had your speedometer re-calibrated? With a bigger wheel the car will travel more for each rotation, thinking that the car is going slower and traveling less distance than it actually is.

Not necessarily. If the tyre profile is lower on the bigger wheel the circumference may be the same or very similar.

I experienced a drop in mpg when I put 17's on, but do also think the weather has a part to play as said above.

I took my spare wheel out as won't go over my brakes to try and lose some weight to improve it again but still struggling to get over 400 miles out of a tank (purely town driving) and drive like a gran most of the time

It's the wider tyre that does it.........we want the wider tyre for increased grip.......increased grip means an increase in rolling resistance.......which means increase in fuel consumtion!!!!......thats before you start getting into the science of "which make is better for fuel comsumpion"....!!!!!....

P.S. I see my fuel consuption change by approx 5 - 10% when I swap my wheels over (195/50 R15 summers, 185/60 R14 winters).......

It's the wider tyre that does it.........we want the wider tyre for increased grip.......increased grip means an increase in rolling resistance.......which means increase in fuel consumtion!!!!......thats before you start getting into the science of "which make is better for fuel comsumpion"....!!!!!....

P.S. I see my fuel consuption change by approx 5 - 10% when I swap my wheels over (195/50 R15 summers, 185/60 R14 winters).......

tyre width hasn't changed? the difference in circumference between 205/45/16 and 205/40/17 is like 15mm, would that be enough to loose 50+ miles from a tank?

Just to confirm, tyre width has stayed the same, its a lower profile tyre so speedometer shouldnt really be affected. Im sure a loss in mpg because of a heavier alloy is expected but surely not around 10mpg.

Also oil change and all filters have been changed recently and this did not solve the problem, mpg has stayed the same.

Iv noticed a drop in MPG too, it's as already mentioned winter time having your lights and heaters on more I guess.

I suppose its a possibility. I changed these wheels over in september so u could say it was the start of the dark seasons but im still not convinced you would get such a noticeable drop.

Im tempted to take it to a garage and get them to give it an inspection having done my own service im wondering if theres something wrong that i wouldnt necessarily pick up on.

Just to confirm, tyre width has stayed the same, its a lower profile tyre so speedometer shouldnt really be affected. Im sure a loss in mpg because of a heavier alloy is expected but surely not around 10mpg.

I'm not at all certain the weight of the wheel has much relevance, something to do with sprung and unsprung weight. I think unsprung weight, which the wheels are, has relatively little effect. I'm absolutely certain I didn't notice any change in mpg when I swopped steels for alloys on my 100bhp TDI estate and I keep meticulous records of mileage and fuel used.

I'm not at all certain the weight of the wheel has much relevance, something to do with sprung and unsprung weight. I think unsprung weight, which the wheels are, has relatively little effect. I'm absolutely certain I didn't notice any change in mpg when I swopped steels for alloys on my 100bhp TDI estate and I keep meticulous records of mileage and fuel used.

Since you're not at all certain, here's some info on unsprung weight for you, and how it has a fairly big effect on this sort of thing: http://en.wikipedia.org/wiki/Unsprung_mass

Also check these out:

http://www.tirerack.com/wheels/tech/techpage.jsp?techid=98

http://www.carbibles.com/suspension_bible_pg4.html

http://answers.edmunds.com/question-Can-someone-explain-the-performance-advantage-of-saving-unsprung-weight-by-changing-to-a-lightweight-wheel-85068.aspx

It's not difficult to be certain that larger, heavier wheels require more force to turn... simple physics!

tyre width hasn't changed? the difference in circumference between 205/45/16 and 205/40/17 is like 15mm, would that be enough to loose 50+ miles from a tank?

No, the actual difference is 5mm in circumference. http://www.tyresave.co.uk/tyresize.htm.

tyre width hasn't changed? the difference in circumference between 205/45/16 and 205/40/17 is like 15mm, would that be enough to loose 50+ miles from a tank?

Didn't quite see that...... .......so if tyre width is the same.....then down to heavier tyre/wheel combo???......or the tyre tread pattern and the actual rolling resistance of that tyre is different to the other tyres????......there will be a difference between a "summer" and a "winter" tyre of the exact same dimensions!!....

Since you're not at all certain, here's some info on unsprung weight for you, and how it has a fairly big effect on this sort of thing: http://en.wikipedia.org/wiki/Unsprung_mass

Also check these out:

http://www.tirerack.com/wheels/tech/techpage.jsp?techid=98

http://www.carbibles.com/suspension_bible_pg4.html

http://answers.edmunds.com/question-Can-someone-explain-the-performance-advantage-of-saving-unsprung-weight-by-changing-to-a-lightweight-wheel-85068.aspx

It's not difficult to be certain that larger, heavier wheels require more force to turn... simple physics!

I won't dispute that larger wheels and tyres would take more power to get them moving but once they are moving I think you will find that the power required to increase the rotation speed is very little different and once the heavier combination is up to speed its increased inertia will keep it rolling for longer. I think that the biggest factor affecting consumption, other than how you use your right foot and increased warm up time in winter, is the rolling resistance of the tyre. Any tyre engineers on here? The biggest advantage of the alloys is the decreased unsprung weight which improves the handling as your quotes show. Any increase in tyre width and overall circumference will increase rolling resistance as the tyre's footprint is increased.

So the increased weight difference between the two sets of alloys is probably the reason why im struggling to get above 400 miles from a full tank? I guess combined with winter conditions its possible, quite surprised though.

I'd reckon it'll go back up to 400+ after the winter

So the increased weight difference between the two sets of alloys is probably the reason why im struggling to get above 400 miles from a full tank? I guess combined with winter conditions its possible, quite surprised though.

I would be most interested in what the difference is in the weight of the smaller wheel and tyre and the larger combination. I suspect it is not a great deal as the tyre on the larger combo will way less, same width, lower section, and counteract the extra weight of the larger wheel. Depending on the construction of the larger wheel it may not weigh more anyway.

No, the actual difference is 5mm in circumference. http://www.tyresave.co.uk/tyresize.htm.

diameter is across, circumference is around

diameter is across, circumference is around

You are so right. Too much red wine last night. 5mm diameter is less than the difference in tread between a worn and a new tyre.

May be the different tyres. Old one harder, new ones softer - more fuel used