Bigger wheels, MPG decrease?

C

Chasabi

Member
 West Yorkshire
Hi all,

I've had my new 18" 224's on for a week now and I've noticed a dip in my MPG. Could the bigger wheels cause this?

In 4 months of ownership the MPG has hovered around 26.9 but I reset it after I put the new wheels on and I now get 23.4 average :o I've also gone from run flat tyres on my old wheels to non-run flat on the new wheels.

I've also just changed all 4 brake pads but they don't seem to be rubbing so I don't think these would be the cause but I could be wrong. Anyone else experienced this? Not sure if it is anything I should be concerned about.
 
The sudden drop in temperature could have a small effect too! Your previous average has been built up over quite a warm spell!
 
Wider and probably stickier rubber than the hard run flats on before will give more drag and slightly more effort to turn a larger heavier wheel. You could try adding a bit more tyre pressure as non-run flat tend to work a bit better on more than the recommended in the door check but lower temperatures and leaving it running to de-ice might have had more effect this last week or so.
 
As above the cold start will likely have an effect as the car is using more fuel to get the engine up to temp. And as also said wider tyres/more grip again could give a similar outcome. I wouldn't go too high though on PSI this time of year is it will give you less grip, plus run flats are terrible for wet grip.
 
I changed the wheels on my 118d a couple of years ago and saw a noticeable decline in MPG, something like 12%.

Asked on the forum and the consensus was was that it was likely related to increased wheel + tyre weight (of about 4kg a corner in my case) requiring greater energy to accelerate the mass, a change in rolling resistance of the tyre, possible greater drag of a wider tyre,and, external temperature changes.

That said it was the same weekend as a service and when I put another arir filter on a week or so later economy improved.

So in my case the initial 12% decrease was down to three elements - a duff air filter with wider, heavier, grippier tyres and a drop in temperature.

Tyre pressures will have an impact too...

Just be grateful fuel costs are falling though :-)
 
Thanks guys. I guess it makes sense really. I've not checked the tyre pressures since I put the new wheels on. What PSI do yo reckon? 225/40 front and 255/45 rears.

I was just sat in the work car park for 20 minutes, having my lunch with the heaters on and the MPG dropped to 14 :|
 
Increasing the alloy wheel diameter and reducing the profile, actually increases the overall diameter of the tyre, this is then harder for the engine to rotate and thus uses more fuel, oh and your car will also be slower and accelerate less quickly.

....but hey on the plus side it looks better :D

oh and when the temperature is colder, efficiency reduces
 
Just how much does this increase the diameter of your wheel by?
If its a lot it obviously affects the speedo reading and if it does how can a manufacturer supply 2 different size wheels for the one model of car?
I assumed that sticking to manufacturers sizes of wheels and tyres that if you went up a size they would be the same diameter and therefor not affect the speedo.
 
Just searched for this
255/30/19 tyre is 635.6mm diameter
255/35/18 tyre is 635.7mm diameter
I dont think 0.1mm would really affect mpg
Also this means fitting larger 19" wheels does not fill the arches more, it must just make it look like they do.
 
Nictrix said:
Just how much does this increase the diameter of your wheel by?
If its a lot it obviously affects the speedo reading and if it does how can a manufacturer supply 2 different size wheels for the one model of car?
I assumed that sticking to manufacturers sizes of wheels and tyres that if you went up a size they would be the same diameter and therefor not affect the speedo.
Click to expand...

if your wheels have a greater rolling diameter, then your odometer will be recording a lesser distance covered for the same journeys-hence your mpg calculations will be skewed (worse)

regards
 
Smartbear said:
Nictrix said:
Just how much does this increase the diameter of your wheel by?
If its a lot it obviously affects the speedo reading and if it does how can a manufacturer supply 2 different size wheels for the one model of car?
I assumed that sticking to manufacturers sizes of wheels and tyres that if you went up a size they would be the same diameter and therefor not affect the speedo.
Click to expand...

if your wheels have a greater rolling diameter, then your odometer will be recording a lesser distance covered for the same journeys-hence your mpg calculations will be skewed (worse)

regards
Click to expand...
True but I dont think 0.1mm will make too much of a difference.
I dont know if the info I posted was accurate or not but the source said a 255/30/19 tyre has a smaller diameter than a 255/35/18.
 
Sars is the engineer but I think there's a typo in what she meant to say (correct me if I'm wrong sars)

Regardless of the size of the rim different aspect ratio tyres are fitted meaning the overall rolling radius is almost identical.

What is different is the rim is bigger and the tyre smaller profile. Putting the extra mass of the rim further from the centre of rotation increases the effort required to both spin it up and slow it down. The rim is also typically heavier too. This translates to lower performance or more fuel used to get to a given speed, harder to brake, etc.

In terms of op's mileage drop this is likely to be a combination of many factors that everyone has posted.
 
cj10jeeper said:
Sars is the engineer but I think there's a typo in what she meant to say (correct me if I'm wrong sars) Regardless of the size of the rim different aspect ratio tyres are fitted meaning the overall rolling radius is almost identical. What is different is the rim is bigger and the tyre smaller profile. Putting the extra mass of the rim further from the centre of rotation increases the effort required to both spin it up and slow it down. The rim is also typically heavier too. This translates to lower performance or more fuel used to get to a given speed, harder to brake, etc. In terms of op's mileage drop this is likely to be a combination of many factors that everyone has posted.
Click to expand...
:o very teckie :) -

just :driving: it :thumbsup:
 
Well I checked tyre pressure last night and all 4 were around 18-20 PSI which probably wasn't helping!

At the moment the MPG has dropped to around 21 MPG but I don't think that is accurate. My usual £40 of V Power lasts about a week. Tomorrow is a week since my last fill up and the fuel warning light hasn't come on yet. I guess if the MPG has dropped a little it isn't quite as much as the computer suggests.

Thanks for all the replies though gents :D
 
thepits said:
cj10jeeper said:
Sars is the engineer but I think there's a typo in what she meant to say (correct me if I'm wrong sars) Regardless of the size of the rim different aspect ratio tyres are fitted meaning the overall rolling radius is almost identical. What is different is the rim is bigger and the tyre smaller profile. Putting the extra mass of the rim further from the centre of rotation increases the effort required to both spin it up and slow it down. The rim is also typically heavier too. This translates to lower performance or more fuel used to get to a given speed, harder to brake, etc. In terms of op's mileage drop this is likely to be a combination of many factors that everyone has posted.
Click to expand...
:o very teckie :) -

just :driving: it :thumbsup:
Click to expand...

May i get even more techie to expand on this?!

The greater moment of inertia of the larger wheel will indeed use more energy to spin up and brake, but it will also maintain energy for longer (like a flywheel), so the energy will only be lost, if you do indeed brake!

Most of the fuel economy would be lost via the drag of the increased tyre width, both with the road and the air. :thumbsup:
 
I changed mine to 19 and didn't really notice any real drop in mpg. I took it for a run up to Birmingham from essex the other day and got 37mpg on the run so was happy with that. I have noticed a big difference in mpg after fitting the quad exhast but thats because i cant stop accelerating to hear it :lol:
 
cj10jeeper, thank you for trying to protect my honour :D

Okay in my scientific little mind some math says otherwise, take a tyre 255/40/18, very simply 18 x 25.4 + 255 x 35% = 546.45 mm diameter, the 255/30/19 is 19 x 25.4 + 255 x 30% = 559.1 mm. There is obviously the rim thickness to consider, however I don't see why they would make it 15mm thicker on a larger wheel?
 
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