CO2 figures v wheel sizes

[Deleted member 65149]

When I bought my new C350 recently I was delighted to discover that despite it having a 3,498cc petrol engine, the published CO2 emissions were 164 g/km, thereby keeping me just inside a slightly lower price category for the annual road fund licence. But even at a higher rate I would still have bought the car - it's fantastic.

But I still don't understand the comparative CO2 figures in the brochure. Why should exctly the same car but on 17" wheels produce a lower figure of 159 g/km? It's the same engine and fuel system, so surely that's what dictates the emissions? And the estate gives rise to further differences: 165 g/km on 17" wheels and 170 g/km on 18" wheels. The differences are similar across the whole range. Why should this be?

To add to my confusion, it can't be anything to do with the outer radius of the tyres on the road because the figures are the same for 17" wheels shod with 225/45 or 245/40 tyres. Or do these work out at the same?

Also, had I chosen a manual gearbox (even I'm not mad enough to do that on a Merc!), then the CO2 figure would have risen to 169 g/km!!

I'll admit that I'm not **** enough to be worried about the specific CO2 figures, I just want to know why they vary so much with the same engine in slightly different configurations of car. Is it a case of the engine having to work harder to cover a km with larger wheels, a manual gearbox and in estate form? If so, I can just about understand the gearbox and body shape variations, but not the wheel sizes. Can anyone explain?

 

[BTB 500]

I suspect the difference on wheels is to do with increased frontal area (and therefore drag) - bigger rims are invariably wider. May also affect rolling resistance.

Car models designed for high mpg nearly always have skinny wheels & tyres.

 

[st4]

Will my CO2 reduce and mpg increase as a result of my lowered stiffer suspension set up in getting added.

 

[Stratman]

Yep, it's rolling resistance.

Mrs. Stratman bought a Fiat Panda on the scrappage scheme. The model she chose has the 'Eco' designation which just creeps into the £30 tax band. The brochure states it's the narrower tyres fitted compared to the standard 1.1 L car that is responsible for the lower rating.

 

[gIzzE]

I had 225.45.17's on my 320d touring auto when I got it and it averaged 48mpg.
Then when I put on some BBS CH rims in 19", wearing 265.30.19 rears and 245.35.19 fronts (I think it was) the MPG plummeted to 39.

I think it was a combination of a slightly different rolling radius as well as the massive increase in rolling resistance. The original tyres were Goodyear Eco tyres which were designed for excellent mpg.

 

[Dieselman]

Larger wheels affect performance and economy for both rolling resistance and mass...in exactly the same way that bigger brakes slow cars down, so do larger wheels.


Common sense, innit...!!