Hybrid fuel consumption vs wheel size question

[ChrisJohn]

Can anyone explain to me why the "official" fuel consumption figures for the three sizes of wheel are so different in the Auris literature? For example, the brochure shows 80.7 for 15"; 78.5 for 16" and 72.4 for the 17".  That's a huge range. I appreciate these are from the official tests but I still cannot understand why there should be such a significant difference.  Sure, the bigger wheels will be a little heavier but if the difference is so great, why are the largest wheels fitted to the top grades?

[FROSTYBALLS]

Two causes - higher weight and wider tyres (more grip, slightly higher drag).

Top grades have the larger wheels for aesthetics.

[cuius]

Had a Mk 1 with 15" wheels - averaged 54.2 over 3+ years - Mk 2 with 17" averaging 51.0 over 2+ years - similar motoring throughout; rural based but with frequent long motorway trips. The reduction is worthwhile given the better ride quality with the larger wheels IMO

[Catlover]

1 hour ago, cuius said:

Had a Mk 1 with 15" wheels - averaged 54.2 over 3+ years - Mk 2 with 17" averaging 51.0 over 2+ years - similar motoring throughout; rural based but with frequent long motorway trips. The reduction is worthwhile given the better ride quality with the larger wheels IMO

Interesting observation you have. 17" wheels on my Auris Hybrid looks good, but much prefer the comfort of a ride in a car with 15" wheels. The tyre deflection of 15" gives more "suspension" then 17" low profile tyres. IMO

[FROSTYBALLS]

It depends how one defines ride quality - which is a compromise between ride comfort and handling. 

Often larger wheels are coupled with low profile tyres, which can have a positive effect on handling, but a detrimental effect on ride comfort.

[ChrisJohn]

Thanks for all the helpful replies. I was fairly ignorant about the meaning of the various measurements for wheels and tyres before making the post and have since been reading up on this complex subject.  At least I think that I now understand what is involved a bit better.  I get the impression that the wheel size and profiles are set to keep the running wheel circumference sizes constant so that odometer/speedometer adjustments are not required when changing sizes. I also feel that the smallest wheel sizes are used on the starting grades to get the headline economy figures for marketing reasons. I note that the Prius is also offered in either 15" or 17" versions with a similar reduction in mpg.

I would be grateful to learn of other users' experience with the various wheel size options offered in terms of fuel economy and ride. Is there a size that really suits the car best?  I am not really interested in the aesthetic aspects. I wonder how many people are aware of the differences in fuel economy when they choose a grade of car?

[tavy]

If you're not interested in the aesthetic aspects, I recommend 15" wheels for best consumption and comfort; my car came with 15" wheels, consumption was 50.44 mpg; after few years I fitted 17" wheels, the comfort decreased, and mpg decreased as well, at 47.88  ...

[YarisHybrid2016]

15" are an option even on the top spec because they offer better fuel consumption, and crucially, lower CO2 (important for CC zone in London - larger wheel models don't make the cut).