Bigger wheels, MPG decrease?

[Nictrix]

I dont know how the website worked out the diameters but it was obviously wrong.
Going on their calculations the sidewall on an 18" wheel would have been over 6".
You can check different sizes here
http://www.tyresave.co.uk/tyresize.html

 

[thepits]

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.

very teckie - just it

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.

What? :? I'm sorry but I must have dropped off - zzzzzzzz -or....

I was too busy to give a fcuk!

 

[RickRob]

Chasabi look what you have done! Bet you wish you'd stayed with the original wheels.

The only saving grace is that the fall in the cost of petrol will have offset you higher MPG!

 

[thepits]

Chasabi look what you have done! Bet you wish you'd stayed with the original wheels. The only saving grace is that the fall in the cost of petrol will have offset you higher MPG!

Yep, It'll soon be back to the old days at 7s 6p per gallon