Tyre size questions - Ford C-Max Forum
 
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post #1 of 4 Old 05-20-2009, 11:16 PM Thread Starter
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Just round the corner from me a bloke has bought a Kia Soul. Slightly smaller than my cmax but its got the biggest set of wheels I've seen [img]smileys/smiley3.gif[/img] on a smallish car - 18", apparently.

Quite apart from ride issues with wheels this big, it got me thinking [not something I do often] about the effect of wheel size on rpm and therefore mpg. My intuition tells me that the bigger the wheel circumference, or diameter, the less revolutions the engine needs to do at a given speed to cover a given distance. There seems therefore a point in having big wheels insofar as the engine needs to work less hard and therefore will use less fuel. Is my reasoning correct, or am I thinking nonsense, as usual?

My second point is, of course, what are the sizes of wheel that will fit a cmax, and, if I had bigger wheels fitted, would I then have to recalibrate everything [speedo, rev counter, computers, etc] which would make the whole mod impossible? Finally, is there anyone out there with a 1.6 petrol [100hp] zetec with 16"ers for comparison? My car has 15"ers and I get 3250 revs at 70mph with one on board on the flat.

Many thanks as always.

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post #2 of 4 Old 05-20-2009, 11:59 PM
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Quite right about the revolutions per mile being less for the bigger wheels. apart from the obvious retro points you mention there is the case that the acceleration will suffer because Torque is the product of Force * Radius

There is then the case that even with the very low profile tyres on 18" rims that some models come with there has to be limiters fitted to the steering to prevent the inside edges of the tyres rubbing on the non moving part

On your second point the tyres on the larger rims are lower profile so the overall rolling circumference is not that much different but they are considerably wider




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post #3 of 4 Old 05-21-2009, 12:04 AM
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I perhaps should also have said that MPH per 1000 PRM is more a product of the final drive (differential raio) and the engine torque than tyre size

By comparison our 1.6 TDCI on 205/55 R16's is only doing about 2500 RPM at the same speed




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post #4 of 4 Old 05-21-2009, 03:45 AM Thread Starter
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Thanks for that, Richard. I have read that the ride in the Kia with the 18" wheels is terrible. The article recommends the 16" option. Nevertheless, I would still be interested to hear from anyone about the above who has a 1.6 petrol on 16"ers.
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