Larger Wheel = Slower Car?

Twiss · Unread post by **Twiss** » 25 Oct 2004, 05:15

Hi You Lot, is it true that if you increase your cars wheel size,lets say from 14" to 15" or even 16" would this slow your car down in speed and acceleration? or is that a load of Tos!!

PeterMann · Unread post by **PeterMann** » 25 Oct 2004, 05:48

If you reduce tyre profile so the overall effect is the same diameter, the only change will be in speed of response to steering adjustments. A larger overall wheel size will decrease acceleration by a small amount, but would tend to reduce fuel consumption on highway runs because the engine is running at lower revs and higher throttle setting (hence a higher effective compression ratio).
Bigger wheels have an advantage on uneven ground, which is why 4wd vehicles are now using truck size wheels, and cars like Ferrari use them to compensate for having such a short suspension travel.

uhn113x · Unread post by **uhn113x** » 25 Oct 2004, 12:45

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Twiss</i>

Hi You Lot, is it true that if you increase your cars wheel size,lets say from 14" to 15" or even 16" would this slow your car down in speed and acceleration? or is that a load of Tos!!
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
It is only the tyre diameter that makes a difference - larger diameter raises the gear ratio, therefore less acceleration. Whether it makes a difference to (top) speed depends on all sorts of other imponderables.
Generally, you will find that the generously-funded manufacturer's design department can be relied upon to choose the correct size wheels and tyres to suit the car! [;)]
What is 'Tos' [?]

Stempy · Unread post by **Stempy** » 25 Oct 2004, 12:49

Yes, if you think about it a bigger wheel/tyre will have a larger rolling circumference, hence the car will travel further for each wheel revolution. It has the effect of raising the overall gear ratio so would give a higher speed for a given engine rpm thus you would feel the effect as reduced accelleration but a higher top speed if the engine has the power to pull it.
If you are concerened you should make sure the new wheel/tyre combination has as close an overall diameter as possible.

TomH · Unread post by **TomH** » 25 Oct 2004, 13:44

so the speedo will under-read as well!

PowerLee · Unread post by **PowerLee** » 25 Oct 2004, 17:41

Have a look at this site, It will help you out.
http://www.miata.net/garage/tirecalc.html

ItDontGo · Unread post by **ItDontGo** » 25 Oct 2004, 18:26

Adding to the radius of a tyre effectively adds weight to the car even if the wheels are the same weight as the smaller radius wheels. I'm sure you're thinking I'm speaking bollocks at this point but by increasing the radius of a rotating component you increase it's angular inertia in a more dramatic way than adding mass - its a quadratic/square relationship and so going from 15 to 16 has less of an effect that 16 to 17 too.
This is not likely to affect a heavy road car too much but it will have an effect on the stop watch on a light car.
The increase in inertia comes from the fact that you are accelerating the wheels round and round as well as the car forward and the mass of the wheel further away from the centre has more of an effect than the mass closest to the centre. A lot more of an effect.

Kowalski · Unread post by **Kowalski** » 25 Oct 2004, 20:00

What about if you replace heavy aluminium "alloy" wheels with magnesium alloy wheels of a larger size?
I did hear the Nascar boys saying that losing a kilo of unsprung mass (wheel / hub etc) was worth 10 kilos of sprung mass (chassis) in terms of the speed advantage it gives. This is more to do with the keeping the wheels on the tarmac, suspension has a harder time keeping a heavy wheel in contact with the ground after a bump than it does with a lighter wheel.