lojk wrote:I know that the smaller 225's would give better acceleration, and probably slightly firmer ride/tighter handling due to reduced sidewall height?
But the speedo would read under wouldnt it?
Taking 225/45/18 as the example, the 225 bit is the width of the tyre, in this case 225mm. The 45 is the height of the sidewall which is a percentage of the tyre width, in this case 45% or 101mm. The 18 is of course the size of the rim the tyre is designed to fit over.
So, a 225/45/18 tyre will have a taller sidewall than a 225/40/18 tyre. For a stock Liberty 2.5i which ships with 215/45/17 tyres, the closest rolling diameter match for an 18" rim to factory is 225/40/18 (you're adding an inch to the rim, so we need to remove roughly an inch from the tyre sidewall - 5% usually does the job while staying within spec).
Where the speedo reads different is if the rolling diameter of the tyre is larger or smaller than the original spec. Legally, the speedo cannot read under what your actual speed is, and from factory most speedos are deliberately slightly off to account for tyre wear, so 100kph on the speedo can equal an actual speed of 96kph. This of course can create a false sense of security for some who say that they sped past a cop at 110kph in a 100 zone and didn't get pulled over, simply because they weren't actually doing 110. If the tyre is smaller than spec, then your speedo will read higher than actual speed and this is a Bad Thing(TM).
Using the 2.5i example again, while 225/40/18 is a good match to the factory 215/45/17 wheels, you can correct the speedo by going for 225/45/18 tyres instead. This makes 100kph on the speedo about 100kph in actual speed. The downside is that your odometer is now slower by 5% (ie: you'll travel 5.25 kilometres for every 5 kilometres indicated).
lojk wrote:It's a tuned b spec so it's pretty quick for day to day stuff already.
Yes, for the most part you wouldn't notice much difference at all unless you went side by side with someone (excusing all other factors such as general state of car, driver competency, etc).
The width will help mostly in lateral grip, when taking corners at speed, though really wide tyres tend to "tramline" easily. The sidewalls help a lot with acceleration and heavy braking. Those with rubber bands as tyres find that they can't accelerate or brake as well as someone else with the exact same car and normal tyres because the sidewall can't flex as much (if at all) to allow the tyre the opportunity to have proper contact with the road (this is why drag cars have monster sidewalls on their tyres - look at a photo of one launching).
There are of course a whole bunch of other factors such as swaybars, camber, centre of gravity, road temperature, design of the tyre tread, and compound used that all impact how well your car grips the road as well, but for simplicity's sake the above forms a large chunk of the science.
lojk wrote:Yep the whiteline sway bars are where i was going to start handling wise.
Excellent. You'll have a lot of fun with that.
I was going to procrastinate, but I put it off...