Basic question I know but presume going from std MY 06 WRX 215/45/17 to 225/40/18 (have i got that right) will effect the overall diameter thus gearing/acceleration/top speed - but how muck?

Also leave less clearance under the arch so limiting amount suspension can be lowered??

Thanks

 

I've just done exactly the same

Gearing,acceleration etc.... can tell no difference from before

Fills the hole pretty much the same because you're increasing the rim size but decreasing the tyre depth

Ride qualty is the same -i thought it would be harsher but it isn't

Handling/road holding will be better-not pushed mine too much yet to see how better it is

Fitted speedline turini 18s with goodyear eagle f1s gs-d3

 

It may be just a tad out but not a noticable amount because you're changing the size of the tyre to keep the rolling circumference the same. Handling is far better than 17's and looks far better to.

 

It's an easy sum.

215/45/17 means the diameter is (17 * 25.4) + 2 * (0.45 * 215) = 625.3

225/40/18 means the diameter is (18 * 25.4) + 2 * (0.4 * 225) = 637.2

So the latter will read less than 2% different. About equal to the difference between new tyres and fully worn tyres, if you don't change your gearing and speedo as the tyres wear out then don't worry about this change either.

 

The speedo is generally over-reading by up to 10% anyway, so a slightly larger overall size will tend to make it slightly more correct. The slightly longer gearing will hardly be noticeable.

The extra weight of the bigger wheels will have a small effect making acceleration and braking performance slightly worse (rotational inertia will tend to be higher, depending on the exact wheels and tyres of course). In this case more than made up for overall by the better tyres.

 

Thanks for the replies, still wrestling with cost vs benifits!

 

another point to think about 18's are noisier than 17's from personal experience..must be something to do with less rubber between the car and the road

 

I just put Kei RSS 18" 225/40 wheels on my MY04 WRX PPP ,I checked the speedo against my GPS road angel which is very acurate, the Speedo is now showing 2MPH than Road Angel.