Upgrade from 17" to 18"

[SegR]

I have a MY05 3.0R with stock 17" rims, 215/45 potenza tyres and non-stock custom-carved gutter upgrades on the rims.

I'm debating whether to repair the rims, get replacement 17" rims or get replacement 18" rims. Note that I'm not particularly interested in lowering the vehicle - while the aesthetics would be awesome, clearance above my steep driveway would not be.

If I were to run 215/45 tyres on 18" rims, I'm going to gain 25mm total wheel diameter... or a bit less with 225/40's. What impact is that extra 25mm diameter going to have on ride, handling and speedo accuracy?

Yes I did many searches and came up with no succinct answers as few people ask these dumb questions :-p

[sti_stig]

From my understanding on physics n all..

In comparison to original wheel size calibration:
* Bigger wheel diameter would mean slower speedo indication than actual speed your doing as you are covering more distance but takes longer to do one revolution of the wheels therefore showing a slower speed but your actually travelling further and faster then displayed. A smaller diameter would mean your speedo would indicate faster whereas your actually travelling slower.

This is my understanding of it correct me if im wrong..

[SegR]

Yeah thanks for the response; I should have removed that part from the question as I figured it out for myself.

If I could be bothered I'd work out how much difference it would really make to the speedo reading but based on the speed reported by GPS my speedo currently over-reads by 5%.

P.S. having an entire video in your sig is really distracting!

[DrivesLikeJehu]

Short answer: Buy the specB 18s from Claremont Wheelpower coz they're a bargain that won't be around forever, and bang 225/45s on them .
Long answer: If the car has ample power for you, and you won't bemoan a very slight, indescernable-without-a-stopwatch drop in "off the mark" acceleration, fix (or reduce at least) the factory speed stuffup with your tyre size. It's the only way. You'll gain slightly better highway cruising/fuel economy, and a lot less people will be passing you when your speedo says you're on the limit.
(My 2c)
Update: just used a calc. 5.24% difference. Perfect speedo . And it'll fill the tyre/guard gap. Visually like lowering, without lowering, LOL.
Of course, 225/40r18 seem to be a more common, cheaper tyre. They improve the speedo error by 1.9%.
This calc is pretty easy;
http://www.1010tires.com/tiresizecalculator.asp
and this is handy;
http://www.kouki.co.uk/utilities/visual ... calculator

Handling: Sidewall height has a large bearing on ride comfort, and cornering/steering response.
215/45/17 = 96.5mm sidewall height.
215/45/18 = exactly the same
225/40/18 = 89.9mm .
However, you need to add the rim width/tyre width calc to the equation. A tyre wider than rim, so the sidewall is diagonal to the rim, gives more flex/squirm/comfort.
so, for the two 18" examples, the 225/40 and 215/45 may feel very similar on a 7" rim, but the 225 would feel firmer on a 7.5" rim, for example.
Just to toss another consideration into the mix, a slightly wider tyre will give you a little more room for error when touch parking.If the tyre is noticably wider than the rim (like my OEM forester ones) minor low speed misjudgements mean a scuffed sidewall, but no rim damage. If the rim is wider than the tyre (like my MY08 3.0RB was) the tiniest curb rub leaves gutter rash. I've gone from 215/45/18 to 225/45/18, and the tyre is just out enough to forgive parking indiscretions. I've lost a smidgeon of standing start acceleration, picked up a tiny bit of sidewall flex (in theory, depending on tyre model chosen), but have pro'lly compensated with a little more tread grip on the road, and improved my speed error from 8% to 5%.
In summary, to answer your question, we'd need to know what rim WIDTH you're talking, and whether we're talking "like with like" on tyre type. Some tyres have very stiff sidewalls for handling, some have soft ones for comfort. A different tyre could cancel out ALL of your careful calculations

[SegR]

Thanks for the info; this confirms the calculations I've been doing manually and adds some good info - I hadn't considered going slightly wider.

As for the rim width... that's to be determined based on what I find for a decent price