Tyre depth profiles....

[Rob]

I've gone for 235 45 17's on the rear as I like the chunky tyre look and it affords a little extra comfort. I presume the speedo signal motor is linked to the rear wheels through the gearbox and as such I will have a slight over-read on the clock of about 3mph at 60? Not bothered, just curious.

With the above profile I am considering either a 40 or 45 profile 215 17" tyre up front.

Does anyone have any advice on which is best in terms of forward tilting stance with the 40's or if the 45's cause other issues etc?

[shinny]

235/45/17... that's an odd size to fit to an SW20! You've thrown your speedo out by 6% and raised the rear by 18mm

These are the sizes that best match the SW20 for 17" wheels:
Front: 205/40/17 and 215/40/17
Rear: 235/40/17 and 245/35/17

[raptor95GTS]

215x45x17 will be really close to rubbing the stock strut suspension hence most folks run 215x40x17. On the rears i've run 245x40x17 to give it a bit more comfort and currently running 255x40x17 because it's a wider tyre and I prefer that look on the 9inch rim. For an 8 or 8.5 inch rim what shinny posts will fit real nice

[Rob]

Mark, how does the extra 5 mm side wall on each side of the tyre equal 18 mm or am I missing something?

As far as I can see, with coilovers I can get a great looking stance at the rear whilst maintaining the ride comfort that 16's give on 45 that I'm used to. These are the 45 rears on:


Yes 6% speedo over read but as it under reads by about 5 mph at 60 I'm out by 1 mpg.

We'll see how it goes, not saying I'm right but wanted to try something more purposeful looking than 35 or 40 that can look weak.

Will most probably stick with 40s at the front as there is less weight to dampen with tyre wall and will help the front pitched stance.[/img]

[drunknmunky]

The profile number on a tire is not a direct mm measurement, the tire wall is measured as a percentage of the tire's width, so for example if you got a 205 50 17, the wall would be 102.5 mm because 50% of 205 is that. Therefore with your 235 45 17, your wall will be 105.75mm. So when you include the rim size, your total rear axle height would be (17x25.4)/2 + 105.75 which equals 321.65 mm.
Hope that makes sense

[raptor95GTS]

the 40 in the 235x40x17 is the ratio of the height of the tyre versus the width. So a 235 x 35 against a 235 x 40 is 24mm (tyre manufacturer dependant of course)

Tyre Size... Radius..... Circumference
--------- ------ -------------
225/40/17 306 mm ...1922 mm
235/40/17 310 mm ...1947 mm
245/40/17 314 mm ...1972 mm
225/35/17 295 mm ...1851 mm
235/35/17 298 mm ...1873 mm

[drunknmunky]

So axle height on Standard size is 225 50 15 so, (15x25.4)/2+112.5 = 303 mm

[Rob]

Every day is a school day!

Thanks for the explanation chaps, that's information I know I've heard before and obviously completely forgotten! .

45 looks fine to me on the rear - the front is an old flat 40 profile that I think looks pants but was used for clearance evaluation. ... and ride height is still at maintenance level.

[drunknmunky]

no problem buddy, and yeah I think it looks fine too! and like you say, should give you a bit more of a cushion on the ride.

[shinny]

OK, tyre radius... that's half the wheel diameter plus the sidewall height. So, in mm:

wheel radius in mm = wheel diameter in inches * 25.4 / 2
sidewall height = profile * width / 100

For the stock 225/50/15:

(15 * 25.4 / 2) + (50 * 225 / 100) = 190.5 + 112.5 = 303mm

For 235/45/17:

(17 * 25.4 / 2) + (45 * 235 / 100) = 215.9 + 105.75 = 321.65mm

For a better fitting 235/40/17:

(17 * 25.4 / 2) + (40 * 235 / 100) = 215.9 + 94 = 309.9mm

For a virtually ideal 245/35/17:

(17 * 25.4 / 2) + (35 * 245 / 100) = 215.9 + 85.75 = 301.65mm



See what I mean?

Of course there's might be more to the decision on tyres than simply trying to match the stock radius, but it's a good place to start. Tyre choice is limited in some sizes and some sizes are cheaper than others if you're really on a budget. Changing the radius is an easy way of essentially changing the final drive ratio, affecting acceleration, gearing and top speed... but fitting the wrong profile tyres is altering how the car drives for that exact reason.

Personally, I don't entirely get the whole "adding profile for comfort" thing... if you want comfort, why did you replace the stock 15"s with 17"s?

[shinny]

45 looks fine to me on the rear - the front is an old flat 40 profile that I think looks pants but was used for clearance evaluation

Don't forget that profile means nothing without knowing the width!

As a very basic rule of thumb, to keep a tyre the same radius you subtract 5% profile for every 30mm width you add. So the 195/55/15 and 225/50/15 stock tyres have pretty much identical radii, despite being different profiles.

As for "looks ok"... well, it can look fine without actually being right. You've changed the gearing Toyota spent ages getting right... you might not care or you might prefer it.. the chances are most people don't even think about that when fitting oversize tyres.

Personally, I think out cars look wrong if the rear tyres have visibly have more sidewall that the fronts. Looks a bit "tractor" to me. I think this "looks right", given the front and rears have identical sidewalls (within 1mm). That's because the radii are deliberately virtually identical to stock tyres, meaning I've not affected the gearing etc. either:




The only good thing I have to say about oversize rear tyres is that it increases rake, which is probably a good think. But then again, you can do exactly that with coilovers too. Oversize fronts are almost always going to be a bad idea.

[Rob]

Same reason you are running BC's over other brand coilovers Mark, because everyone hates a harsh ride ! 😀

I want 17's for looks and I am happy for it to cost me a speedo inaccuracy that is already covered by the manufacturers built in under reading.

I very much doubt that the handling will be at all compromised and if it is I will change to 40's.

I personally think the MR2 looks better for having beefier rears as it has a beefier back end and slender front. To me 40's look a little bit elastic band.

You can also say that Toyota never intended for us to have much stiffer reduced height side walls but we do it for the looks.

[craig]

255/40/17 look awesome on an MR2, and they'll be getting fitted to my Enkei RP01 rears - 9x17 ET38. Arches should be getting rolled too.

[Rob]

They do, I went a bit safe with 235 width as I've had a mild arch roll and full respray!

Beefy rears on SW20 look purposeful.

[raptor95GTS]

I went 17's to get a tyre choice! and yeah 255x40x17's look right meaty esp Pilot Sport 2

[shinny]

Same reason you are running BC's over other brand coilovers Mark, because everyone hates a harsh ride ! 😀

I want 17's for looks and I am happy for it to cost me a speedo inaccuracy that is already covered by the manufacturers built in under reading.

I very much doubt that the handling will be at all compromised and if it is I will change to 40's.

I personally think the MR2 looks better for having beefier rears as it has a beefier back end and slender front. To me 40's look a little bit elastic band.

You can also say that Toyota never intended for us to have much stiffer reduced height side walls but we do it for the looks.

My rears are 35 profile and are a perfect match for the stock radius Also, there are many more benefits to lowering than just aesthetics! (Comfort is not one of them)

The thing that gets me about the tractor look (sorry, a "beefier rear end") is it immediately makes my mind scream that something is wrong; "Why have they fitted the wrong size tyres?", my brain asks. And if they're drag radials then my brain is placated as I can see there's a genuine performance reason behind it.

My choice is to run tyres that both look "right" (read "could be factory fit") and are "right" (read "use correct radii"), which has the excellent side effect of not altering the car's gearing and hence theoretical acceleration. Your choice may be different for whatever reasons... and that's fair enough

[jimGTS]

Lovely wheels!!



Nout wrong with 40s at the back, dont need to be 45s!

With my Works offset/width and ride height, it looks pretty neat
Et22 and 9j along with a good 40-50mm drop, if not more

[Rob]

Having checked the tyre calculator my 45 profile is within 1mph of 255 40 at 60mph so ours should look pretty much the same in terms of radius on a 17x9 wheel Jim & Raptor.

[Rob]

Thanks for everyone's constructive input, you learn something every day and I will post up what I find out when the car is back on the road.....

[rev3gtturbo]

the 40 in the 235x40x17 is the ratio of the height of the tyre versus the width. So a 235 x 35 against a 235 x 40 is 24mm (tyre manufacturer dependant of course)

Tyre Size... Radius..... Circumference
--------- ------ -------------
225/40/17 306 mm ...1922 mm
235/40/17 310 mm ...1947 mm
245/40/17 314 mm ...1972 mm
225/35/17 295 mm ...1851 mm
235/35/17 298 mm ...1873 mm

... and that does not take into account the tyre tread depth wearing from 8mm down to 2mm
(although I replace mine at 3mm).

When the car has the oem wheel and tyre sizes the speedo is not allowed to indicate a speed slower than the car is actually doing, so I guess the speed calculation assumes the car has new tyres.