Austarion

car feels more torquey now? would changing from 17s to 15s do this? i can't see the overall diameter being that much different. but thinking it could be the change in wheels cos it handles like crap now (traction). could just be that i havn't driven in a long time tho. will have to test when i clear defect and put the 17s straight on after a drive and see how it feels

Put them side by side and see what the diff in height is. Probably more than you expect.

Yes check it because it can be quite deceptive just looking at them. Measure the rolling diameter and compare them.

so if the rolling diameter is bigger, i'll be missing out on off-boost performance, but getting better traction and handling?

no, the reason why you appear to be getting better traction is that your putting less torque to the ground. If the rolling diameter is bigger it will have a highter top speed and be slower accellerating.

What are the different tyre sizes of the 17's and the 15's?

17s have 235s front and back, so i thought thatd be helping with the traction, plus the low profile i can corner a lot better. 15s have 195 or 205s i think, and like to squeel on corners. can feel a bit more tyre roll in them, but, the car feels torque'ier. as far as i can tell, havn't driven in a while

CussCuss wrote:no, the reason why you appear to be getting better traction is that your putting less torque to the ground.

all the better reason to rev higher :D

CussCuss wrote:If the rolling diameter is bigger it will have a higher top speed and be slower accellerating.

i'm not really fussed over my top speed :( might have to attempt both of best worlds with even lower profile tyres on the 17s?

what about the other number in the tyre size, the 17's will be 235/??/17 and the 15's will be 195/??/15

the middle number is the one from the edge of the rim to the edge of the tyre so then you can see the difference.. 235 and 195 are only how wide the tyres are (what you see when looking from the front or back)

and if your interested in how much out your speedo will be, on this site you can put in the stock tyre size and new tyre size and it will tell you how much out the speedo will be..
http://www.miata.net/garage/tirecalc.html

yes my post was in the fact that the tyre widths and compounds were equal, you will get better traction with wider tyres ofcourse. The 17's will also increase the weight that the car has to spin if they are heavyer (usually are) think of it like putting more weight on the flywheel.

Putting a 3.9 LSD would offset the 17's to a degree, if you want the 17's on there i would be putting near identical rolling diameter (whatever is closest without going over) and a 3.9 LSD.

as Starionboy said, we need to know the profile of the tyres and the width of the wheels then i can give you an ideal combination.

Also, you may want to try weighing one of the wheels and comparing them, the 17s could be having an effect like a heavier flywheel

Chris 83JA wrote: the 17s could be having an effect like a heavier flywheel

also having an effect of making the car look sexy :D

current ones on the 15s are x60h and the 17s had 40s i beleive. yeh i could imagine the simmons weighing a bit more... the sacrafice for 3peice dish :D

as for me getting a a 3.9.... ok :D only if its lsd tho :D and $500 :D

If you have 195/60/15's, then with the 235/40/17's the radius will be out by 0.8% (over). Ie, if your speedo reads 100kph, then you will be actually travelling at 100.8 kph.

If they are 205/60/15's, then the 17's will be 1.1% less the 15's radius. Ie, your speedo will be over by 1.1% (actual speed will be 98.9 when speedo shows 100 kph).

Either way, it is pretty much the same acceleration and speed-wise. Traction is a different story ;)

shweeeet