[tcrzero]

I am looking into new rims for my zx3. I really like the look of 18's, but I've heard they make the car feel sluggish and they make your odometer and speedometer read wrong. I know all these would be true if the overall wheel dimension changed, but you would use lower profile tires with 18's, so wouldn't overall dimension be the same? do 17's have any of these effects?

 

[jptech2004]

welcome to the jet. you are correct in your thinking. in order to keep the speedo accurate, you must get the right plus size tire recommened for the car. for 17s, the proper size tire is 205/45/17. i run 215/40/17 and my speedo reads 1mph too fast. for 18s, the proper plus size tire is 215/35/18.

 

[Zoomer69]

Larger wheels have greater rotational inertia, partially due to greater weight, partially since that weight is distributed farther from the center of the wheel. That will mean your acceleration will suffer.

Go as light as possible if you do decide to go to 18's; that'll minimize the negative effects.

 

[tcrzero]

Thanks for the help guys. Question on one other thing, which is heavier, the differance between a 17/18 wheel, or the difference between a 45/35 tire? I thought it would be actually lighter to go with 18's because the extra rim would be lighter than the extra tire. and if overall diameters are the same performance would be better. am I wrong?

 

[whiskeytangofoxtrot]

The smaller wheel is lighter given you are using the same brand wheel for each size.

Most track racers use 14, 15, and sometimes 16 inch wheels for that very reason.

Also, the way I've always figured plus sizing is this:

If you start with 205/50/16 and you'd like to go to 17" wheels, you have to use a 215/40/17 to maintain the proper rolling diameter. Add 10 to the first number and take 10 from the second. I saw this method a long time ago on the Discount tire direct website.

Now, as far as why the above poster is experiencing speedo inaccuracies with that size and his 17s... I have to chalk that up to the fact that not all tire companies make their tires to the same size specs.

 

[Qdoba Addict]

Thanks for the help guys. Question on one other thing, which is heavier, the differance between a 17/18 wheel, or the difference between a 45/35 tire? I thought it would be actually lighter to go with 18's because the extra rim would be lighter than the extra tire. and if overall diameters are the same performance would be better. am I wrong?

Larger diameter tires aren't always lighter than the smaller ones because the larger one needs more reinforcement due to the small sidewall. For example I'm looking at some Falken Ziex ZE-512, the 16" 205/50/60 weighs 19.4 pounds and the 18" 215/35/18 weighs 22.2 pounds.

 

[P.]

The smaller wheel is lighter given you are using the same brand wheel for each size.

Most track racers use 14, 15, and sometimes 16 inch wheels for that very reason.

Also, the way I've always figured plus sizing is this:

If you start with 205/50/16 and you'd like to go to 17" wheels, you have to use a 215/40/17 to maintain the proper rolling diameter. Add 10 to the first number and take 10 from the second. I saw this method a long time ago on the Discount tire direct website.

Now, as far as why the above poster is experiencing speedo inaccuracies with that size and his 17s... I have to chalk that up to the fact that not all tire companies make their tires to the same size specs.

well add 10 minus 10 will get you close... if you do the OD math on the matter you will find that tire rack is correct. take your sectional width (205) divide by 25.4 = 8.07= width of tire. take 8.07 and multiply by aspect ratio .45 =3.63= height of 1 sidewall in inches. multiply 3.63 by 2 (technically 2 sidewalls, 1 top, 1 bottom)= 7.26. add rim diameter to 7.26=(17")=24.26" top to bottom.

a 215/40/17 is 23.77" OD
a 205/50/16 is 24.07" OD
a 215/35/18 is 23.9" OD
a 215/35/19 is 24.9" OD

at least thats how i did the math to check mine. I went with the 215/40 since my stock tires rubbed and this ratio is slightly smaller OD.