Tire size variance...

[TxRC51]

I've been checking spec's on overall tire diameter trying to put together a package that will fit. At what point does the difference in overall diameter affect the anti-lock brake system? 1/10th of an inch is what I'm being told to stay within as a working range. I'd like 18's all the way around with as close to as possible, a matching aspect ratio. I had 275/40/17 & 275/35/18 on WS6 T/A & never really liked the visable difference in aspect ratio of that combo. Yeah, I know Gravana & Group A Wheel have pretty much calculated this with their packages, but, I like to explore every possibility.

[miscreant]

I've been checking spec's on overall tire diameter trying to put together a package that will fit. At what point does the difference in overall diameter affect the anti-lock brake system? 1/10th of an inch is what I'm being told to stay within as a working range. I'd like 18's all the way around with as close to as possible, a matching aspect ratio. I had 275/40/17 & 275/35/18 on WS6 T/A & never really liked the visable difference in aspect ratio of that combo. Yeah, I know Gravana & Group A Wheel have pretty much calculated this with their packages, but, I like to explore every possibility.
I'm nut sure exactly what you are asking, because there are only a couple combinations that work in 18" to begin with.

First, 1/10th of an inch is incorrect. Typically with Bosch systems, you want to stay around +-3% variance to keep the abs from screaming. Basically between 25" and 26.4". However, at ~26" you start to get to close to the spring perch for comfort. And at ~25" the tires start to look too small.

I'm assuming you are wanting to go staggered, so your combinations really are:

245/40R18 and 275/35R18 (both ~25.6"-25.7")
or
235/40R18 and 265/35R18 (both ~25.3"-25.4")

You can also go 235/40R18 and 275/35R18 with a tad more sidewall in the rear to give a "raked" look. You could also go 285/35R18 in the rear, but then you need a 9.5" wide wheel really.

Again, I'm not sure exactly what your asking or if I'm on track, but there ya go.

[TxRC51]

You understood me just fine. Thank You for the input. I was trying to describe my preference for matching Sidewall Aspect Ratio's. Pulled this from Tire Rack site:

Sidewall Aspect Ratio
Typically following the three digits identifying the tire's Section Width in millimeters is a two-digit number that identifies the tire's profile or aspect ratio.

P225/50R16 91S

The 50 indicates that this tire size's sidewall height (from rim to tread) is 50% of its section width. The measurement is the tire's section height, and also referred to as the tire's series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall. We know that this tire size's section width is 225mm and that its section height is 50% of 225mm. By converting the 225mm to inches (225 ÷ 25.4 = 8.86") and multiplying it by 50% (.50) we confirm that this tire size results in a tire section height of 4.43". If this tire were a P225/70R16 size, our calculation would confirm that the size would result in a section height of 6.20", approximately a 1.8-inch taller sidewall.



I've been comparing spec's, so for I like the 235/40R18 (25.4) - 275/35R18 (25.5) combo the best. Might go 245/40R18 (25.7) - 275/30R19 (25.5). Worried about tire rubbing, so a 235/40R18 (25.4) - 265/30R19 (25.3) might be the way to go.

[miscreant]

I've been comparing spec's, so for I like the 235/40R18 (25.4) - 275/35R18 (25.5) combo the best. Might go 245/40R18 (25.7) - 275/30R19 (25.5). Worried about tire rubbing, so a 235/40R18 (25.4) - 265/30R19 (25.3) might be the way to go.
Keep in mind that if you go 18" up front and 19" in rear, your sidewalls will not match. The rear sidewalls will be .5" shorter than the fronts.

[TxRC51]

Keep in mind that if you go 18" up front and 19" in rear, your sidewalls will not match. The rear sidewalls will be .5" shorter than the fronts.


Exactly, that’s my point. I like the more balanced, or even look of matching sidewall height. I never truly liked the 275/40/17 – 275/35/18 combo on my T/A. Getting the widest rear tire possible with closely matching sidewall heights is kinda frustrating.

[sccaGTO]

It may be hard to tell from the e-mail Isent you, but the 245/40 & 275/35 18" combo looks equal in height. I look at mine & just see short sidewalls all around. Looks very balanced. Still waiting for Hotchkis sway bars so I can unleash this beast.

[ronrosa]

It may be hard to tell from the e-mail Isent you, but the 245/40 & 275/35 18" combo looks equal in height. I look at mine & just see short sidewalls all around. Looks very balanced. Still waiting for Hotchkis sway bars so I can unleash this beast.

I think he is talking about the difference in sidewall height due to 18" rims in the front and 19" rims in the back. To match tire diameters with these rims, the back tire sidewall will be shorther than the front.

This is the setup I have. Yes, the 1/2" difference in sidewall is noticeable. But so is the difference in rim size. IMHO, the increase in size of the rims looks good. Part of the reason I went this route was the look of a bigger rim in the back, plus I was nervous about pot holes and wanted more tire sidewall in the front.

Follow the link in my sig to see pictures of my setup. You can decide for yourself if the difference in sidewall height is objectionable or not.

I can match sidewall size, but the front tire will have about 1" smaller diameter than the rear. Speedo may read a little faster than you are actually going. I think I calculated 103 mph with actual speed of 100 mph. When my current tires wear out I may try out matching sidewall heights. The pot hole situation is not the problem I thought it would be and I will be switching back to the stock rims + snow tires for winter.

[sccaGTO]

I think he is talking about the difference in sidewall height due to 18" rims in the front and 19" rims in the back. To match tire diameters with these rims, the back tire sidewall will be shorther than the front.
You may be right. He had a 275/40/17 & 275/35/18 combo.
IF he chooses 18" or 19" rims front & rear, he can stagger them to appear the same height.

[TxRC51]

You may be right. He had a 275/40/17 & 275/35/18 combo.
IF he chooses 18" or 19" rims front & rear, he can stagger them to appear the same height.


You guys understood my twisted question. I prefer, as close to as possible, matching or appearing to match, sidewall heights. I like the balance in appearance. Like Ron, I like the larger rear wheel too. Thats why I put the Z06 wheel package (17/18) on my WS6 T/A. My first set of tires didn't have matching overall tire diameters and gave the ABS absolute fits! I'm leaning towards the 18/19 combo, but have not ruled out 18/18. I've compiled alot of tire spec's (Goodyear F1 GS D3) and I'm playing with wheel tire combo's until I get one I'm satisfied with. Plus looking at almost every pic on this board checking wheel/tire combos. So far I have 2 18/18 and 2 18/19 combo's that will work.


I have looked closely at pic's of both your cars (Thanks SCCA for the e-mail) and that's why I'm leaning 18/19.

[ronrosa]

My first set of tires didn't have matching overall tire diameters and gave the ABS absolute fits!

How much of a difference in diameter did you have ? Did the ABS problem occur all the time or only on hard braking ?

If I match sidewalls, my tire diameter in the front will be around 1" smaller. Do you think that would cause a problem with the ABS ?

I like my setup. The difference in sidewall doesn't look too bad to me, but it's always good to have options. Just wondering if my next tires had matching sidewall heights if it would cause a problem with the ABS.

[TxRC51]

How much of a difference in diameter did you have ? Did the ABS problem occur all the time or only on hard braking ?

If I match sidewalls, my tire diameter in the front will be around 1" smaller. Do you think that would cause a problem with the ABS ?

I like my setup. The difference in sidewall doesn't look too bad to me, but it's always good to have options. Just wondering if my next tires had matching sidewall heights if it would cause a problem with the ABS.



I don't know the actual difference in diameter on that 1st set, but under hard braking, the ABS freaked out! Fronts were 275/40/17 and I think rears were 275/40/18. Me being a rookie at dealing with different wheel sizes thought that 275 front and 275 rear were the ticket. The Z06 Vette ABS system compensates for different diameter sizes. My WS6 T/A threw up all over itself...LOL I'll assume our GTO's ABS system is calibrated with matching wheel diameters front and rear just like my T/A. Too large of a difference will certainly cause ABS issues.