ok i am having a hard time understanding how the different measurements affect the cars sensors. i understand how tire diameter affects speed, basically the bigger the tire diameter from stock the faster you will actually be goin compared to the speedo, and you have the opposite effect with smaller wheel diameter from stock. but how does the width of the tire affect the sensors? or does it affect it at all? in my mind i feel like the faster you go the more grip is needed to maintain your speed so a wider contact patch will will yield better acceleration or a higher speed compared to the stock tire, right? this is really bugging me and if anyone is able to help please do so. the thing that brought this to my attention was the tire calculators, which i figured out that they are not necessarily accurate, but the tire width seemed to affect the resulting speed.

 

Section width doesn't have a direct impact on the speedo.

If you increase your section width but keep the same ratio of sidewall height to section width then you'll get taller tire and a greater outer diameter.

The sensors have no way of knowing about the tires' width except how it affects the diameter based on the ratio of the sidewall height to section width.

Tire sizing is done with a ratio.
Stock size
225/45/18 means:
Tire width: 225mm
Sidewall height: 45% of 225mm (101.25mm)
Inner diameter: 18"
Total height: 18" + 7.96" (101.25 mm in inches x 2 since you have the sidewall on either side of the wheel) = 25.96"

Wider than stock but maintaining stock ratio
265/45/18 means:
Tire width: 265mm
Sidewall height: 45% of 265mm (119mm)
Inner diameter: 18"
Total height: 18" + 9.36" (98mm in inches x 2 since you have the sidewall on either side of the wheel) = 27.36"

Wider than stock with shorter ratio than stock
265/35/18 means:
Tire width: 265mm
Sidewall height: 35% of 265mm (92.75mm)
Inner diameter: 18"
Total height: 18" + 7.3" (98mm in inches x 2 since you have the sidewall on either side of the wheel) = 25.3"

 

ok, so i shouldnt really care about the width as long as the diameter is the same, or close to. see thats what i originally thought but then i came across some tire calculators and i messed with the width and on every single one of them it changed the speed, which didnt make any sense to me, and i guess i looked to far into it. i know what all the numbers mean its just like i said i think i may have looked to far into it.

 

Google "tire calculator"

 

google width

 

Well as you can see, the tire stomach on this guy totally engulfs the speedo

 

i know about the calculators, and the first guy pretty much straightened out my confusion.

 

Also : you can have different widths front and rear IE staggered . But ensure the diameters are within 10mm of eachother or you will get the DSC doing random braking on even the slowest of corners .

 

It's funny that I see this thread today. I actually just put stock wheels and tires back on my car since I sold my Volks and I am awaiting my new wheels and tires.

Anyway, i did a test on teh way back into town on Sunday.

With my staggered Volks and 245/40-18 tires I was about 3MPH faster per my Magellan GPS and my phone. With my stock wheels and 225/45-18 tires I was still 2MPH faster.

 

wouldnt a stretched tire decrease the overall diameter of the wheel and therefore cause differences that the calculators would not pick up, being that the side wall is now angled instead of standing straight

 

Well when I was running 245/40-18's on stock wheels I was still 3MPH faster, nothing changed when I installed the Volks (18x8 Front and 18 x 9 rears with same tires all the way around).

Going back to stock only affected it 1MPH which basically means it affected nothing at all.

 

right, I know the difference would be negligible, just checking to see if I understand what I was thinking

 

I will let you know when my 18 x 9.5's come in I will be running 245/40-18's on them as well.

 

^just curious but what did you order?

 

These bad boys.

http://www.flickr.com/photos/9krpmrx8/5416539655/http://www.flickr.com/photos/9krpmrx8/5416539655/ by http://www.flickr.com/people/9krpmrx8/, on Flickr

 

^damn nice choice!

 

Thanks, I'm excited. It's just taking too damn long to get them

 

^yeah I need to get a good set sometime soon, settled on rotas after wrecking my car and busting my fn01r-c's

 

MUCH better - have to say I was never a fan of the red highlights in your prvious wheels

 

Yeah I am hating driving on stockers right now and factory exhaust (sold my HKS). It's like I'm driving someone elses car. I need my new exhaust and wheels on ASAP.

 

Yeah I was never in love with them but I got comments everyday about them. But I have to say the quality of them does stand out when the car is all cleaned up and blin-gin'

 

no kidding im driving on an old set of 18.7.5s right now and its killing me....Cant wait till this weekend when I can put on my 17x9.5 rotas

 

Those tire calculators are for a theoretical size since all manufacturers sizes vary some.

Best way to compare is to look up tire your running on Tirerack.com, and under specs see how many revs the tire makes per mile, then, look up the revs for the tire you want to run.

My stock 225/45r18 SPT's were 803 revs per mile, and I put on 225/40r18 Hankook Ventus V12 with 828 revs per mile.

803 plus 3.1% (approx) = 828 revs on new tire.

So, I set my Scan gauge for 3% lower on speed to account for speedo being off. This can be verified by using interstate mile markers for several miles with distance calculator on the Scan gauge.

One more thing, I think we've seen drag radials get larger circumference when spinning, but street tire have firm belts inside to prevent that, so the belt would keep revs per mile very close unless you're spinning on purpose.

 

Just curious, what are the exact tire size measurements on your 18 x 7.5 wheels? I'm getting ready to buy some XXR wheels and they will be 18 x 7.5 and I want to make sure the tires I get will work without any issues.