verbs

It's time for new tires and I'm torn between tire sizes. I have 3.73's on my car right now in an M6.

I have 17x9.5" wheels, and am looking for either a 265/40/17, 275/40/17, or 285/40/17.

Here is my dilemma. I get in so many races at high speeds (more so than ones from a stop) that I'm now debating which would be my best top speed tire size.

A 265/40/17 is going to be the lightest of the group, (less rotating mass) and will probably get me off the line quicker in a race from a dead dig and will be best from a roll. However, this tire will cause me to have to shift out of 4th gear way too soon in a top speed race.

A 285/40/17 has the opposite effect (heaviest of the group i.e. most rotating mass), will be the worst from a roll or stop, however, it will allow me to stay in 4th gear the longest.

Is the added rotating mass of this tire (285/40/17) worth it in exchange for the extra gearing???

I'm trying to get opinions on which tire will best suit my needs for high speed runs.

NoGo

iTrader: (2)

I'm not a tire expert by any means, but being both tires are of the same radius, 17, and the same series, 40, wouldn't they both give you same gearing: both will have the same shift point in all your gears (with the exception of the 285 at exceptionally high speeds if your getting picky).

The 265 I imagine will give you more kick at the higher speeds being there is less rotating mass, and traction shouldn't be an issue in 4th gear.

<small>[ April 03, 2002, 03:51 PM: Message edited by: NoGo ]</small>

verbs

A 265/40/17 is going to be shorter in height than a 285/40/17.

The way to figure how tall a tire is from the rim to the tread (sidewall height) in millimeters is to multiply the width, or in this case say 285 by the middle number in the tire rating/100 or in this case, a 40 series/100= .4

So multiply 285x.4 and you get 114 mm in height vs a 265x.4 getting you 106mm, so the difference in height of the sidewall in this case is 8mm.

I think I got that right <img border="0" title="" alt="[Confused]" src="images/icons/confused.gif" /> if so then i deserve a <img border="0" alt="[chug]" title="" src="graemlins/gr_chug.gif" />

JimMueller

iTrader: (9)

WTF? I just wrote a long post, then it bailed on me because it says I had a parenthesis in an HTML tag, when I didn't have any URL references. Nice.

I played with CarTest to see what the changes would do. To make a long story short, the 275 width will get you to top speed a few feet shorter than the 285's, and much sooner than the 265's. They all had a top speed of 192.x. These were the assumptions:

- Add one pound of weight for each add'l 10mm of overal tire width (based upon Bridgestone Pole Position site)
- Calculate contact patch / overall width percentage
- Everything else remains equal

$0.02

Dope

iTrader: (2)

The differences are astronimically small. There's only approximately a 1/2" tire diameter difference between a 265 and a 285. That's like 2mph at MOST. The weight is probably within one pound or two too. Cost, looks, and traction from a stop are far more important and noticable things to consider.

Dope

verbs

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Dope:
<strong>The differences are astronimically small. There's only approximately a 1/2" tire diameter difference between a 265 and a 285. That's like 2mph at MOST. The weight is probably within one pound or two too. Cost, looks, and traction from a stop are far more important and noticable things to consider.

Dope</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">I agree on everything you said. I was considering this too....

If you are gonna race someone top end....lets say you shift like this with the different tire sizes (I'm just throwing numbers out just as an example)

Lets say that 2 equal cars are racing from a roll, with one car having 265's and the other having 285's..

If the race starts from 50mph and both cars shift at lets say 6000rpm, you can look down below to see at what mph when the two cars have to shift at. (Just guesstimates)

2nd gear:
265's-->68mph
285's-->70mph

3rd gear:
265's--> 88mph
285's--> 90mph

4th gear:
265's--> 128mph
285's--> 130mph

The car with 285's spends more time in second gear being that it will start lower in the power band in second gear than the other car with 265's. Is that an advantage to get to spend more time in a lower gear or a hinderance being that it starts lower in the power band than the other car. Throughout every gear, the car with the 265's will be higher up in the power band at any given time (with the exception of the few seconds where the car with 285's is just getting ready to shift out of the lower gear, and the car with 265's has already shifted into the higher gear).

Does anyone follow me??? It's not so much the advantage of the 285's staying in 4th gear an extra few mph before you have to shift to the crappy 5th gear vs. the 265's, but also the fact that with the 285's, you have a slight disadvantage being that through most of the race, the 265's will be higher in the power band in every gear....

OK I think I'm rambling....

So are these factors so small that they will cancel each other out anyway and am I worrying about absolutely nothing???