I am keeping a note book of information and will time to time have some dumb questions that I know ya'll can answer. This one is the tires. I am thinking about putting 18's on the rear. I think that might make the lines just a little more appealing. Two questions?

1. When you edit the PCM I assume the tire size is for the rear, correct?

2. What are your opinions on 18 rear and 17 front?

Thanks in advance

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Well, it works just fine for the C5 Corvettes ;)

You will easily be able to fix your speedo with an Edit, but I would bet you a beer you screw up the ABS unless all the tires on the car have the same loaded radius (rollout).

When you move to a bigger wheel, just use the +1, +2.... sizing rule. Put a lower profile tire on the the wheels to maintain the same overall diameter.

The plus one, plus two method doesn't work when you're trying to fill the wheel wells though. I had some early concerns regarding the pictures of the rear wheel openings being larger than the front and skewing the appearance. Since seeing the M/T video footage I see it's much less noticable. I am however upgrading to 18's to help with the handling and yes I will use the +1 method so as not to confuse the computer.

I can't settle for .80 G's!!! If you read the rest of M/T December issue you'll find the testing for SUV of the year. The Porsche Cayenne, Infiniti FX45 and the BMW X3 all tested over .80 G's. I can't have a TRUCK tailing me through the twisties now can I?!

Originally posted by Commissar@Nov 2 2003, 02:05 PM
When you move to a bigger wheel, just use the +1, +2.... sizing rule. Put a lower profile tire on the the wheels to maintain the same overall diameter.
and the that rule would be??!!??
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When you move up in rim diameter you move down in aspect ratio. So moving from 17 to 18's you want to move down from 45's to 40's. Tire rack offers a great tool for this. Also you can use the overall dimension of the wheel. I believe that the 245/45/17's on the GTO equate to 25.7" diameter. That would equate to 245/40/18's and 245/35/19's in "up sizes". Also if there is enough width room you could run 275/35/18's or 275/30/19's. That would be over 1" wider than stock.

Hope that helps. :D

Originally posted by cvp33@Nov 2 2003, 03:58 PM
Hope that helps.* :D
Yes thank you it helps. I should have said that I will of course wait until I actually see the car before deciding. I believe I understand. Checking out the Z06 tires the fronts are listed as the same P245/45ZR17. Options are P275/40ZR18 or P295/35ZR18. The +1, +2 rule. The circumferences are 80.679 inches, 83.761 inches, and 82.090 respectively.

Further questions:

1. If you upgrade to all 18 inch wheels the the computer may be off as much as 3% without editing, right?

2. Would the ABS be screwed up because the load would be the same but the radius is different so the Torque (radius X Load) would differ by 3% in #1?

3. If you go with wider tires then to keep the same load would you have to lower the tire pressure, raise it, or keep it the same? It seems that if the footprint of the tire is greater then to equal the weight on the tire you should lower the pressure but then that would increase the footprint even more on ad infinitum.

4. For a quicker takeoff would it be better to have skinny tires or fat tires? The more footprint the more friction so it seems to me there is an optimum tire size that balances friction which must be overcome with power versus enough friction to keep from breaking loose.

5. Can you translate what the .80 g's means in real life. What does it feel like if going 60 MPH. Do you have to lean your body? Do you slide into the door? Is that the g's where the tires start to slid?

6. Since I am planning on doing some running at the local track to air it out once in a while is it better to have lower profile of higher profile tires? What about tire pressure?

Thanks again in advance.

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From the sounds of your car knowledge, I would suggest taking driving classes before taking this car out to the track.

You do not want to end up like this guy:

http://www.bimmerforums.com/forum/showthre...threadid=119890 (http://www.bimmerforums.com/forum/showthread.php?s=&threadid=119890)

Another guy who bought a fast car and didn't know how to handle it, it was also suggested to him to take driving classes (which he opted not to do)

Originally posted by Commissar@Nov 2 2003, 09:23 PM
From the sounds of your car knowledge, I would suggest taking driving classes before taking this car out to the track.

You do not want to end up like this guy:

http://www.bimmerforums.com/forum/showthre...threadid=119890 (http://www.bimmerforums.com/forum/showthread.php?s=&threadid=119890)

Another guy who bought a fast car and didn't know how to handle it, it was also suggested to him to take driving classes (which he opted not to do)
Actually they won't let me on the track until I take a high performance class. It is a pretty good deal. The class is $400 and that includes 2 hours of track work. They even supply a car for an additional $200 if you don't have your own. Once you take the class they charge $150 for a 1/2 day of running (up to 10 people). That include 5 experts watching you do the workout and critiquing what you do. All in all pretty good value.

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That class sounds pretty good Tails.....it actually sounds like a good Christmas present idea for your wife. :P

I don't know if I'd want to be that tough on my new GTO right away. However, I would love to put a few hard miles on my Stang before trading it in or selling it. :D

Yes it is definitely on my list to my wife. Actually the only thing on the list. My kids got the email from the Pontiac Mall forwarded to them. I am not going to be hard on it and certainly won't be taking it on the track until I get a couple k on the odometer. I don't see the top speed going much over 100 mph at the track until I get some experience. I have the first few hundred break-in miles already mapped out in the area going amongst family and friends who will get to see it whether they want to or not. My wife also has the number of several therapists on hand....

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Tails,

There are far more qualified posters than I to answer this list of questions but I'll give it a go.

#1 - If you use the plus one method for sizing up your diameter will be the same. Therefore there will be NO difference to the computer.

#2 - See answer #1

#3 - You should always follow the manufacturer's recommendations for tire pressure. At the track (drag racing) you can play with your tire pressure to help your 60 ft. times however other than purpose built drag radials I doubt you'll see any gains on a "street" radial.

#4 - Friction at speed is indeed the enemy, however on take-off it's your friend. The quick answer is fat in back for traction, skinny up front for reduced rolling resistance. At the track, if you do decide to run a fat drag radials remember you are putting more stress back through the drive train. If your car is not purpose built you could be spitting parts in short order. Nothing worse than seeing a brand new car spit a drive shaft or grenade a rear end.

#5 - G's stands for gravity. If a car can pull 1 G on the skid pad for instance. Than a 200 lb. driver in a hard right turn would feel 200 lbs. of pressure pushing from his right side. Indy and Cart cars routinely reach over 5 G's because of down force created by the car itself at speed. In the real world our cars have to count on 'sticky' tires and suspension geometry. It's the point at which a car can no longer hold the road. .80 g's is not stellar, but will cover most drivers.

#6 - Profile of tires is dependent on what "track" means. As said before in a drag racing situation you want more sidewall for "spooling" a tire and transfer of torque to the ground. In a road course setting you want consistent contact patch and NO flexing. That calls for a low sidewall.

Again I'm no expert, but I'm fairly confident of the information above.

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