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Originally Posted by Pirate_d00d
Ok I am rapidly gaining interest as well for autocrossing my 6. What size wheel and tire is too large? When does size cause a loss in handling/cornering performance?
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It depends on the class that you plan to compete in as the answers are a bit different.
For a stock class you are only allowed to run wheels(aftermarket or not) that are the same dimensions as the stock item. Offset can be +-1/4"(@5-6mm) from stock. This is to allow aftermarket wheels that sometimes do not have the *exact* stock offset(sometimes the stock offset is just goofy).
With stock classes you have to use stock springs. The car will have a lot of motion because of this. Having the stock size tire will have a lot of weight transferred onto them and have them quickly overheat. Autocross is also about as much grip as one can afford. Because of these 2 factors, you will see aurocrossers stuff as much tire as can fit onto the stock dimension wheels. For the MX6 the stock wheel dimension is 15x6.5". The tire of choice for the car is a 225/45/15 R-compound tire. Its not unusual to see even more extreme tire combinations such as this (225/50/14 on 5" wide miata or civic wheels, 285/30/18 on RX8/nissan 350z 8" wide wheels). Yes, those fitments will have extreme camber to the sidewalls which will lead to sloppy handling. R-compound tires have very stiff sidewalls however and the "squirm" associated with these fitments can be minimized a bit.
Speaking of stiff sidewalls..your tire changing guys will not be none too pleased about doing this(some may even refuse). You might want to have a $20 bill to smooth things over after they do the work or they may come after you with tire irons.
In general the more tire the better, especially when it comes to heavier cars. There is a diminishing point of returns however where the tire is just too wide(or cant fit on the bead of the wheel). For example.while the RX8 and 350z can fit large rubber on the 8" wheels, the winning combo for the RX8 is actually a 245/40/18 tire. There have been tests with the 285/30 as well but it proves no faster(or a bit slower) and the extreme camber of the sidewalls dull the handling of the car. On the 350z however the added weight of the car makes the 285 a marginally better choice despite the steering feedback handicap.
In other classes that allow alternate wheel widths then the standard is to get the maximum wheel width that that class allows and then find the tire that will be mazes out on that wheel. For this you want the maximum tread width, the shortest aspect ratio and the most "square" sidewall shoulder to help with steering response.
Tire diameter is also another variable in both preparation classes. Its very common to use that to adjust the gearing of the car. Smaller diameter than stock will gibe a boost to available torque and lower the cars center of gravity without penalty to the suspension geometry. It will lower the cars terminal speed at a given RPM, but if the car as long leg (the KL is a revver), you can use that to gain back the loss of mph. If the cars gearing is such that you find yourself "between" gears you may have to do more shifting.
For my 1st gen GS car I used 225/50/15 and 225/45/15 r-compounds on 15x6" wheels. The shorter 225/45/15s (2" shorter than the stock 205/60/15 tires) were the faster tire even though I had to shift to 3rd to gain the speed back that I needed (2nd would top out at 53mph and I needed to target 60-62mph).
hope this helps
Gavin