93redFD

Im having trouble understanding why 18" wheels are not good for racing. Is it that the over-all diameter is more or is it weight? I measured a 18" wheel, 235/40/18 compared to a 225/50/16 and I come up with about a 1/2 difference(from top to bottom of wheel, in inches). Does it just change the final gear or what.
Im really debating on 18" or 17", Before I had very bad traction problem with 18X9.5 in the rear(mainly becuase of the spinmo xsta supra's) Im thinking of using a 17X9.5 and putting a 275 drag radial for the rear.

SleepR1

Depends on what type of racing you're doing. Straight-line drags really call for stock 16s or Supra MkIV 16 x 9s (with wheel spacers, and bored out center bore). The 9 wide x 16s mate really well with fat Micky Thompson ETs. For high speed road course lapping, 17s or 18s work well. Currently 10 x 18 with 285/30-18s are the hot setup--IF-- you have 2.5-inch coil springs and ride height adjustable shocks. This is the widest contact patch available for maximum cornering grip and handling. The next best is 9 or 9.5 x 17s with 255/40-17s. These will fit on most stock setups given the proper offset. With 10 x 18s you pay in extra unspring weight for more grip (a fair trade-off if you have 360 rear-wheel hp). For stock or mostly stock cars, 9 x 17 with 255/40-17 costs a few more lbs of unsprung weight in exchange for MUCH more grip than the stock setup. 17s/18s aren't good for drag racing because the the tire profile for most of these sizes (except 275/40-17) are very low, and act as stiffer springs (yes the tire sidewalls act as springs also). Stiffer is not necessarily what you want when trying to hook up for a good holeshot

93redFD

sleepR1, thanks man.. You answered alot of my questions, but whats the disadvanteges of accelaration with 18. Also i dont want to jack the car up and higher than it already is. If a over all diameter is a 1/2 inch more, it would lift the car a 1/2 off the ground right.
talk me out of getting 18's

OC_

18" are now almost like a standered in Grand touring type racing. Just look at all the 18" race slicks, it seems only avon makes a good slection of 17" race slicks now.
tall wheels have lower profile tires and low profile tire sidewalls roll over less durring hard cornering.

high profile tires a good for drag racing because you can let some of the air out so the tire gets a wider contact patch on the ground.

this is fantastic for trying to find a new tire size and to make sure the new tire is as tall as the OE size.
http://www.miata.net/garage/tirecalc.html

SleepR1

The disadvantage of 18s is the rotational inertia...although, theoretically, overall tire diameter should be close to stock (25 inches)--IF--you plus sized CORRECTLY. If you didn't, and increase your overall tire diameter by 1/2-inch then you do increase rotational moment, and thus increase the drag on the motor. Rotational inertia increases as diameter increases. Combine that with the weight of an 18-inch tire at the periphery of the wheel, and this increases the drag even more. Increased rotational inertia, and stiff sidewalls are why many drag racers stay with 16s, and choose a size like 265/45-16 on 9 x 16s. Some slicks come in 25 x 10 x 16. This would be ideal for a 9 x 16 wheel, since the overall tire diameter is 25, and the overall width is 10, so you'd be perfect with that setup for drag (and for road race too )