Downsizing wheels/tires
 

One of Davard's posts in a thread in Wheel/Tire section got me wondering...

Lots of people throughout the season kept saying to go down a size (at least one) in rims and go with smaller diameter tires which serves two purposes:
- decrease weight
- change gearing

Is there a limit where you can go too small for tires? Davard mentioned 205/40/17 being too small for a MP3/Mazdaspeed weight car.. I know you can fit 14" wheels on a 3rd gen but is that just insane to put tires as small as that on it for competition? 225/50/15 would be nice, and it's not impossible to find some reasonably light 15" wheels.

Would it be better to just go wider and not worry about sizing down? Would a tire size that would give a noticeable difference in gearing be too small as to not support the car properly?

I'm trying to plan for next season as to wheels, tires .. and whether or not the five or six of us in Toronto who will be competing should all try to get the same size tires or not.
(There are a couple 1st gens, a couple 99 SE's, a couple P5's .. and me with my MP3).

IMHO this depends if you're talking about a tire for pure race use or a tire that will see street use.

The main problem with using a lower-profile tire on your existing 17" wheels is the risk of damage on the street. A 205/40 tire has a VERY short sidewall, IMHO too short for a 2800 lb. car. One bad pothole and you bend a wheel. :( I'm actually convinced that this is one of the main reasons that Mazda used a 45-series tire on the MP3. (The other main reason is styling- it keeps the wheelwells full of tire and looks better.) :)

However, a 205/45R16 or 215/40R16 tire on a 16" wheel WOULD give you a gearing advantage and lower the car by 1/2", improving handling. A 225/50R15 would probably work too, but the taller sidewalls will hurt handling (this depends a lot on the type of tire tho) and the slightly larger diameter will not help either.

If you are talking autocross tires as opposed to street tires, the trend right now is small diameter and W-I-D-E. :) A 225/45R13 tire would work if 13" wheels will fit (I don't know if they will). 14" wheels and 225/50R14 would work too, although you may have trouble finding 14" wheels that are wider than 6" (it's much easier to find wide wheels in 13" and 15" diamaters). 225/45R15 tires would work too.

I'm talkin' pure race tire. And you're right ... 14x7 is super hard to find.. might have to look into some kinda steel wheels (like those Diamond ones in another thread) 13's will not fit on an MP3.

14 x 7 wheels. Panasport, Borbet Type-T, Circle, CCW or Revolution are your choices. Don't know if they will fit over your brakes, though. You might be better off with 15's, but not much is available wider than 7" in that size, either.

225 Hoosiers are pretty wide, no matter what the rim size. But that will also take you out of stock class here in the US (Don't know about Canada).

Current Ontario rules are:
Any size wheel & tire as long as it sits inside the fender without modifications. And, any DOT approved tire is OK. There is currently a 2% bonus for people running street tires instead of r-compounds, but that might disappear next season.

So, offset is my limiting factor in finding a suitable wheel.
Now I have to go and find out what backspace means because all the USA manufacturers seem to really like using that instead of offset.

15x6"
 

I have a 2000 ES that I race with 225-45-15's (Kumho and Hoosier make them available at www.tirerack.com) on 15x6 Miata Wheels. They are light 12lbs and fit perfectly. Now with a MP3 you would need a 5 lug weel- maybe a MX6 wheel would fit, or any other aftermarket wheel in the 15x6 to 15x7 range.

Just a little precaution- the 225 on a 6" wheel is tight, so make sure the shop installing them has the best equipment- I usually go to Sears.

I would say- go with the Kumho's and maybe some light 15"x7"s wheels- Borbet wheels are pretty light- so are stock Mazda Wheels.