Optimal Wheel Size for Stock Power

[Tanuki]

Originally Posted by Kairos

The problem is not that they are 18's, it's that you bought 28+ lb. wheels. Get a 18x9 that weight 18 lbs and I guarantee you'll see a huge difference. I believe the stockers are 20-22 lbs so you added alot of extra weight to rotate.

I didn't buy the works for this car in particular, and I think they are closer to a 22-24lb wheel, probably not a 28+, but I get your point. I think the front tires contact patch is a balancing act as well.

[Mitch]

Originally Posted by D1cker

a light 17x9 with a 245/40/17 will be ideal for a car that sees the track and autocross as well as the street, if it's more of a street car, go with 235/40/17.

18's look better but the 17 will be better for performance.

I think the size is just a matter of opinion. I think 18s make the brakes look ridiculously small. I'm running 17x8 RPF1s with 255/40 Star Specs. I'll probably go to 245/40 next time just to see if I can feel the difference.

[D1cker]

Originally Posted by Mitch

I think the size is just a matter of opinion. I think 18s make the brakes look ridiculously small. I'm running 17x8 RPF1s with 255/40 Star Specs. I'll probably go to 245/40 next time just to see if I can feel the difference.

The 255/40 works as well, but with an 8" wide wheel you can't really utilize it. When I was doing time trials in miatas, a 225 on a 9" wheel would be faster than a 225 on an 8" wheel, The 235 is abour as big as I'd want to run on an 8 before running into diminishing returns (excluding autocross, which I'm not really into)

[Mitch]

Originally Posted by D1cker

The 255/40 works as well, but with an 8" wide wheel you can't really utilize it. When I was doing time trials in miatas, a 225 on a 9" wheel would be faster than a 225 on an 8" wheel, The 235 is abour as big as I'd want to run on an 8 before running into diminishing returns (excluding autocross, which I'm not really into)

My setup is for autocross, so there ya go. 245 may be the sweetspot for me.

[white_knight]

P

[Spartan65]

I will be looking for a 18x7.5 with a 42-45 mm offset

OEM
17x7 5-100 48mm offset with 215/45-17 tires
Sidewall 3.8 in
Radius 12.3 n
Diameter 24.6 in
Circumference 77.3 in
RPM 819.0

Option 1
18x7.5 42-45mm offset wheel with 235/35-18 tires
Sidewall 3.2 in
Radius 12.2 n
Diameter 24.5 in
Circumference 76.9 in
RPM 824.0

Option 2 = Expensive as all Hell
19x7.5 5-100 42mm offset 235/30-19 tires
Sidewall 2.8 in
Radius 12.3 in
Diameter 24.6 in
Circumference 77.1 in
RPM 821.0

I learned over the years that if you keep the RPM close and the weight down you only lose by the drag created from the width, any deviation from this has direct effect upon gearing and power bleed.

[Tanuki]

How will it affect the trip/odom/mpg comp reading?

[D1cker]

Originally Posted by Spartan65

I will be looking for a 18x7.5 with a 42-45 mm offset

OEM
17x7 5-100 48mm offset with 215/45-17 tires
Sidewall 3.8 in
Radius 12.3 n
Diameter 24.6 in
Circumference 77.3 in
RPM 819.0

Option 1
18x7.5 42-45mm offset wheel with 235/35-18 tires
Sidewall 3.2 in
Radius 12.2 n
Diameter 24.5 in
Circumference 76.9 in
RPM 824.0

Option 2 = Expensive as all Hell
19x7.5 5-100 42mm offset 235/30-19 tires
Sidewall 2.8 in
Radius 12.3 in
Diameter 24.6 in
Circumference 77.1 in
RPM 821.0

I learned over the years that if you keep the RPM close and the weight down you only lose by the drag created from the width, any deviation from this has direct effect upon gearing and power bleed.

The problem with this is that almost no one makes a 235/35/18. 245/35/18 will be a lot more readily available or 225/40/18

[D1cker]

Originally Posted by Tanuki

How will it affect the trip/odom/mpg comp reading?

It depends on the diameter of the tire. If you go with a 235/40/17 the speedo will read just a smidge high, where if you go with a 245/35/18 it will read a bit low.

Google a tire calculator and you can input different tire sizes to get an idea.

[JimmyMac]

Better yet, use this calculator. Just type in stock spec on the left, and what you want to try on the right. Remember stock is:

215/45-17x7+48

Here is the link:
http://rimsntires.com/specs.jsp

[Spartan65]

Originally Posted by D1cker

The problem with this is that almost no one makes a 235/35/18. 245/35/18 will be a lot more readily available or 225/40/18

Falken makes the Ziex, probably the least expensive.
FALKEN ZIEX 912 ALL SEASON 235/35R18 140-165$ per
and the
BFGoodrich g-Force T/A KDW-2 approx 180-195 per

[Spartan65]

Originally Posted by Tanuki

How will it affect the trip/odom/mpg comp reading?

18x7.5 ET 45
When speedometer reads 60mph (96.6km/h) actual speed will be 61.8mph (99.5km/h): 3% faster.

19x7.5 ET 45
When speedometer reads 60mph (96.6km/h) actual speed will be 59.9mph (96.4km/h): 0.2% slower.

[Tanuki]

Thanks for the info on those calculators.

[Spartan65]

Originally Posted by D1cker

It depends on the diameter of the tire. If you go with a 235/40/17 the speedo will read just a smidge high, where if you go with a 245/35/18 it will read a bit low.

Google a tire calculator and you can input different tire sizes to get an idea.

Seriously a smidge?, you sound like the guys that tried to sell tires with me when I was at DTC. I retired (sorry about the pun) from DTC, three words " I know Tires"

http://www.miata.net/garage/tirecalc.html