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Old 12-08-2008, 10:15 AM   #1
juanmedina
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Default which car will be faster?

we have 2 cars they are identical, they have the same power mods, same suspension mods, same everything but wheels and tires.

Car A: has some 18x9.5 wheels that weight 22 lbs and some 245/40/18 tires that weight 27 lbs

Car B: has some 17x8 wheels that weight 14 lbs and some 215/40/17 tires that weight 18 lbs.

**tires are the same brand and compound**

Which will win on Auto cross, how about road racing, and drag?

Idk I know one will more surface contact but the other will have less rotational mass.

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Old 12-08-2008, 10:26 AM   #2
soulbazz
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grassroots did a pretty good article on this (using a miata) a few months back ... maybe you can find it on their website. it has test data and I found it pretty useful
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Old 12-08-2008, 10:29 AM   #3
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Assuming the tires perform the same with sidewall/width ratio, slip angle etc...same driver, you really can't call it. Best guess is:

B would win in all counts of forward acceleration so long as traction isn't an issue.

B would loose in braking and cornering grip. Although A does have more work for the brakes to handle.

Last edited by Homemade WRX; 12-08-2008 at 11:37 AM.
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Old 12-08-2008, 10:30 AM   #4
leecea
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There is also a significant gearing change due to tire diameters.

IMHO, there are too many unknowns.
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Old 12-08-2008, 10:35 AM   #5
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well that's why I said tire B would win acceleration, although I guess pending power levels/traps that could be requiring another gear shift...
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Old 12-08-2008, 10:54 AM   #6
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Quote:
Originally Posted by juanmedina View Post
we have 2 cars they are identical, they have the same power mods, same suspension mods, same everything but wheels and tires.

Car A: has some 18x9.5 wheels that weight 22 lbs and some 245/40/18 tires that weight 27 lbs

Car B: has some 17x8 wheels that weight 14 lbs and some 215/40/17 tires that weight 18 lbs.

**tires are the same brand and compound**

Which will win on Auto cross, how about road racing, and drag?

Idk I know one will more surface contact but the other will have less rotational mass.



Code:
Stock Tire - 215/40R17    
Tire 1 - 245/40R18         

Section Width: 8.46 in 215 mm 
Section Width: 9.64 in 245 mm 
 
Rim Diameter: 17 in 431.8 mm 
Rim Diameter: 18 in 457.2 mm 
 
Rim Width Range: 7 - 8.5 in 
Rim Width Range: 8 - 9.5 in 
 
Overall Diameter: 23.77 in 603.75 mm 
Overall Diameter: 25.71 in 653.03 mm 
 
Sidewall Height: 3.38 in 85.852 mm 
Sidewall Height: 3.85 in 97.79 mm 
 
Radius: 11.88 in 301.75 mm 
Radius: 12.85 in 326.39 mm 
 
Circumference: 74.67 in 1896.6 mm 
Circumference: 80.77 in 2051.5 mm 
 
Revs per Mile: 875.0 
Revs per Mile: 809.0 
 
Actual Speed: 60 mph 100 km/h 
Speedometer1: 55.4 mph 92.4 km/h 
 
Speedometer Difference: - Speedometer Difference: 8.158% too slow 
Diameter Difference: - Diameter Difference: 7.55%
Better driver wins, handsdown.
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Old 12-08-2008, 11:31 AM   #7
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All being equal, and same driver, I'd call it too close to tell. Too many other variables to say which will win based on just tires/wheels.

--KC
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Old 12-08-2008, 12:08 PM   #8
juanmedina
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so it seems that car should be about equal. Car A been faster on the acceleration department while Car B is faster on the corners and braking.

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Old 12-08-2008, 12:20 PM   #9
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I've covertly put a "rallydecals.com" sticker on car A......now who wins?

jack
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Old 12-08-2008, 12:25 PM   #10
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Add a jackstand sticker to car A next to the rallydecals.com and its a no brainer.

-Tom
who won on thinner but shorter tires before because of gearing
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Old 12-08-2008, 12:40 PM   #11
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Yea, i think theres alot of other factors as well.. the track being a huge one.. but i'd go for more meat on the tires winning.
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Old 12-08-2008, 01:12 PM   #12
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Quote:
Originally Posted by STi-MAN View Post
Yea, i think theres alot of other factors as well.. the track being a huge one.. but i'd go for more meat on the tires winning.
I wouldn't.

--kC
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Old 12-08-2008, 02:02 PM   #13
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As I said, there are two many unknowns. The OP doesn't even say what car or what mods. Just that they are equal.

Imagine two versions of this test being run, one with stock Miatas and another with modified Z06s.

Will a Z06 accelerate faster on 215s?

Even if you assume it's a Suby, the differences between a stock NA Impreza and a 500whp STi will change the equation. Given enough power, the 215s can't get the power down to take advantage of the gearing and weight reduction.
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Old 12-08-2008, 02:52 PM   #14
juanmedina
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Quote:
Originally Posted by leecea View Post
As I said, there are two many unknowns. The OP doesn't even say what car or what mods. Just that they are equal.

Imagine two versions of this test being run, one with stock Miatas and another with modified Z06s.

Will a Z06 accelerate faster on 215s?

Even if you assume it's a Suby, the differences between a stock NA Impreza and a 500whp STi will change the equation. Given enough power, the 215s can't get the power down to take advantage of the gearing and weight reduction.
I am talking about my car.

Power mods: vf39, sti TMIC, gimmick turbo inlet, APS CAI, Perrin EL and Uppipe, TGV deletes, Phenolic spacer, Buschur Racing catless TBE, 860cc injectors, and a E85 tune that is not finallized .

Suspension mods: megan racing street coilovers, cobb adjustable swaybars and kartboy endlinks.

The car should weight about 2890-2910 with the heavy wheels and tires without me, I weight 150 lbs. Also, it has a stock 07 WRX tranny.

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Old 12-08-2008, 02:58 PM   #15
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Quote:
Originally Posted by juanmedina View Post
I am talking about my car.

Power mods: vf39, sti TMIC, gimmick turbo inlet, APS CAI, Perrin EL and Uppipe, TGV deletes, Phenolic spacer, Buschur Racing catless TBE, 860cc injectors, and a E85 tune that is not finallized .

Suspension mods: megan racing street coilovers, cobb adjustable swaybars and kartboy endlinks.

The car should weight about 2890-2910 with the heavy wheels and tires without me, I weight 150 lbs. Also, it has a stock 07 WRX tranny.

Oh, well, in that case, the larger tires/wheels will only emphasize your asstastic gearing and final drive.
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Old 12-08-2008, 03:48 PM   #16
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also, how good of a driver are you? That would make a big difference between the two. Are you good enough to be able to fully utilize the extra tire? I'm really only asking in terms of road racing.

However, based on what you've given me, and assuming you are an average, but not great, driver. I would guess the thinner tires would win in autocross and drag, while the wide tires would win at road racing.

Hopefully, Im right. Because that was pretty much the basis I used to make my decision on wheels/tires for my soon-to-be-road racing RS.
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Old 12-08-2008, 08:05 PM   #17
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Small differences in alignment, shock valving, and tire age could make a difference. The devil is the details.

With that said, it should be close, but one car will be better than the other depending on venue. One has less rotational mass and shorter gearing while the other has wider rubber - Course dependent.
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Old 12-08-2008, 08:27 PM   #18
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Quote:
Originally Posted by Jaberwocky View Post
Small differences in alignment, shock valving, and tire age could make a difference. The devil is the details.

With that said, it should be close, but one car will be better than the other depending on venue. One has less rotational mass and shorter gearing while the other has wider rubber - Course dependent.
Yeah,

also short gearing doesnt nec.. equal faster track time.. depending on different sections of a track, the car with the taller gearing could be faster then that with the short since no time will be lost for a shift.
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Old 12-08-2008, 10:06 PM   #19
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For that year WRX on most SCCA courses I'd want a short tire, but 215 is too skinny. Why are you limiting yourself to those two choices?
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Old 12-08-2008, 10:16 PM   #20
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17 pounds per wheel of unsprung weight is a lot..
I'd say the longer one is better for autocross

I lot depends on how much power the car has.
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Old 12-08-2008, 10:45 PM   #21
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Quote:
Originally Posted by juanmedina View Post
Car A: has some 18x9.5 wheels that weight 22 lbs and some 245/40/18 tires that weight 27 lbs

Car B: has some 17x8 wheels that weight 14 lbs and some 215/40/17 tires that weight 18 lbs.
Car C: Has 17x8 wheels that weigh 14 lbs and 245/40R17 tires that weigh 20lbs?

Car C wins hands down.
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Old 12-08-2008, 10:47 PM   #22
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I would say for road racing, car A would be faster despite being heavier. Thats why in, say time attack, I would want the widest tire that would suit my needs. 215 or 245, it seems like a no brainer.

For drag, I would say if the car doesnt spin the tires, car B would be faster, just because of the lighter weight and less rotating mass.

Autocross???
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