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Old 03-04-2008, 04:51 PM   #1
metoo
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Default Tires: Wider and bigger, but how bad is that really?

I have 225/45/17s on now (24.97 diameter, 24 pounds for a Fuzion ZRi), but thinking seriously about 18’s for my daily driven, non-competing, GT30R powered STi. Though I don’t compete anymore, I do drive it like an STi. The 18’s I have chosen are pretty much the same weight as the 17s that I have now. So I have two choices of tire (Fuzion ZRi):

225/40/18 = 25.09 diameter, 24 pounds (Fuzion ZRi), same contact patch (psi dependant 3.67 fewer revs/mile
235/40/18 = 25.40 diameter, 25 pounds, (Fuzion ZRi),10mm more patch (psi dependant) 13.6 fewer revs/mile

So the question is which do I choose? On paper, there is virtually no difference in the 225/18 and the 22/17. But, it would appear the 235 will lower my gas mileage due to the engine having to work harder to accelerate it because of its diameter, its extra weight, extra wind resistance because of its width, and extra rolling resistance because of its larger contact patch. Would I even notice these negatives in the real world? If not, the advantages of the 235 would win out: wider look, better handling, speedometer correction, fills the wheel well more.

Without a lot of time and effort to do a thorough test, it’s hard to really know. I think that if you are a conservative driver with few stops in your daily commutes, you won’t notice a difference in acceleration or gas mileage. I think if you give the pedal a push more often than not and have a lot of 90 degree turns and stops in your daily commutes, you just may see the difference. What do you think?
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Last edited by metoo; 03-04-2008 at 06:05 PM.
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Old 03-04-2008, 05:00 PM   #2
fulie
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I am no tire guru but what you said seems to make sense
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Old 03-04-2008, 05:35 PM   #3
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235/40/18 will be better as it will be closer to stock diameter.

i debated going from a 225/45/17 to 235/40/18, but decided 255/35/18 is a better option.
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Old 03-04-2008, 06:02 PM   #4
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Dude, you drive like your middle name is Andretti and your last name is Garlits, so I don't know how you think gas mileage is even going to be a factor

What I do know is that -you- (since I know you) will not like the pizza-cutter look of a 'too narrow' tire. You should stuff the 235's on there, or if you will just roll your rear fender-lip, some 245's. Fat is where it's at.

You're also discounting the benefit of having a slightly taller final-drive ratio with taller tires. Your motor turns fewer RPMs at the same actual mph with a taller tire. I'm betting it'll even out any 'wind resistance' differences, mang!
That's not to say you should stuff some 32" tall tires on there either though!


wind resistance... really!?... no,.... no man,..... sheeet-NO man! (not that you'd notice, anyway)

srsly... most of the tire's 'frontal area' is covered by bodywork.. the bit that's exposed is under the car where the airflow is already dirty... I doubt there's really any issue with 10mm of tire width difference w/respect to wind resistance.

Just my view is all.
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Old 03-04-2008, 06:13 PM   #5
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Quote:
Originally Posted by nhat View Post
235/40/18 will be better as it will be closer to stock diameter.

i debated going from a 225/45/17 to 235/40/18, but decided 255/35/18 is a better option.
Oops, that was a typo, I meant 235/40/18. I've now corrected that in the original post.

255? Boy that would look nice, on my '04 with stock struts and springs, I'm not sure they would fit. Even if they would, I'd probably have to choose a different rim. Then there is the whole straight line acceleration issue that I would be concerned about. The car handles fine as is for my use. I'd love teh look, but fear the potential draw backs of mileage and acceleration....not to mention the cost of replacing them every 15kmiles. So for show and racing, yes. Daily driving, no.

Last edited by metoo; 03-04-2008 at 06:22 PM.
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Old 03-04-2008, 06:22 PM   #6
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Quote:
Originally Posted by jamesohoh7 View Post
Dude, you drive like your middle name is Andretti and your last name is Garlits, so I don't know how you think gas mileage is even going to be a factor

What I do know is that -you- (since I know you) will not like the pizza-cutter look of a 'too narrow' tire. You should stuff the 235's on there, or if you will just roll your rear fender-lip, some 245's. Fat is where it's at.

You're also discounting the benefit of having a slightly taller final-drive ratio with taller tires. Your motor turns fewer RPMs at the same actual mph with a taller tire. I'm betting it'll even out any 'wind resistance' differences, mang!
That's not to say you should stuff some 32" tall tires on there either though!


wind resistance... really!?... no,.... no man,..... sheeet-NO man! (not that you'd notice, anyway)

srsly... most of the tire's 'frontal area' is covered by bodywork.. the bit that's exposed is under the car where the airflow is already dirty... I doubt there's really any issue with 10mm of tire width difference w/respect to wind resistance.

Just my view is all.

For most of the reasons you say I should go wider, I'm saying are the reasons that I should not go wider.

Taller gearing: Yes, like on a bike, traveling at a given speed in a taller gear means less turns of the pedals, but it means more work on your legs. Same with the car.....more work for the engine. Does this equate to lower fuel economy?

I drive with a heavy foot, yes. But that coupled with the extra strain on the engine due to the larger/heavier tires make fore noticeably lower gas mileage?

Dirty air flow under the car. I'll give you that one. I really can't compare it to cycling where the wheels are completely exposed.
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Old 03-04-2008, 06:34 PM   #7
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Quote:
Originally Posted by metoo View Post
For most of the reasons you say I should go wider, I'm saying are the reasons that I should not go wider.

Taller gearing: Yes, like on a bike, traveling at a given speed in a taller gear means less turns of the pedals, but it means more work on your legs. Same with the car.....more work for the engine. Does this equate to lower fuel economy?

I drive with a heavy foot, yes. But that coupled with the extra strain on the engine due to the larger/heavier tires make fore noticeably lower gas mileage?

Dirty air flow under the car. I'll give you that one. I really can't compare it to cycling where the wheels are completely exposed.
Well, I'll say that when I've run the taller tires (ran some 245/45/17 Azenis in '06), I really didn't notice any gas mileage change at all. But!, I too drive fairly briskly when I'm in the STi. My truck, not so much.

I think that, sure, at a point, 'lugging' the motor puts some unwanted strain on it. But, I think with the difference in height you're looking at, it's not a huge deal. You need to find out what the HP requirements are to cruise at a given steady speed. I think it's not terribly more than 15 or 20 hp to cruise at 55 or 60 (just a wild-ass guess)... the rest of the power the motor is making is 'gravy' (until you punch it).

Think about this: a C6 'vette has a ridiculously tall 6th gear that GM put in there so the car would get crazy-good highway mileage. It must work, and it apparently doesn't over-strain things. Apples-oranges a bit, yeah, but I think the concept is the same.

EDIT: for drags or just acceleration, I definitely see how taller, wider + heavier == performance hit. I guess it depends on what your primary goals are as to what you want to 'optimize' towards.
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Old 03-04-2008, 06:36 PM   #8
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You might want to look into tire options first before you become dead set on a wheel. It might become pretty important what brands and models are available for each size. Feel free to look at various widths, 225, 235, 245, and see what's available at around 25" total diameter. Consider wheel width too and what works well, 7.5", 8", 8.5", even 9". Consider tire cost, wheel cost, installation limitations from shop to shop, availability, sidewall protection, and ride comfort.

I have a personal preference towards wider as it offers both more grip and lower force per unit area meaning better longevity. I mean, I have a piddly NA Forester, and I'm running 245s. I do it because I autocross some, but I wouldn't shy away from it if I wasn't because I know the tires will last forever just because there is so little load on them through normal use. You get very little heat buildup and very little wear.

I also have a preference for a decent amount of sidewall height. At the end of the day, you still have to find the ride acceptable, and it has to be capable of soaking up the normal bumps and thumps while keeping the rim safe. A stiff carcass will provide a responsive ride whether it's a 235/40/R17(or 235/45/R17) or a 235/35/R18(or 235/40/R18), yet one will provide half inch more sidewall to soak up those bumps.

Besides, if you were really picking about handling responsiveness, bushings will become an important focus and a big source of the slop and delay found.
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Old 03-04-2008, 07:32 PM   #9
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Quote:
Originally Posted by metoo View Post
255? Boy that would look nice, on my '04 with stock struts and springs, I'm not sure they would fit. Even if they would, I'd probably have to choose a different rim. Then there is the whole straight line acceleration issue that I would be concerned about. The car handles fine as is for my use. I'd love teh look, but fear the potential draw backs of mileage and acceleration....not to mention the cost of replacing them every 15kmiles. So for show and racing, yes. Daily driving, no.
you've got a gt30r powered sti and you're worried about mileage and acceleration? you're quite the conundrum. i'm pretty sure your car has more than enough hp and tq to make the rolling resistance of an extra 30mm of tire on a 255 negligible

tires in 235/40/18 aren't going to break your bank, on average i'd say they're $10-15 more than tires in 235/45/17. of course, you'd have to buy another set of wheels but i'm sure you can justify that purchase as i did

another option is 245/40/18 since you're on stock springs and struts but those are taller by .7" and your acceleration will suffer.

going to a 255/35/18 will increase the price of your tires by about ~50-90% but imagine the grip and how it would look on your car plus it's spot on with the 225/45/17!

if you do buy 18s, go with something in 18x8.5 or even wider
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