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Old 05-08-2007, 09:25 PM   #11
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AAAAaaaand, your odometer would be off as well. It could show something like 250 miles on a fillup, when you really drove 270. Easy to compensate for, but something to keep in mind.
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Old 06-29-2007, 09:18 AM   #12
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Originally Posted by Phoenix View Post
Larger Tires raise the vehicle - What effect on FE does the potential increase in aerodynamic drag have?
Larger Tires have more tire to road contact- More Friction?
Larger Tires weigh more- Doesnt spinning weight like that have a significantly bigger impact than stationary weight?
Tire to Rim/Wheel Proportion- If spinning wieght is something to worry about, whats lighter, a huge (17") alloy rim with short sidewalls or a small rim (14") with tall sidewalls?
??? ??? Thanks for the input!
From what I THINK (Im no expert), but I would reason that larger tires will raise the car and prote more drag under the car. Most people, as I gather, are trying to keep the car nice and low. You are opening up a can of worms when going to a larger tire. Plus you are putting more strain in your engine, gear ratios and the like. My personal, unprofessional and possibly wrong opinion, is to stick with your stock size that you car was built for

WIDER (I assume you meant wider, not larger) tires have more contact and from my common knowledge, skinny tires are more FE and also have less drag. Dont go too skinny and lose control and safety

LARGE sidewall will always be lighter. Low profile tires with tiny sidewalls is all rim which is way heavier than tires. Unless you using some super sweet expensive allow rim then I would assume that steel rim are much heavier than tire material. I would again, stick with stock size, or MAYBE go a little skinnier.

Get good quality tires with decent LRR numbers and moderately increase psi unless you are kamikaze daredevi and willing to try anything. Remember that tires built just for LRR wear very fast. Lots of really angry Pruis owners changing tires after 20K miles
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Old 06-30-2007, 10:26 AM   #13
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Decisions, decisions

Pheonix, I've just been through this tire-size decision process, and I can confirm that you can be drowned by the options. Tire width and height, rim size and mass, rolling resistance, traction, speed, load and temperature ratings; there certainly are a lot of variables.

Trebuchet and the racers are correct that less mass is easier to accelerate. But I ruled out light-weight alloy wheels, as it would probably take forever to recover their cost in fuel savings. So what is left for FE? Friction, aka rolling resistance. And as you and StanleyD noted, skinnier tires are usually more FE. So I looked for skinnier tires with a low rolling resistance.

DracoFelis and StanleyD's comments about changing the tire diameter are significant, because this also changes your effective final gear ratio. I don't have overdrive, and I considered larger diameter tires. But I decided to keep my tire diameter the same and only change the tire width. ( I am double thinking this decision, as changing tire size is much cheaper than a gear or transmission swap to achieve higher gearing... but too late)

My Cavalier came with 195/65R15 tires (the "big 15's") though most Cavaliers came with 14" wheels. I could not find 185/70 15" tires, but I did find 185/75R14's. They are not common, but available online. This size maintains the same diameter as the stock tires within 2 rev's per mile. I found 175/80r14 tires online in Europe, but not in the USA. This is interesting to me since they are more into mileage than the U.S.A. See the pictures below for a comparison of my old and new tires. Remember, the new tire is not inflated, so were not comparing apples to apples here. But I wanted to show them anyway - they look like bicycle tires compared to my old tires. I hope, when mounted, they maintain that rounded cross section at the tread, instead of being flat like the old ones. That would reduce road contact even more. I'll post another shot once I get them mounted.



These are the General Ameri G4S. I only found two sources of RR data online. The data from 2002 lists the Continental Ameri G4S as having the second lowest RRC at 0.0078. Since Continental makes the General tires, I'm hoping it is the same tire with just a different branding slapped on. I've written Continental asking if this is the case. My tires have a treadwear rating of 520 and a 60,000 mile tread warranty. So they should last a little longer that the Prius OEM tires, which have a treadwear rating of 160.

When I tell my friends that I got some tall, skinny, LRR tires, many respond with "won't that affect your traction". Well, here's where I get to rant. <standing on soap box> The current fad of wide, low-profile tires is silly. That style of tire is only good for traction on dry pavement. ie, racing or trying to pull 1g in the turns. Narrow tires concentrate the weight of your car on a smaller footprint, giving better traction in rain and snow. And THAT's when I'm concerned about traction. < off the soap box now> The 195's on my Cavalier hydroplane easily and were terrible in the snow. I expect much better performance from the new, narrower tires. I will report my findings as soon as I get some 14" wheels.
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Old 06-30-2007, 01:07 PM   #14
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Keep the original size tires and wheels, but get aluminums. It'll stay correct and have less rotational mass.

I saw a set of wheels and tires on ebay the other day, we're 8 pounds each(that's wheel AND tire).
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Old 07-04-2007, 01:57 PM   #15
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Update: Continental replied that, yes, the General G4S's are the same tire as the 2002 Continental G4S. Woohoo! I've got some of the lowest Low Rolling Resistance tires out there. Go Team OPEC Busters!
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Old 07-04-2007, 02:43 PM   #16
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i noticed absolutely no difference in MPG switching from a WIDER tire from my stock ones. exact same rim side both steel rim one set is chrome plated tho. same tire sizes except one set is slightly wider by about an inch or 2
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Old 07-04-2007, 05:14 PM   #17
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I just switched from the stock 14 steelies with racing disc hubcaps (threw 1 due to the cr-p road I live on) to 17's on my Corolla. Gas mileage has taken a SERIOUS nosedive. I'm flat out embarrassed. But dang, the car looks sooo good - I still think I will have to be admitted if I continue the horrible mileage. Good thing is the rims will fit whatever car I have, I may swap them to whatever is the least driven car. Actually, the Corolla may be driven less very soon. I'm almost done building my other car.
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