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Old 05-07-2007, 01:39 PM   #11
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Suprapsu,

I'm currently doing this with my '89 CRX HF. I'll let you know how it turns out. I'm swapping out my stock/OEM Honda 13" x 4.5" steel wheels for a set of the superlight Mazda Miata BBS 14" x 6" rims. I've already had the center bore on the Miata rims machined to fit the Honda hub. Now it's just down to ordering the tires. ...and I've just posted that question here on another thread. I'll keep you posted as to the changes.

***However, I'll mention in the meantime that I'm currently experimenting evein before installing the Miata rims. I've got a random set of '93 Mitsubishi Mirage steel rims (13") with 185/80-13's on the CRX HF right now. I've been driving with them on the car for about 2,000 miles now. They're heavy. And the 80 aspect ratio is ugly. I think the sidewalls on this no-name tire flex a lot. They grip great, but aside from that the car is a pig as far as handling now. My speedometer, with these large tires, is off by approx. 5 miles per hour...which I LOVE! Finally I can go the same speed as other cars without my RPM's shooting through the roof. I have it all figured. My mileage multiplier is 1.0741 (so if my odometer reads 300 miles, I've actually gone 322.23 When I use this to figure my mileage, there is a definite increase in Highway MPG. I'm just not sure about city MPG yet.
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Old 05-07-2007, 01:42 PM   #12
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Originally Posted by Bill in Houston View Post
AND, I think that for a given tire diameter, a smaller wheel diameter gives better mileage. But I can't prove it.

For example, if the max that your car will allow is a certain 215/80R15, then that will have lower rr than the same diameter 215/??R18.

I think that it has to do with the sidewall stiffness...
What you're referring to is the tendency for manufacturers to make LRR 13/14/15" tires. Most 16"+tires usually aren't marketed at the economy crowd, so there's not much LRR up there, as well as having a smaller sidewall to tread ratio like you mentioned. In any event, assuming rolling resistance is the same between tires, going w/ a bigger tire/wheel combo should help out efficiency a bit.

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Originally Posted by MorningGaser View Post
Bigger tires mean less torque at the wheels, which means more engine effort to turn those tires, which can translate into less MPG.
It's usually the opposite, all things being equal. the more the engine works at a given speed, the greater it's efficiency, up to a point.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 05-07-2007, 08:24 PM   #13
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Ive been researching this myself recently and have come to the conclusion that the smaller the WHEEL(as in the metal part) the better (assuming that the outside diameter of the tire stays the same,) so if FE is your goal, stay with the 13" wheels. This is because that given a fixed outside diameter, increasing the diameter of the wheel adds proportionately more weight to the wheel/tire combo than add more sidewall to the tire. Keeping the weight down, especially unsprung weight, and rotational weight, of which the wheel and tire are both, has a much bigger impact than the same weight in the trunk.

If you do want to try lowering engine RPM via larger outside diameter, than just buy larger tires for you existing rims, but this raises some of the same questions I asked about in another thread. If I new how to link the thread I would, but I don't so just find the one in this same forum category called "Tire and Rim/Wheel Changes - Overall Effects on FE"
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Old 05-07-2007, 10:23 PM   #14
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if you are going for better gas mileage, and I assume that you are, I would stick with 13" rims, because it's really hard to find narrow tires for larger rims, finding 4.5" wide rims is almost impossible, they only came on a few of the pre-1988 civics as light weight steel rims on things like the HF, and standard on the pre-1984 civics, weighing around 12-13 pounds each, or as civic vx alloy rims weighing 11pounds each, a 13" rim can take a 155mm wide tire, where the narrowest you can get in the USA on a 14" rim is 165mm, but with limited options, 15" gives you even less selection.
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Old 05-07-2007, 10:32 PM   #15
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I think I smell an experiment brewing.

I believe that changing to 13" to 15" wheels would show a increase in mpg (just remember to set it so the odometer wont be off).
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Old 05-07-2007, 10:43 PM   #16
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it really depends on your tire profile/sidewall hight, the larger wheels tend to be wider, and thus weigh more, low profile tires don't tend to weigh less, so you are increasing your rotating mass, and eliminating your option of usieng a narrow tire that has been proven to roll easier, and weigh less.
want an experiement? find simaler weight tires that are simply taller.
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Old 05-08-2007, 08:31 AM   #17
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Did no one read my post? I mentioned some of these things that are still being discussed.

With each engine having its own speed/RPM range for peak efficiency, if I start to get lousy gas mileage at a lower RPM on the highway (with my actual speed being the same) then I can assume that I've now got to drive faster to get my car back into that peak RPM/efficiency range which used to be lower due the smaller diameter wheel/tire combination that I had on the car with my stock HF.
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Old 05-08-2007, 09:44 AM   #18
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Oh yes, now that you mentioned it, imle cars built during the 80s, and even some economy cars in the 90s, have nearly optimal gearing for FE assuming stock configuration. Otoh, cars a lot of built in the 90s are not in the BSFC peak for their engine/car/transmission. YMMV... It's best to find a BSFC map in order to figure out what will be best.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 05-08-2007, 02:53 PM   #19
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hmm well my car is 27 years old (1980 chevette) and going at highway speeds (55mph) its at 2800 rpm@55mph. at 60 its 3K and ive driven on the interstate once for about a mile with it to see what the rpms would be. lets just say its not pretty. this car was obviously geared towards mostly city. i need some new tires desperately but im a tad concerend about having them too big. the current tires are 165-80R13 i was thinking maybe 195-80 R13 as i do mostly highways speeds (45+) but some city where it can be conjested but usually can cruse easily at 35-40 mph.

am i correct that those tires i plan to get will have the same width just a taller sidewall?
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Old 05-08-2007, 02:58 PM   #20
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It's best to find a BSFC map in order to figure out what will be best.
Where would I look for one of those? Just searching in Yahoo didn't really give me good results. I am most interested in my k24 engine in my Element... Thanks!
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