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Old 06-05-2008, 07:20 AM   #1 (permalink)
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Lowering your car by getting shorter tires?

Something tells me I'm asking a very stupid question but here:

http://www.tirerack.com/tires/sizes/...sp?diameter=15

Also check this out to find out what i'm talking about:

http://ecomodder.com/forum/showthrea...ling-2813.html

you can buy tires that are taller or shorter (distance between wheel and tire tread)....

which would be most fuel efficient, a very tall tire or a very short one? assuming same tire width, rolling resistence, etc.

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Old 06-05-2008, 08:35 AM   #2 (permalink)
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That's fine for a non-drive tire, but a smaller drive tire will have you doing more RPMs on the highway which will surely cancel out any aero benefit.
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Old 06-05-2008, 10:12 AM   #3 (permalink)
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so what if i get really really tall tires then?
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Old 06-05-2008, 10:13 AM   #4 (permalink)
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Then your highway revs will go down.
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Old 06-05-2008, 10:58 AM   #5 (permalink)
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However it does slightly depend on your cars engine some are very in efficient at lower revs.. some are more efficient in lower revs.. but yes generally the slower you can get the RPMs for any given speed the better MPG you'll get as each RPM your pulling in a set amount of air and fuel into the cars engine..

The nice thing about tirew sizes are you can experiment pretty easily and cheaply.. no re gearing the transmission etc.. just swap out the tires, and you could even just do the drive tires if you didn't want to invest in a whole set.. at least initially.

However taller tires will raise the car that may be a consideration.. and your speedo will be off.. however a gps can tell you how far its off and you can then remember that 55mph on the speedo is actually 60 or whatever the gps gives you..
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Old 06-05-2008, 08:48 PM   #6 (permalink)
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blackjackel -

If I had a rear-wheel drive, I would get "one size smaller than stock" front tires and "one size larger than stock" back tires. I *think* I would also lower the front end shocks and leave the rear stock. The lower fronts would (hopefully) stop some airflow from going under the car, and the larger rears would make all my transmission gearing "taller" (lowering my RPM at speed X).

Changing the angle might create a larger frontal area, but I think the positives would outweigh the negatives.

I'd have a Cooooooool-Rod that looks like a Hot Wheel!!!!!

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Old 06-05-2008, 09:40 PM   #7 (permalink)
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One thing to keep in mind when changing tire sizes: Your vehicle was engineered to handle, brake, etc with a specific wheel set in mind. Changing your wheels changes the engineering.
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Old 06-05-2008, 10:28 PM   #8 (permalink)
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Another thing to consider, you won't be able to trust your speedo anymore for an accurate judgement on how fast you're traveling because smaller/larger tires means that your axil will spin faster/slower for a given velocity and will throw off the speedo by sometimes as much as 10mph or more. Happened to my friend, he had a Land Rover Defender 90 (sweet offroad vehicle I must say) and he put some larger mud tires on it and when he drove it, he drove past one of those real time speed signs and noticed it was about 12mph off what his speedo was reading, so from then on he had to adjust accordingly. Just know by how much you need to compensate on the speedo reading so you don't go getting yourself in trouble with the law is all I'm saying.
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Old 06-05-2008, 10:52 PM   #9 (permalink)
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Or if you have a scangauge, you can set your speed multiplier to whatever you want and just look at that.
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Old 06-06-2008, 12:03 AM   #10 (permalink)
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After much much much study, research, and deliberation, the results of which can be found here ( http://ecomodder.com/forum/showthrea...ling-2813.html ) I have decided to get new wheels and tires. My tires have another 5,000 miles on em, 10,000 if I'm really really lucky.... so.. I will be doing the following:

from 15" wheels to 14" wheels.

Tire size from 195-65-15 to 185-70-14

I used this tire calculation site to calculate the changes in measurements, its not much at all: http://www.miata.net/garage/tirecalc.html

my odometer would be off by .9% (if it says i'm going 60 MPH i'll be really going 60.5 MPH)

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
195/65-15 5.0in 12.5in 25.0in 78.5in 807 0.0%
185/70-14 5.1in 12.1in 24.2in 76.0in 834 -3.1%

The new tire would be .4 inches or 10 millimeteres narrower, or approx 1.5 inches if you count all tires combined.

I'm going from a GoodYear integrity to BridgeStone B381
This is going from a rolling resistence of 0.00955 to a RR of 0.00615 or a difference of 0.00340 in rolling resistence!

I expect at least 5 more MPG, if not more.

Any thoughts?
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