View Full Version : berry stupid question
09-15-2005, 02:46 AM
so, lets say my car comes with 15" wheels, and I decide to put in bigger or smaller rims/wheels on the car. Do I have to adjust anything on the car telling it I put on different size wheels? because I was thinking....if I put in different size wheels then what it came with, I would either be going more miles or less miles then I actually am because the circumfrence is different on the wheels ie. 15", 16", 17". am I corrent? or am I just blowing out smelly gas? somebody do explain...I am one confussed chinaman
09-15-2005, 03:09 AM
The odometer will be slightly off......no biggie.
09-15-2005, 03:11 AM
Well if you're speaking strictly mathematically, when the engine has turned a certain number of rotations, a car with with larger wheels (= larger circumference) would have gone farther than one with smaller wheels. But then again, I don't think it would necessarily be more efficient cause it would take more torque to make big wheels perform the same as small ones. So, although I can't really compare fuel efficiency, you would indeed get more for your car when you sell it used, cause your odometer would indicate less miles than you actually drove. But the difference is most likely to be miniscule, so just go with what looks cooler.
Someone confirm this for me?
09-15-2005, 03:28 AM
Someone confirm this for me?
circumference = 2piR = pi*D
C of 17 inch wheels = ~53.41
C of 15 inch wheels = ~47.12
difference = ~6.29 inches, or roughly a half a foot
So, if everything was calibrated for 15 inch wheels each time your car "thinks" it has made one revolution you have really moved a half of a foot further. So after you have made roughly 10,000 revolutions on the tire (nearly 7 and a half miles on the odometer), your odometer would be short by one mile.
This is all assuming there is no slip or tire spin and the tires are of the same profile.
Traveling at what your speedo reads as 60 mph would mean you were really going about 68 mph.
The larger tires will also have a larger moment of inertia so they will be harder to get going (and stop!).
09-15-2005, 07:55 AM
It is not really a big deal unless you make a big change to the tires. By this I mean putting oversized wheels on your jeep or truck. If you do that you'll need to recalibrate your speedometer and you'll want to upgrade your brakes. But if you're just ricing around with your Honda then there is no meaningful difference.
09-15-2005, 10:21 AM
you can go bigger rims, but then, go low profile tires. This will get you the total radius (thus circumference) to be the same as the OEM rim/tire combo.
09-15-2005, 10:38 AM
Tirerack and those other places make buying new wheels and tires easy.
09-15-2005, 10:46 AM
No but you have to get correct tires so the overall diameter (D+2*TH) stays the same otherwise your speedometer and odometer are off. If the diameter is larger, your speedometer is lower than your actually speed thus you have lower mileage and vice versa.
09-15-2005, 11:48 AM
I heard you can take it to a car repair shop and they can modify the system to show the proper speed.
09-15-2005, 12:47 PM
Of course there are several ways to address this issue...
As folks have already stated... you can do +1, +2, or +3 tire sizing to bring the tire size down in relation to the wheel diameter going up. Doing this... and keeping the diameter of the tire to within +/- .75" of the original tire diameter won't throw off your odometer/speedometer.
However, with the advent of "tuner" chips that can re-program your car's computer, you can sometimes just adjust the computer directly to the new tire size... which will fix the odometer/speedometer.
Now... just be careful that when you upsize your wheels... you still have the proper braking force to stop the larger wheels. As far as imports go... if you only go up +2 you probably will be ok with the the standard brakes... however if you go +3 or more, then you might want to consider better calipers and pads.
As far as gas mileage goes... well, I would say that this really depends on the weight of the new wheels/tires in relation to the old wheels/tires. If they are the same... then the difference will probably be marginal at best. However, say you bought some really light wheels (carbon fiber) and you reduced your weight overall by 100+ pounds... then yeah, you might see an extra 1 MPG or so in fuel savings. But honestly, most folks who change their wheels ain't doin' it for the fuel savings. :D
09-15-2005, 01:04 PM
However, say you bought some really light wheels (carbon fiber) and you reduced your weight overall by 100+ pounds...
And that is unsprung weight. Getting that down can really help performance.
Let's get a little more information here. Are you looking to make your own "monster truck" or street racer?
09-15-2005, 01:09 PM
Speaking of unsprung weight, I can't wait to buy tweels.
09-15-2005, 07:40 PM
lol...thx for ALL the help guys...
I'm looking to put 16" or 17" rims on my car from the default 15". I was just worried about the speed readings and the Odometer readings. we'll see...I might just stay with the 15". I just want rims instead of the steel black rims with plastic silver colored wheel covers.
09-15-2005, 10:02 PM
You need a Tire size calculator (http://www.miata.net/garage/tirecalc.html)
There are a ton. Google for others-this is not the best. I believe our speedomters and odometers are calibrated for 185/65's. The widest you can go is 205, but don't get larger than 45/50.
Poke around on www.7thgencivic.com and ask around. I have Falken Ziex 190/60's on the stock EX 15" 7-spoke alloys.
09-16-2005, 01:46 AM
the mathematics above is the final gear ratio if u wanted to know
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