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Discussion Starter #1
Hi,
I've been hearing that say when you change the rim size on a car, the speedometer either reads too fast or too slow. When I get my IS, I was thinking of putting on 16" rims for the winter and trips to the mountains, and maybe 18" ones during the summmer. The speedomter would be off a little in both of these cases, correcT? Does that make the odometer wrong? I can live w/ some error in the speedo but really wouldn't want the odometer to read the wrong mileage. What else is effected when change tire size (different circumference of rim + tire).

Also, I was wondering, how do manufacturers with a car with different options that offer anywhere from 15" to 17" rims for the car deal with this problem? Are the speedos recalibrated for each or is the difference just so insignificant that it doesnt matter?
 

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I assume the car is calibrated to whatever wheel size you order it with. So, if you order the 16" wheel option, your speedomter should be calibrated. I assume it will be a little off when you put the 18" wheels on ...

Originally posted by ISx300:
Hi,
I've been hearing that say when you change the rim size on a car, the speedometer either reads too fast or too slow. When I get my IS, I was thinking of putting on 16" rims for the winter and trips to the mountains, and maybe 18" ones during the summmer. The speedomter would be off a little in both of these cases, correcT? Does that make the odometer wrong? I can live w/ some error in the speedo but really wouldn't want the odometer to read the wrong mileage. What else is effected when change tire size (different circumference of rim + tire).

Also, I was wondering, how do manufacturers with a car with different options that offer anywhere from 15" to 17" rims for the car deal with this problem? Are the speedos recalibrated for each or is the difference just so insignificant that it doesnt matter?
 

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If I'm not mistaken it is actually a combination of tire and rim. If you keep the overall circumference of your different wheel sets the same it should not change anything. And please, someone let me know if I've been misinformed.
 

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Discussion Starter #4
I believe you are correct. My biggest concern is whether the odometer will start being inaccurate IF the circumference changes
 

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From my experience with my Integra, as Rich was saying, as long as the whole wheel set size (wheel + tire sidewall) remains very close to factory, then you shouldn't have any problems. For example, factory is like 215/45 17, and if you wanted to go to 18's and beefier tires, than you might get away with going 225 or 235/ 40 18.

Again, this is from personal experience with my Teg (which I upsized from 15 to 17 w/o any problems )
 

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Yes, the last 3 posts are correct.

It is the overall diameter of the tire that effects the speedo calibration.

The 16" wheels with 55 series tires are probably very close in overall diameter to the 17" wheels with 45 series tires...
 

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Yes - it counts rotations of the driveshaft and assumes that each rotation covers a specific distance. With larger diameter tires you cover more distance per rotation so you go faster than the speedo/ODO think you are going. Before you go out and put on bigger tires keep in mind that it will take you longer (in 1st gear) to rev the engine to the "sweet spot" of the power band so 0-60 times will probably be longer with larger tires. Top speed may end up being higher though...
 

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If you go to Tire Rack's web site, www.tirerack.com, hit the "Tire Tech" link and then the "Calculating Tire Dimensions" link you will find the formulas for calculating the diameter of a tire from its size numbers. As long as a tire diameter is within 1/2 inch of the original factory tire diameter, the speedometer and odometer will be within approximately 2% of their original accuracy.
 

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I think changing your wheel/tire size also changes your gear ratios. It was on the show Trucks! and I read that SCC project Sentra felt slightly sluggish wearing upsized rims and tires.
 
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