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i thought of something. we all know about the speedometer thing, which isn't a big deal if you're off by 2 mph or something, but what about odometer? technically you could have driven your car further than your odometer says if you have really big tires. for examples sake, think of tires 1 mile in diameter; you would travel 3.14 miles for every tire rotation, yet your car would think you would have traveled less than 10 feet. on the other hand, and more importantly, if you have mexican tires that are like 12", you could end up "paying for" a lot of odometer mileage you didn't travel. and yes i think even 1" would add up to thousands of miles over the life of the car.

really, shouldn't the odometer be based base off of rpms, not miles? think about it, it makes more sense, because it would include idling, and the size of the tires would matter not.
 

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you are correct, your odometer will not be correct if you use a tire/rim combo that isn't in size relation to stock circumference. What you're talking about is done on some commercial equipment / landscaping/ tractors etc. They have hour ratings that turn based on how long the machine is turned on. That is how they determine it's usage.
 

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well, yes and no, because if you change the size of your tire diameter wise, you change how far the wheel would go when making a full rotation for every rpm. And if they set it at calculating distance by rpm, it still would be off when changed to a new wheel, due to the same concept of having different distance per rotation.
 

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when i put on my RPF1s a while back with 225/40/18 tires. my speedometer was 6 mph off. i was doing 60 and my girl called me and asked me why i was going so fast, didnt know i was really going 66. thats what she was doing when she was following me.
 

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Discussion Starter · #6 · (Edited)
well, yes and no, because if you change the size of your tire diameter wise, you change how far the wheel would go when making a full rotation for every rpm. And if they set it at calculating distance by rpm, it still would be off when changed to a new wheel, due to the same concept of having different distance per rotation.
so you're saying that 3000 rpms with 15" tires will be a different speed/distance than 3000 rpms with 20" tires?

hmm had the same question but anyone know the speedometer differnce when going from 16 inch to 17?
google it, there are calculators that will tell you exactly. you input the exact tire sizes, and it tells you down to the fraction of a mph. i remember doing a calculation for fun, and using the same exact tire but going from 17" to 18", it was around 4%. so 2 mph if you're doing 50 mph.

may i ask why you have 16's on your car in the first place? si's have 17's. or are you doing 16" steelies for winter?
 

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