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BadNick · 05-06-2004, 07:06 AM

On a car site, I'd just simply answer. Here, I feel like I should first philosophize; am I a paranoid maniac? So first you have to hear my speech

: some famous woman once said this way back when women were real smart: "here, take this apple and taste" ...errrr that's the wrong story ..."give a man a fish and feed him once, give a man a fishing pole and feed him for life".
This is why men go fishing all the time instead of doing what they are suppossed to be doing.

Here's how to figure it out:
The total tire outside diameter is directly proportional to the circumference, which is directly proportional to speedometer reading. So if you figure the percentage +/- difference in total diameter between the two sizes, that would be the same % difference in the indicated speed. The "60" or "40" numbers are aspect ratio, which is the ratio of tire sidewall height to tire width.

So if you have a 195/60 X 15" setup, the sidewall is 195 X 0.6 = 117mm; you have two sidewall heights plus the wheel diameter adding up to the total diameter (keep all units the same, inches or mm); 117mm = 4.606"; 4.606 + 4.606 + 15 = 24.21" total

If you have a 205/40 X 17" setup, the total is 23.46".

23.46/24.21 = .969

So your real speed with the 17" setup will be about 3% lower than the 15" setup. Assuming your speedometer is correct with the 15", if your speedometer says 150mph on the 17's, you will actually be going 145.5; at indicated 50mph, you will be going 48.5mph.

The above does not factor in how the wheel width effects the sidewall height of a tire, since the same tire width on a wider wheel usually results in a slightly lower sidewall height. But this is not usually very significant so I ignored it.

Lower sidewall height gives you less rubber to absorb road shocks, so it will probably feel a little harsher but even the 17" you show have quite enough sidewall in my opinion for normal comfort. IMO, most cars look way better with bigger wheels, up to a reasonable point. I'm not a fan of 24" setups on civics.

05-06-2004, 07:06 AM	#5 (permalink)
BadNick Riding the Ocean Spray Location: S.E. PA in U Sofa	On a car site, I'd just simply answer. Here, I feel like I should first philosophize; am I a paranoid maniac? So first you have to hear my speech : some famous woman once said this way back when women were real smart: "here, take this apple and taste" ...errrr that's the wrong story ..."give a man a fish and feed him once, give a man a fishing pole and feed him for life". This is why men go fishing all the time instead of doing what they are suppossed to be doing. Here's how to figure it out: The total tire outside diameter is directly proportional to the circumference, which is directly proportional to speedometer reading. So if you figure the percentage +/- difference in total diameter between the two sizes, that would be the same % difference in the indicated speed. The "60" or "40" numbers are aspect ratio, which is the ratio of tire sidewall height to tire width. So if you have a 195/60 X 15" setup, the sidewall is 195 X 0.6 = 117mm; you have two sidewall heights plus the wheel diameter adding up to the total diameter (keep all units the same, inches or mm); 117mm = 4.606"; 4.606 + 4.606 + 15 = 24.21" total If you have a 205/40 X 17" setup, the total is 23.46". 23.46/24.21 = .969 So your real speed with the 17" setup will be about 3% lower than the 15" setup. Assuming your speedometer is correct with the 15", if your speedometer says 150mph on the 17's, you will actually be going 145.5; at indicated 50mph, you will be going 48.5mph. The above does not factor in how the wheel width effects the sidewall height of a tire, since the same tire width on a wider wheel usually results in a slightly lower sidewall height. But this is not usually very significant so I ignored it. Lower sidewall height gives you less rubber to absorb road shocks, so it will probably feel a little harsher but even the 17" you show have quite enough sidewall in my opinion for normal comfort. IMO, most cars look way better with bigger wheels, up to a reasonable point. I'm not a fan of 24" setups on civics.