True wheel size

I have an Accord Coupe (6th Gen) and my original dimensions are 195-65-15. I'd like to upgrade to 18's and have heard different versions of what the "ideal" size would be. I've heard 215/35/18, 225/40/18...etc. How do I know which size is right? What is the calculation done to figure it out?

I think I remember this, but someone correct me if I'm wrong. The first number is the max tire width (mm) - call it W. The second number is the sidewall height as a % of the width - call it H. The third number is the ID of the tire in inches - call it D.

OD can thus be calculated (in mm) as
OD = 25.4*D + 2*H*W

For your 195/65/15 :
OD = 25.4*(15)+2*0.65*195 = 634.5 mm

For a 215/xx/18, the profile you need (to be the same OD as the 15s) would be :
H = (OD-25.4*D) / (2W) = 0.412 => so a 215/40 would be close

For a 225/xx/18, the profile you need would be :
H = (634.5-25.4*18) / (2*225) = 0.394 => a 225/40 is still the closest you can get

Of course, you can put it all in 1 equation, but I thought subscripts might be confusing.

I don't see any need for a correction.

Stock: total height = 24.98"
215/40/18: total height = 24.77"
225/40/18: total height = 25.087"

The 225 will be slightly taller than stock, but it will also be 3cm wider, or 1.18" wider. This is where your real problem could be. 215 will be 0.787" wider than stock.

As has been said, we should be able to fit 19" without rubbing, but my guess is we'd need 215/35 for that to keep it from getting too tall.

b

[4jacks]

NO NO NO ... BUY A LARGER WHEEL !!!!!

If you buy a slightly larger wheel then you get to cheat your odometer, for every mile you drive it will only read .95 of a mile. or some figure like that.. If you buy a smaller wheel then you will be adding miles that you did not drive onto your odometer!!!!!

Here is a great link to calculate all your wheel size questions..

215/35/r18 don't even come close to rubbing on my 02 civic.. I'm getting 225/40/18's next time around !!

here's the link http://www.2k1civic.com/stuff/tire_calc.cfmhttp://www.2k1civic.com/stuff/tire_calc.cfm

Originally posted by 4jacks
[b] If you buy a smaller wheel then you will be adding miles that you did not drive onto your odometer!!!!!

Or put on 10" wheels and roll back your odometer; it's just as dishonest.

Originally posted by 4jacks
NO NO NO ... BUY A LARGER WHEEL !!!!!

If you buy a slightly larger wheel then you get to cheat your odometer, for every mile you drive it will only read .95 of a mile. or some figure like that.. If you buy a smaller wheel then you will be adding miles that you did not drive onto your odometer!!!!!

Here is a great link to calculate all your wheel size questions..

215/35/r18 don't even come close to rubbing on my 02 civic.. I'm getting 225/40/18's next time around !!

here's the link http://www.2k1civic.com/stuff/tire_calc.cfmhttp://www.2k1civic.com/stuff/tire_calc.cfm

And increase your rotational mass, and change the accuracy of your speedometer.

I certainly would never buy one thing over another just on account of its effects on my odometer. Let's just say that the change is .95, and you put 100k miles on your car. It'll read 95k miles on the car....which is negligible when compared to 100k, thus it will have little or no effect on the value of the vehicle. If you put 50k on the car, then it'll read 47.5k, which is still pretty negligible and won't really affect the value. See a common theme here?

But, based upon the calculation that I just did, you'd be changing it by a factor of roughly +/-.005 from stock whether you go with 215/40/18 or 225/40/18, respectively. Based upon that order of magnitude, for every 1000 miles you drive, you'd be changing the odometer by 5 miles.....that's 500 miles for 100k.

b