This will result in a VERY narrow sidewall.
If you tell us what size tires you have now, I'll calculate what you will
end up with if you get 20" rims. Assuming you have either the 225 or 235s
now, you will end up with a 25 or 30 sidewall.
The tire has a number, 235/40x17. This means the tire WIDTH is 235mm, the
sidewall is 40% of the width, and the rim diameter is 17 inches. The width
is a fixed number for the most part. Issues affecting width are the
proximity of the shock/springs and the fenders. The diameter is defined by
the rims, and the sidewall is a flexible number that causes changes in the
gearing - which affects the speedometer and performance, both of these
things can be corrected by changing the gear set in the differential.
So, when you change the diameter of the rim, you have to alter the height of
the sidewall to maintain the overall diameter so that you do not run into
issues with the gearing -- speedometer and performance.
Multiply the width (235) by the sidewall height -- more accurately termed
aspect ratio -- (.40) to get one sidewall. Double the number to get the
other sidewall. Now, convert the number from millimeters to inches by
dividing by 25.4. This will tell you the total sidewall in inches, finally
add the rim diameter to determine the overall diameter.
235 x .40 = 94 x 2 = 188 / 25.4 = 7.4 + 17 = 24.4. You currently have a tire
that is 24.4 inches in diameter, you want your 20" rims to come out to as
close to the same number as possible. If your difference in the new tire is
a bit larger, then your speedometer will slow down and you may expose
yourself to speeding tickets. If the new tire is a bit smaller, the
speedometer will speed up and read a bit faster than you are really going.
As a practicle matter, the speedometer reads fast already, and a half inch
or so larger tire will make the speedo dead nuts accurate, and you will not
get tagged for speeding if yo keep your eye on the instruments.
With the 20s, the 225/25 will be 24.43 inches, the 235/25 will be 24.62. We
already figured out that the stock 235/40s are 24.40. So, either of these
sizes will fit fine if the only consideration is the gearing. The problem,
and this is a huge problem in my mind, is that the 20 inch tires will only
give you 2.2 inches between the rim and the pavement. I predict lots of
broken rims with this set up. If you currently have the 225/45x17s, then
your diameter is 24.9 inches, and the equivelent in a 20 is the 225/30,
which gives a bit more protection between your rims and the road -- you get
2.6 inches, which isn't very much.
wrote in message
> Can I put 20" wheels on my 2006 BMW 325i?
> >> Stay informed about: 20"" Wheels on 06MY 325i? 
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