Larz wrote:My wheels are 18" and my M is RWD, but there is a member here with a X model and 19" wheels had no issues regarding fitment with the 245/55 tires. As for speedometer accuracy, a member called Psyopper worked out that my car is going 1 mph more than the car indicates on the speedo AND with increased MPG.
Tire size sidewall or profile measurements are actually an aspect ratio of the height to the width. In laymans terms, this second number is a percentage of the width. Our stock 245/45 have a tire width of 245mm and height of 45% of 245mm, or 110mm.
Your 255/45 has a sidewall height of 115mm (45% of 255mm). The overall diameter increases by 10mm. Because of the larger circumference your car goes a little bit further than stock with a single revolution of the tire. In this case, when your speedo reads 60 mph you are actually going 61mph.
With 255/45 tires, the car will actually travel 1 mile + 70 feet for each mile it used to travel. One would assume that you would see an increase in fuel economy, and it would be more pronounced in highway driving where you could be taking advantage of that small increase in final drive ratio.
Well, this is too theoretical a calculation actually - using the tire sizes in this way gives a precise value ... that may not be correct!
Reason is that different brands and models of tires (even the same size tire for different models from the same manufacturer) will flex differently, have a different tire profile, and all this can affect the actual
effective rolling diameter of the tire.
Plus, if the tire/wheel combo is a stretch (even if minimal), the tire pressure is low, or other factors, it can cause the shape of the tire profile to change, etc.
So, it is far better (and simpler) to use the "revs/mile" specifications that you can occasionally find at Tire Rack (or the manufacturer web site) for tires. This can easily show you the speedometer error in any given case - just compare the stock tire (brand and size) revs/mile to the replacement tire revs/mile and that ratio can then be used to calculate the actual speedo error decently under similar conditions.
Yes, it may be close to the theoretical calculations based on tire size. But I have seen identical size tires have different revs/mile, so have a
minute speedo error, even though the theoretical calculation above would say "no error". Is this enough to matter? Probably not in most cases.
Anyway, if the revs/mile for the replacement tire is higher, it will turn more and give you an speedometer reading that is higher than your actual speed - in the same ratio as the revs/mile ratio of the tires.
Z