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Location: New Orleans, LA
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There are a lot of questions posted about which size to use, and how wide of a wheel to use, especially when plus-sizing on the US-spec 6. So, here is an easy guide to refer to when looking for replacement tires. First, let's breakdown what the size numbers/letters mean:
P = Passenger Tire (does not appear always)
215 = Section width (in millimeters)
50 = Section height or aspect ratio (ie. sidewall is 50% of section width)
R = Radial construction
17 = Rim diameter (in inches)
93 = Load Index
V = Speed symbol
All of the components of your tire size are important, but when choosing a replacement or plus size, many of those numbers will change. You may have a wider tire with lower aspect ratio and a larger wheel. However, the last two characters are the most important.
For the US-spec model, you MUST choose a tire with at least a 91 (or greater) load index and a speed symbol of H or greater (H, V, Z, W, Y) [NOTE: Winter tires are excepted from this stipulation]. These last two characters refer to the internal construction of the tire and it is important to maintain the same or greater load and speed capability of your tires as per Mazda's recommendation.
What follows is a guide, by rim diameter of the acceptable replacement sizes for your 6, which rim widths are acceptable for that size and the minimum inflation pressure for that size on your 6. Please refer to this guide before posting questions about what will work on your 6 (for simplicity, I am excluding the P designation and the load index/speed symbol, see above for requirements). This list has been thoroughly researched. If you don't see a size listed here (ie 215/45R17), then it is not recommended for the US-spec 6.
16" (original equipment wheel is 16x7)
205/60R16 5.5"-7.5" 32 psi *original equipment size*
215/55R16 6.0"-7.5" 33 psi
215/60R16 6.0"-7.5" 32 psi
225/55R16 6.0"-8.0" 32 psi
235/50R16 6.5"-8.5" 32 psi
17" (original equipment wheel is 17x7)
215/50R17 6.0"-7.5" 32 psi (93+ load index), 33 psi (91 load index) *original equipment size*
225/45R17 7.0"-8.5" 35 psi
225/50R17 6.0"-8.0" 32 psi
235/45R17 7.5"-9.0" 32 psi
245/45R17 7.5"-9.0" 32 psi
18" (original equipment wheel is 18x7)
215/45R18 7.0"-8.0" 35 psi (6s, 6i), 38 psi (Mazdaspeed6) *original equipment size*
225/40R18 7.5"-9.0" 38 psi
225/45R18 7.0"-8.5" 38 psi
235/40R18 8.0"-9.5" 38 psi
245/40R18 8.0"-9.5" 38 psi
225/40R19 7.5"-9.0" 36 psi
235/35R19 8.0"-9.5" 39 psi
245/35R19 8.0"-9.5" 36 psi
225/35R20 7.5"-9.0" 40 psi NOTE: This fitment is BORDERLINE, but acceptable. Make certain you are choosing a reinforced tire (LOAD RATING 90).
NOTE: Section widths of 235 and above will probably require fender rolling. Check the offset of your wheels carefully before purchasing replacement tires. I was going to also include the overall diameter for each tire, but this can vary from manufacturer to manufacturer by .1-.2". Suffice to say that these tires fall within the "-2%, +3%" rule, meaning they are not more than 2% smaller or 3% larger than the original equipment tires, which have an overall diameter of 25.5 inches.
TAKE NOTE: When selecting replacement wheels and tires, the relationship of the wheel width to tire section width is very important! For maximum handling benefit, mount the tire on the widest wheel that size will allow. Mounting a tire on the narrowest width allowed will give better ride comfort, but won't give the best handling. In many cases, handling may be worse than before by mounting a wider tire on a narrow rim (ie. 235/40R18 on an 8 inch wide wheel vs. 215/45R18 on an 8 inch). Installing wider tires on your 6 will likely yield a handling improvement only if you have a wheel wide enough to take advantage of the extra tire width.
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