a 255 tire is a section width of 10". Thats the tire at the wides point, usually mid-sidewall to mid-sidewall. On a 7" wide rim, that's budging 1.5" past the rim of both side.
I'll run 215/70s to 235/60s on 7" wide, no bigger. By squeezing a 255 wide onto a 7" rim doesn't buy you clearance. An 8" wide rim is a much better bet for safety reasons. Go to the chart at the very bottom of this post.
FWIW:
A tire flexes like crazy! the lower the profile, the less flex. Look at the following video with a very low profile tire on a FWD race car. Look how that little sidewall still lets the tire flex all over the place. Now put a big'ole 60 series sidewall....on a marginally too narrow rim. You can see how you may peel the tire right off the rim.
http://www.youtube.com/watch?v=UCCdxfBu66U
From the Yokahama website:
It is important to note that there are some differences between P-Metric and Metric approved rim width ranges (in general, Metric tires have a slightly narrower range). The TRA also has developed a measuring rim for each tire size that enables all manufacturers to measure their tires on the same size wheel. For 50-series tires and higher, the measuring rim width is 70% of the tire's section width rounded off to the nearest 0.5." For example, a P255/50VR16 tire, which has a design section width of 10.04", is measured on a 7" wide wheel.
Mid-Range Rim Widths
Provide a balance between handling capabilities and ride quality. The wheel's width influences handling and ride quality. Always choose a rim width within the range of the tire's acceptable rim width specification.
For tires that have an aspect ratio lower than a 50-series, the measuring rim is 85% of the tire's section design width. Therefore, a 255/40ZR17 tire with the same 10.04" section width would be measured on an 8.5" wide wheel. The measuring rims for Yokohama tires are listed with their dimensional data in this guide.
Now here is the recomended tire size / rim size chart. Note the 255/60 isn't given! A 235/60 is; this is why I like the 235 on the 7" wide rim. If you sort interpolate the chart.....255/60 on a 7" wide rim is too wide.
Below is a quick reference chart listing each Yokohama tire size and the acceptable rim widths for that size. Choosing a wheel near the middle of the range will give a balance between ride quality and handling. A wider wheel will improve handling at the expense of ride quality, while a narrower wheel will improve ride quality at the expense of handling. Consider these compromises when selecting wheels.
Metric Rim Widths
Aspect Ratio Metric Size Designations Approved Rim Width Ranges
--------------------------------------------------------------------------------
35 Series 275/35R17 9.0-10.5
335/35R17 11.0-13.0
345/35R15 11.0-13.5
40 Series 285/40R15 11.0-13.5
215/40R15 7.0-8.5
245/40R17 8.0-9.5
255/40R17 8.5-10.0
265/40R17 8.5-10.0
235/40R18 8.0-9.5
45 Series 195/45/R15 6.5-7.5
215/45R15 7.0-8.5
255/45R15 8.5-10.0
205/45R16 7.0
225/45R16 7.5-9.0
245/45R16 8.0-9.5
215/45R17 7.0-8.5
235/45R17 8.0-9.5
255/45R17 8.5-10.5
50 Series 175/50R13 5.0-6.0
195/50R14 5.5-7.0
205/50R15 5.5-7.5
215/50R15 6.0-7.5
225/50R15 6.0-8.0
195/50R16 5.5-7.0
205/50R16 5.5-7.5
225/50R16 6.0-8.0
235/50R16 6.5-8.5
215/50R17 6.5-7.5
55 Series 205/55R14 5.5-7.5
185/55R15 6.0-6.5
195/55R15 5.5-7.0
205/55R15 5.5-7.5
205/55R16 5.5-7.5
225/55R16 6.0-8.0
Aspect Ratio Metric Size Designations Approved Rim Width Ranges
--------------------------------------------------------------------------------
60 Series 185/60R13 5.0-6.5
205/60R13 5.5-7.5
185/60R14 5.0-6.5
195/60R14 5.5-7.0
205/60R14
195/60R15 5.5-7.0
205/60R15 5.5-7.5
215/60R15 6.0-7.5
225/60R15 6.0-8.0
235/60R15 6.5-8.5
65 Series 185/65R15 5.0-6.5
195/65R15 5.5-7.0
205/65R15 5.5-7.5
215/65R15 6.0-7.5
70 Series 165/70R10 4.5-5.5
175/70R12 5.0-6.0
175/70R13 5.0-6.0
205/45R16 7.0
185/70R13 5.0-6.5
185/70R14 5.0-6.5
195/70R15 5.5-7.0
205/70R14 5.5-7.5
185/70R15 5.0-6.5
205/70R15 5.5-7.5
215/70R15 6.0-7.5
82 Series 155R12 4.0-5.0
155R13 4.0-5.5
165R13 4.0-5.5
175R14 4.5-6.0
185R14 4.5-6.0
165R15 4.0-5.5
P-Metric Rim Widths
Aspect Ratio Metric Size Designations Approved Rim Width Ranges
--------------------------------------------------------------------------------
75 Series P205/75R15 5.5-7.5
P215/75R15 5.5-7.5
P225/75R15 6.0-8.0
P235/75R15 6.0-8.0
80 Series P155/80R13 4.5-5.5
P165/80R13 4.5-6.0
P175/80R13 4.5-6.0
P185/80R13 5.0-6.5
Aspect Ratio Metric Size Designations Approved Rim Width Ranges
--------------------------------------------------------------------------------
35 Series P315/35R17 10.0-12.0
40 Series P275/40R17 8.5-10.0
50 Series P215/50R13 5.5-8.5
P235/50R13 6.0-9.0
P245/50R14 6.5-9.0
P225/50R15 6.0-8.0
P295/50R15 7.5-11.0
P225/50R16 6.0-8.0
P245/50R16 6.5-9.5