What's the perfect rear axle ratio?

What better is there to do during a snowstorm than try to decide what is the perfect rear axle ratio for a car that I don't have and won't even be available for 8 months? Here are some numbers for cruising RPM and speeds at maximum RPM in each gear. Note that the 3.55 ratio with the 2005 tires is approximately equivalent to a 3.27 on the previous years 245/45's.

Based on 760 revolutions per mile (235/55 x 17)
--------------RPMs--------Speeds at 6250 RPM
-------5th@60--5th@80--1st-2nd-3rd--4th--5th
3.27----1541----2054----45--75--113-151--243
3.55----1673----2230----41--69--105-139--224
3.73----1758----2343----39--66--99--132--213
3.9-----1838----2450----37--63--95--127--204
4.1-----1932----2576----36--60--90--120--194
4.3-----2026----2702----34--57--86--115--185
4.56----2149----2865----32--54--81--108--175

Based on 820 revolutions per mile (245/45 x 17)
-------------RPMs-------Speeds at 6250 RPM
-------5th@60-5th@80- 1st-2nd-3rd---4th--5th
3.27----1662---2217--- 41--70--105--140--226
3.55----1805---2406--- 38--64--97---129--208
3.73----1896---2528--- 36--61--92---123--198
3.9-----1983---2644--- 35--59--88---117--189
4.1-----2084---2779--- 33--56--84---112--180
4.3-----2186---2915--- 31--53--80---106--172
4.56----2318---3091--- 30--50--75---100--162

351CJ said:

There is no such thing as a perfect axle ratio. Any gear selection is a compromise.

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Of course, but then there would be no discussion...

But I don't quite agree with your #'s. For 235/55R17 I get 742 revolutions / mile (27.177" dia) . For 245/45R17 I get 785.3 revolutions / mile (25.6811" dia).

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Actually, I don't agree with the numbers either! I took the revolutions per mile using those called out in various manufacturer's specifications. The diameters and revs per mile in their charts do not agree using the normal conversion. I assumed that the revolutions per mile were more likely to be correct. Of course, those numbers also vary considerably for the same size tire even within brands. I'll be happy to edit the numbers if I knew exactly which to use. I'm not a fan of perpetrating disinformation.

[Going back and looking at the numbers, I probably exagerated the difference. It looks like the 235/55's run around 765 or so, and the 245/45's about 812, based on my highly unscientific sampling of specs. It occurs to me that the difference may be in the loaded radius versus the unloaded diameter of the tires.]