What's the perfect rear axle ratio?

[Pompey]

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]

There is no such thing as a perfect axle ratio. Any gear selection is a compromise.
You do make a good point, that is with the larger diameter tires on the 2005, it has the effect of lowering the engine RPM at any given speed.

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).

 

[Pompey]

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

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).

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.]

 

[david97gt46]

3.73 there is no better. good drag starts low highway cruising rpms, best of both worlds

 

[BSRacing]

3.73s on the new stang is like putting 3.55s on a 96-04...

Don't fear the gear... How about a 4.30s (GT) and 4.56s (SVT) on the new stang for all around driving and 4.88s (SVT) for drag... If you really look at it, is like 4.10s on a 96-04 GT and 4.30s on a 96-04 Cobra...

I had 4.10s in my 99 GT and could have gone with more gear... The best investment in my car period... I had 4.56s in my 01 Cobra... Best investment with the dry shot...

 

[ZStanger]

I'd say 3.73's to 4.10's... Gets decent gas mileage, but can accelerate pretty hard when needed...

 

[Sir Hacksalot]

I was planning on 4.10's regardless

Hax