Originally Posted by YaXMaNGTO

Out of curiousity, how power will the Kaaz diff handle?

And who would want a 3.90 ratio and why?

Originally Posted by Zeon

I took the time to do some calculations for rpm at different speeds for the different differential ratios available through Fluid as per this thread, so I figured I would just pass it on to everyone in case they were interested. Below are my assumptions, followed by my calculations.

WARNING: All values are calculated meaning this does not necessarily directly correspond to real world situations.

Assumptions:
Gear Ratios (I was only worried about the 2004 6-speed)
Taken from: link to transmission thread I grabbed ratios from
1st = 3.76
2nd = 2.269
3rd = 1.645
4th = 1.187
5th = 1.000
6th = .843

Constants:
mms to inches conversion factor = 1/25.4
MPH to inches conversion = 1056/pi (info grabbed from this thread on another forum)
Tire Size = 225/45 R18
From this we infer:
nominal width of tire = 225
ratio of height to width of tire = 45%
rim diameter code = 18

** I just wanted to note that there is a difference in my calculated values when compared to the thread I linked above. I calculated the tire diameter below to be about 26", I cannot say if this is correct or not as I haven't measured my wheels/tires to compare. The thread I linked appears to be using a tire diameter value of 24.223113865" for some reason which wasn't explained in the thread. I have confirmed my calculation with different tire size calculators online, so I'm just going with it. So just keep in mind that the actual speed could be slightly off as I mentioned in my warning. **

Formulas:
Tire Diameter = (nominal width of tire * ratio of height to width of tire * 2 * mm to inches conversion factor) + rim diameter code
Note: the ‘2’ in the above formula is to take both sidewalls into account for the total wheel + tire diameter

MPH = (RPM * tire diameter) / (gear ratio * differential ratio * MPH to inches conversion)

From that we can derive the formula for finding RPM at a given speed

RPM = (MPH * gear ratio * differential raio * MPH to Inches conversion) / tire diameter

Results:

Maximum MPH per gear
-------------------------------------------------------------
Gear Differential ratio
- 3.9 4.1 4.33 4.44 4.77
1 47 45 43 42 39
2 79 75 71 69 64
3 108 103 98 95 89
4 150 143 135 132 123
5 178 170 161 157 146
6 212 201 191 186 173



RPM @ MPH
-------------------------------------------------------------
30mph
Gear Differential ratio
- 3.9 4.1 4.33 4.44 4.77
1 5693 5985 6321 6482 6964
2 3436 3612 3815 3911 4202
3 2491 2619 2766 2836 3047
4 1797 1890 1996 2046 2198
5 1514 1592 1681 1724 1852
6 1276 1342 1417 1453 1561

45mph
Gear Differential ratio
3.9 4.1 4.33 4.44 4.77
1 8540 8978 9482 9723 10445
2 5154 5418 5722 5867 6303
3 3736 3928 4148 4254 4570
4 2696 2834 2993 3069 3297
5 2271 2388 2522 2586 2778
6 1915 2013 2126 2180 2342

60mph
Gear Differential ratio
3.9 4.1 4.33 4.44 4.77
1 11387 11971 12642 12964 13927
2 6872 7224 7629 7823 8404
3 4982 5237 5531 5672 6093
4 3595 3779 3991 4092 4397
5 3028 3184 3362 3448 3704
6 2553 2684 2834 2906 3122

70mph
Gear Differential ratio
3.9 4.1 4.33 4.44 4.77
1 13285 13966 14749 15124 16248
2 8017 8428 8901 9127 9805
3 5812 6110 6453 6617 7109
4 4194 4409 4656 4775 5129
5 3533 3714 3923 4022 4321
6 2978 3131 3307 3391 3643

I have no idea how bad the formatting is going to be for this, so I have attached a zip containing the .xls file where I have made a little basic calculator. In this simple calculator, you can change all values that are highlighted in yellow and it will automatically update the correct information.

I hope this is helpful to anyone out there considering a differential gear change and if someone finds an issue with any of my calculations PLEASE let me know and I will be sure to update the .xls and this post.

Originally Posted by rotarygod

Why? I don't know. I do know that every RX-8 should come with a 4.10 though as what they have now is worthless and no faster except when it comes to burning up your gas.

Originally Posted by rotarygod

Why? I don't know. I do know that every RX-8 should come with a 4.10 though as what they have now is worthless and no faster except when it comes to burning up your gas.

Originally Posted by Fluid Motorsports

Pulled this from the other thread in my Section.

Thanks Craig for Running the numbers

Originally Posted by chancejat

yeah....that explains why mazda race teams use 5.12s.....

Originally Posted by kersh4w

mazda race teams use 5.12 gears due to the rules. not their own preference.

a fully stock rx8 will lose to a stock rx8 w/ 4.10 in a drag race.

Originally Posted by chancejat

yeah....that explains why mazda race teams use 5.12s.....

Originally Posted by Zeon

Please forgive my nubness, but why do you say the 4.1 rear would beat a 4.44 rear 8? I thought that the lower the final drive the longer the gears were, but acceleration suffered from the lengthening of a gear. Would it be faster just due to the fact that it wouldn't take as many shifts to go the distance?

Originally Posted by rotarygod

I wouldn't say that the 4.10 would be faster in acceleration than the 4.44. If it is slower though, it's not enough to matter and most likely any wheel spin/driver error is more than enough to make up the difference. The 4.44 equipped cars accelerate faster than the 4.77 equipped cars due. Shifting takes time. More is not always better. There comes a point where more is worse and uneccessary. It's somewhere right around a 4.10 or so as I find the 4.44 completely worthless and the 4.77 downright terrible. I'm sorry to all of you RX-8 owners out there but all of your differential ratios suck and your cars drive is a bit substandard as a result. Fortunately you can fix it. I'll keep my 4.10 in my RX-7 anyday. Even with less power and even with only a 5 speed.