RPM calculator with drag coefficient?

[mike_ekim1024]

I've found multiple gear rpm calculators and they're useful, but they calculate 62mph max in 2nd, but other sources quote 58. I'm assuming there's things like drag that aren't factored in?

[StormTrooper]

2nd gear is not drag limited.....

Might be manual vs auto.

[troek]

the MT lower spec 86s in japan have a longer final drive as well.

[Porsche]

Originally Posted by mike_ekim1024

I've found multiple gear rpm calculators and they're useful, but they calculate 62mph max in 2nd, but other sources quote 58. I'm assuming there's things like drag that aren't factored in?

Drag coefficient is not a factor in gear-speed-rpm calculations. Drag comes into play when we ask whether the car with this gearing and these tires can actually attain the CALCULATED speed.

For example, on OEM Michelin tires the calculated speed in 2nd gear at 7400 rpm = 58.645 mph. The gears are not variable, they're set. However, the actual tire diameter under load will vary. The figure above for 2nd gear is calculated employing Tire Revolutions Per Mile = 844. This number comes from the Michelin site and is mirrored in the tech specs for this OEM tire on the Tire Rack site.

If the tire makes 844 revs/mi, then we can calculate the real world rolling diameter, and we will find D=23.89589".

Research the make, model, and size of tire you have mounted and note the revs per mile (rpm).

I have found that employing the Michelin spec of 844 rpm works out accurately for my stock car. The calculated speeds for a given rpm are what I see on the speedometer on the road.

Calculated top speed in 6th gear = 167.284 mph on the OEM tires.

Here is where drag becomes an issue; the car cannot pull 7400 rpm in 6th in the real world. It would require more power.

The OEM BRZ/FR-S can pull redline, however, in all of the other gears.

Note that some online calculators may not be programmed correctly and will yield incorrect answers. I've observed this on some bank loan calculators of all things!

[mike_ekim1024]

Originally Posted by Porsche

Drag coefficient is not a factor in gear-speed-rpm calculations. Drag comes into play when we ask whether the car with this gearing and these tires can actually attain the CALCULATED speed.

For example, on OEM Michelin tires the calculated speed in 2nd gear at 7400 rpm = 58.645 mph. The gears are not variable, they're set. However, the actual tire diameter under load will vary. The figure above for 2nd gear is calculated employing Tire Revolutions Per Mile = 844. This number comes from the Michelin site and is mirrored in the tech specs for this OEM tire on the Tire Rack site.

If the tire makes 844 revs/mi, then we can calculate the real world rolling diameter, and we will find D=23.89589".

Research the make, model, and size of tire you have mounted and note the revs per mile (rpm).

I have found that employing the Michelin spec of 844 rpm works out accurately for my stock car. The calculated speeds for a given rpm are what I see on the speedometer on the road.

Calculated top speed in 6th gear = 167.284 mph on the OEM tires.

Here is where drag becomes an issue; the car cannot pull 7400 rpm in 6th in the real world. It would require more power.

The OEM BRZ/FR-S can pull redline, however, in all of the other gears.

Note that some online calculators may not be programmed correctly and will yield incorrect answers. I've observed this on some bank loan calculators of all things!

Oh wow, this is gold thank you! Might seem basic, but I didn't realize using diameter vs revs per mile would make a difference. These calculator sites should mention the accuracies of the input types.

[mike_ekim1024]

There's been other posts regarding this, but here's some numbers on auto final drive 4.56 and 4.88. I used revs per mile now to be more accurate.

Gear   MT      AT      4.10      4.56      4.88
1st    3.626   3.538   102.49%   92.15%    86.11%
2nd    2.188   2.060   106.21%   95.50%    89.24%
3rd    1.541   1.404   109.76%   98.69%    92.21%
4th    1.213   1.000   121.30%   109.06%   101.91%
5th    1.000   0.713   140.25%   126.10%   117.84%
6th    0.767   0.582   131.79%   118.49%   110.72%

Final   Ratio   MPG
4.10    1.00    34.00
4.56    1.11    30.57
4.88    1.19    28.57

Final  Gear   RPM    Speed
4.10   2nd    7400   62
4.10   6th    2200   65
4.10   6th    2300   70
4.10   6th    2500   75
            
4.56   2nd    7400   56
4.56   6th    2400   65
4.56   6th    2600   70
4.56   6th    2800   75
            
4.88   2nd    7400   52
4.88   6th    2600   65
4.88   6th    2800   70
4.88   6th    3000   75

I bought the 4.56 as a tradeoff on MPG / acceleration. If I was just going for performance 4.88 or even higher would be awesome. It's a daily driver with spirited mountain drives and some autox.

With 4.56, the first 2 gears are lower than manual and about the same at 3rd, while with 4.88 the first 3 are lower and about the same at 4th. 4.56 is about %11 difference overall, while 4.88 is 19%.

Now I just need someone to install it

[czar07]

Just out of curiosity what is the 0-58 time for a stock car? I know 60/62mph takes a bit longer due to the gear change, but 58 is pretty much 60..

[Porsche]

Originally Posted by mike_ekim1024

Oh wow, this is gold thank you!

You're most welcome!

Might seem basic, but I didn't realize using diameter vs revs per mile would make a difference. These calculator sites should mention the accuracies of the input types.

It might help if they would suggest that people obtain the actual rolling diameter of the tire when mounted on a vehicle. That squishes the tire down, resulting in a smaller effective radius/diameter than when the tire is completely unloaded.

The diameter specs I see for tires appear to be for the tire mounted on a rim, but not compressed under load on a car. The difference in diameter is significant.