Given two of these pieces, how do I figure out the missing one? What is the formula?

For example: I saw one post that said 60mph = 8.3K RPMS in 2nd gear. How was this figured?

I'd like to know since we can't really depend on the speedo when accelerating quickly.

 

Rear axle RPM = Engine RPM / gear ratio / reduction gear ratio / final drive ratio.
Speed = Rear axle RPM * Tire circumference.

If you measured the tire circumference in inches, you'll end up with speed in inches/minute. Just convert units to get miles/hour.

 

[QUOTE]Originally posted by CRitchie
[B]Given two of these pieces, how do I figure out the missing one?

 

There is a slight correction for flattening of the tire as it roll under weight. You can find this correction listed for most tires (figure it from published circumference or diameter versus published revolutions per mile).

 

So using the above formula:

7900 RPMs
2nd Gear Ratio: 2.045
Primary Gear Reduction: 1.160
Final Drive Ratio: 4.100

Tire Circumference of SO2: 78.168 inches

Comes to 60.1MPH

Does that sound right?

 

CRitchie, the number you were after was 57.6mph (in 2nd gear at 7.9k revs).

Though, you don't need to be so comlex to work out what you're after. S2000 reaches these speeds in each gear at 9k revs:
---------------
> > 1st - 69km/h = 43mph
> > 2nd - 105km/h = 65.6mph
> > 3rd - 144km/h = 90mph
> > 4th - 186km/h = 116mph
> > 5th - 223km/h = 139.4mph
> > 6th - 266km/h = 166mph
---------------

Therefore, to work out what speed you're doing at 7.9k revs all you need to do is 65.6 / 9 * 7.9 = 57.6mph.

 

So is the above "complex" formula theoretical? Based on it 9000 RPM in 2nd gear should reach 68.5MPH.

[I'm not trying to be argumentative. I'm just an anal computer type that is very curious about such things.]

Where did you get your numbers from?

 

Under certain condition, the rev limiter can cut off at >9000RPM.

Using the "complex" formula:
MPH/1000 RPM
1)5.0
2)7.6
3)10.6
4)13.5
5)16.1
6)19.3


Due to digital speedometer (sample and hold), the best way to read the instantaneous speed under hard acceleration is based on RPM.
For example if you try to time your 0-60 with stop watch.
Stop the clock when the 2nd gear RPM hit 8000RPM.
You may never see the digital actually display 60mph since (under hard accel) it will be jumping like 47, 55, 58, 64...

[Edited by asin on 05-12-2001 at 07:04 PM]

 

CRitchie, if you know the gear ratios, final drive, revs and tyre size, then you can work out the 'speeds'. I verifyed them myself a while ago and they worked out fine for me. I can't verify your ratios to see if you have the right numbers but your rear tyre circumference is correct. Numbers should add up.

Though, the speeds I quoted are from numerous magazines. A lot of them print the speed/1k revs for every gear and some actually print the speeds exactly ... I'm sure I got some of them scanned here so if you like I'll send it to you.

 

Using these numbers I still get 7900 revs at 60 MPH. What am I missing?

I pulled the ratios from Dan Carney's book and verified them with several sites on the web. They all say the same thing. I used the numbers that I list in a previous post.