Formula for calculating theoritical redline speed ???

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E9KNZ047

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Something that has been puzzling me for awhile and perhaps other also. Is there a generally accepted formlua for calculating the 'theoritical' speed at redline for an E9 or any car. Rather than risking life and limb on publics roads to determine something we all like to consider our cars maybe capable of is it possible to take a RPM reading and MPH (or KPH) reading at a specific point (100kph ??) and considering other factors (diff ??, gearbox) to get a reasonably accurate indication of what the speed is at redline.
Figure there must be but thought I would ask others better informed.
 
At a guess

Given
RPM= Engine revolutions per minute
GR= Gear ratio of selected gear at the gerabox, eg 1st, 2nd, 3rd, 4th
PSPRPM= Calculated Prop shaft RPM
DR=Diff Ratio
RC=Rolling circumference


Speed = (RPM * (GR)* (DR)*(RC))*60

For example

Max RPM= 6500
Top Gear ratio = 1:1
Diff Ratio= 3.25:1
RC= 15" rims with tyres with a height of 3.5 inches.
RC = Diameter of 15+3.5+3.5 = 22inches*PYE( 3.1415926535897932384626433832795)
RC= 91.115038378975451246178154432149inches :roll:
Sooooo

Speed= (6500*1*(1/3.25)*91)
Speed= (6500*1*0.307*91)
Speed= 181590.5 inchs per minute!!
Speed= 2.866012 miles per minute
Speed= 171 miles an hour! :lol:

Think that's right :roll:
Malc

Ops always was rubbish at maths!
 
You must correlate rpm of engine to rpm of wheels times circumference of wheels. This relationship will of course change depending on what gear you're in.

For example- a 1.0 (4th) gear ratio x 3.25 diff= 3.25 overall ratio. If engine turns at 6,000 then wheels will spin at 6,000/3.25= 1846 RPM

If you are running 195 70 14's the outside diameters are 25" (varies slightly from manufacturer). Circumference is 25 X 3.1416= 78.54" Speed is 78.54 X 1846 = 144,984 in/min. MPH would be- ((144,984/12)/5280))(60)= 137 MPH

This does not account for rolling efficiency of tires. I'd guess that loss is between 1-2% on average.
 
Here is my naive understanding, I am sure I am overlooking all kind of second order effects.

For a manual transmission car the speed to RPM ratio should be constant
at a given gear, so if I see that I do 80MPH @3000 RPM, then it would
do 160MPH @6000 RPM.

The question is if the engine can get me to that speed at all. I don't
remember when the power band goes down, or where all the
friction forces dominate.

On an automatic the ratio is not constant because of the viscosity coupling
of the torque converter (unless it has the lock feature of the 4 speed ZF).
 
Arde has it

You wanted "theoretical" max speed.

It will be twice your road speed when in top gear, at half of max rpm.

Example: if redline is 6500 rpm and my car goes (say) 65 mph in fifth gear at 3250 rpm, it will top out at 130 mph (twice as fast) at 6500 rpm twice as fast).

This neglects possible speedo error or change in tire circumference. Inspect your tires (and brakes!) before your land speed record attempt. And put me in your will for the remains of the coupe, 'kay?

And half max speed is easy enough to achieve on the highway.

Also note that the overdrive fifth has a ratio of 0.81 while 4th is 1.00, so for a given rpm, your speed in fifth will be 19 percent higher if fifth if the rpm is unchanged.

But I don't think most stock motors will pull to 135 mph any more.
 
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