how fast was i going
TECH Regular
Joined: Jan 2005
Posts: 475
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From: Orlaanndoooo
Back on topic.............
You can't use a gear/speed calculator to see how fast your going.
To really calculate it, it's way beyond the scope of simple forum math.
How the calculators work is by the following:
The engine is turning a certain RPM, and the tires are being turned at a multiple slower. To calculate that multiple, divide the engine RPM by your transmission gear, and then by your rear end gear. That is your tire RPM, then using the diameter of your tire, you calculate speed.
Pretty simple, right?
Problem is that that there is driveline loss, wind friction, tire friction, tire rolling resistance, etc. These intangibles really add up.
Case in point, an M6 at redline in 6th is supposed to be going something like 250mph, which we all know is bunk.
Only way to truely calculate it out is to use your car's torque curve, entered into a spreadsheet to calculate force generated at the tire/pavement contact surface. From there, using all the drag/friction factors (which are all toss-up since brands of tires all have different characteristics), you can approximate your speed.
Either that, or you could get some ***** and look at the speedo while trying it out.....
You can't use a gear/speed calculator to see how fast your going.
To really calculate it, it's way beyond the scope of simple forum math.
How the calculators work is by the following:
The engine is turning a certain RPM, and the tires are being turned at a multiple slower. To calculate that multiple, divide the engine RPM by your transmission gear, and then by your rear end gear. That is your tire RPM, then using the diameter of your tire, you calculate speed.
Pretty simple, right?
Problem is that that there is driveline loss, wind friction, tire friction, tire rolling resistance, etc. These intangibles really add up.
Case in point, an M6 at redline in 6th is supposed to be going something like 250mph, which we all know is bunk.
Only way to truely calculate it out is to use your car's torque curve, entered into a spreadsheet to calculate force generated at the tire/pavement contact surface. From there, using all the drag/friction factors (which are all toss-up since brands of tires all have different characteristics), you can approximate your speed.
Either that, or you could get some ***** and look at the speedo while trying it out.....
Now, if he had said "I have xxx hp, y.yy rear axle, z.zz trans gear, and aa.aa diameter tire, what's my top speed?"...then you would be correct in saying it is not simple math.
things like tire friction, wind resistance, and driveline loss relate to potential speed.
if you have a known rpm, known gear ratio and a known tire size you will be able to find out exactly how fast you are going.
if you have a known rpm, known gear ratio and a known tire size you will be able to find out exactly how fast you are going.
if its an automatic, a percentage must be thrown in to calculate for engine speed vs driveshaft speed. the torque converter is not 100% efficient.
given converter slippage my car only runs 143mph top end, where as a calculator has it going 152. small differences, but that is just another variable that will change in every calculation.
TECH Fanatic
Joined: Sep 2004
Posts: 1,124
Likes: 1
From: Las Vegas
Mechanically...
(RPM x Tire Dia x 0.002975) / Final Drive Ratio
I take it you are running Ecsta V700's ?? Because I dont know of any other 305/35/18 on the market currently. 25" Dia...
So by the math, that would be 191.8187 MPH.
... Minus inclusive parasitic losses and the CD of the chassis. So figure 15 to 18% for a typical AOD.
Funny thing is... a simple change in rear tire width will make a huge difference in the CD of a chassis.
(RPM x Tire Dia x 0.002975) / Final Drive Ratio
I take it you are running Ecsta V700's ?? Because I dont know of any other 305/35/18 on the market currently. 25" Dia...
So by the math, that would be 191.8187 MPH.
... Minus inclusive parasitic losses and the CD of the chassis. So figure 15 to 18% for a typical AOD.
Funny thing is... a simple change in rear tire width will make a huge difference in the CD of a chassis.
TECH Regular
Joined: Jan 2005
Posts: 475
Likes: 0
From: Orlaanndoooo
They observed 5,300 rpm in 6th gear given 4.11 r & p and 305/35/18 tires...a specific calculation. Horsepower, driveline loss, and drag coefficient have already done their battle, and are not involved in the answer to their question.
As pointed out earlier, they are not asking for a potential top speed, nor are they asking for their horsepower. Had they asked for either of these, the equation would be complicated by all the neat little forces acting against each other.
