Formula for vehicle speed derived from.....

[Chepsk8]

What was the formula to derive vehicle speed from RPM?

I know it has something to do with gears, tire size, etc...

thanks!

[Ray]

Try here: Wallace Racing - Automotive Calculators

A bunch of different calculators.

Ray

[smithtlw]

Tire circumference (inches) = pi * Tire Diameter (D)
GR = Rear Gear Ratio
TR = Transmission Ratio

Ground Speed = (pi * D * RPM) / (GR * TR) - units are inches/minute


There are 60 minutes/hour and 1mile = 63360inches

Ground Speed (miles/hr) = (pi * D * 60 * RPM) / (63360 * GR * TR)


(pi * D * RPM)
--------------------
(1056 * GR * TR)


Final Answer:

Ground Speed (MPH) =

(D * RPM)
-------------------------
(336.135 * GR * TR)

Example:

for a car with a transmission in 5th gear with a ratio of .82 at 3,000 RPM with rear tire diameter of 26 inches and rear end ratio of 3.73 to 1 :

Speed@3,000 = (26 * 3000) / (336.135 * 3.73 * .82)

Speed@3,000 = 75.87 MPH

[Tickwomp]

Thanks for the formula! I had fun playing with figures to see what my calculated speeds would be at different RPMs and gear.

Looks like my 1st gear speed at 6200 RPM will be 46 MPH and my 5th gear will at 6000 RPM will be 160 MPH. Can't wait!!

Originally Posted by smithtlw

Tire circumference (inches) = pi * Tire Diameter (D)
GR = Rear Gear Ratio
TR = Transmission Ratio

Ground Speed = (pi * D * RPM) / (GR * TR) - units are inches/minute


There are 60 minutes/hour and 1mile = 63360inches

Ground Speed (miles/hr) = (pi * D * 60 * RPM) / (63360 * GR * TR)


(pi * D * RPM)
--------------------
(1056 * GR * TR)


Final Answer:

Ground Speed (MPH) =

(D * RPM)
-------------------------
(336.135 * GR * TR)

Example:

for a car with a transmission in 5th gear with a ratio of .82 at 3,000 RPM with rear tire diameter of 26 inches and rear end ratio of 3.73 to 1 :

Speed@3,000 = (26 * 3000) / (336.135 * 3.73 * .82)

Speed@3,000 = 75.87 MPH

[rick8928]

Looks like my 1st gear speed at 6200 RPM will be 46 MPH and my 5th gear will at 6000 RPM will be 160 MPH. Can't wait!!

If your car sees 160MPH, it's going to be heading down vertically - with a very abrupt stop in its future! lol

[Cone Basher]

Rick: Be nice, that's theoretical top speed which we all know is aero limited. My Coupe specs out over 200 but it will take over 600 hp to go that fast and I do not have that. The new GT 500 Mustang claims +200 but it packs over 600 hp.
CB

[trevor]

mine sees 161mph at VIR. It takes a long time from 140-160. To 140 is nothing.

[Thumper135]

I saw the episode of TOP GEAR “UK Version” where they took the Bugatti Veyron to its limits and they were talking about the formulas for horse power just to get through the AIR to reach really high speeds……… could that be next?


It just boggles the mind………………

[Tickwomp]

Not trying to start anything, but that seems like a pretty bold statement when you don't even know what I'm running.

Hey Gord Levy, can you pipe up with what my motor made? And yes, I know the difference between theorectical top end and reality, especially when you consider the testicular fortitude it would take to bottom this thing out.

If your car sees 160MPH, it's going to be heading down vertically - with a very abrupt stop in its future! lol[/QUOTE]

[Gordon Levy]

You car with that engine I built is very capable over 160+. It's well over 600hp. The question is whether you have the intestinal fortitude to drive it that fast.

[Chepsk8]

Thank you for the formulas and extra info!

I am trying to figure out what my roadster is doing at 6200 RPM (Limiter) with 3.55 gears and 275/17 tires

[cwhoofgator]

Tell me your final drive ratio in the trans and tire revs' and I'll give you what it COULD run if you have the HP..

[tad240]

Originally Posted by Gordon Levy

The question is whether you have the intestinal fortitude to drive it that fast.

Got that right - at least in my case. About 135-140 for me at VIR and my intestinal outlet was puckered mighty tight. I kept having thoughts and visions of the windshield flying off. (Not sure if that would really happen but it definitely flexes at speed)

You really get the true sensation of speed in these cars.

[Ray]

Use this: Wallace Racing - Calculate Rear Tire Diameter needed

to input tire size into this:RPM/MPH From Gear Ratio Calculator by Wallace Racing

Ray

[trevor]

dan, what gear are you in, I am guessing 4th? 133.5 mph

[Chepsk8]

Originally Posted by cwhoofgator

Tell me your final drive ratio in the trans and tire revs' and I'll give you what it COULD run if you have the HP..

I leave it in 4th (1:1)

[Chepsk8]

Originally Posted by tad240

Got that right - at least in my case. About 135-140 for me at VIR and my intestinal outlet was puckered mighty tight. I kept having thoughts and visions of the windshield flying off. (Not sure if that would really happen but it definitely flexes at speed)

You really get the true sensation of speed in these cars.

Heck yeah! Mine at speed is a wild ride

[cwhoofgator]

Originally Posted by Chepsk8

I leave it in 4th (1:1)

Make and size of tires?

[smithtlw]

Formula for "nominal" tire diameter based on the specs:

Dia (in) = (Nominal Width/25.4) * (Aspect Ratio / 100) * 2 + Wheel Diameter

Example:

Diameter of a 315/35/17 tire = (315/25.4) * (35/100) * 2 + 17
Dia = 12.402 * .35 * 2 + 17
Dia = 25.7 inches

Obviously, different tire models are likely to vary from this number a bit and the best would be to accurately measure your rolling radius to accommodate for the squish.

[Chepsk8]

oops!

Kuhmo V710 275/45R17

[trevor]

133.5 mph at 6200rpm with 275/40-17s

[Laser Stan]

Here's a spreadsheet that will let you insert tire size, trans ratios, rear end ratio and then calculate speed from rpm in any gear, not accounting for HP limitations.