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Old 04-19-2006, 03:19 PM   #1
jacobsen1_
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Default Speed by RPM and gear?

How do you determine how fast a car can go by it's gears and the engine's RPMs? I have a 2004 WRX with a 5MT so my gears are as follows(from the sticky/FAQ):

Manual transmission gear ratios:
1st - 3.454
2nd - 1.947
3rd - 1.366
4th - 0.972
5th - 0.738
reverse - 3.333
final drive ratio - 3.90

I checked the stickies/faqs, so if I missed it sorry.

Thanks,
Ben
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Old 04-19-2006, 03:55 PM   #2
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o divide the engine RPM by the product of the gear ratio and final drive ratio to get the wheel RPM

o multiply the wheel RPM by the tire circumference to get rolling speed in inches per minute or meters per minute

o convert to mph or km/h

e.g. 7000rpm in 2nd with 3.9 FD => 7000/(1.947 x 3.90) = 922rpm at wheel; tire diameter 0.63m x pi => tire circumference 1.98m so the wheel speed is 922 x 1.98 = 1824m per minute x 60/1000 = 109.5km/h (about 68mph)
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Old 04-19-2006, 04:25 PM   #3
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Quote:
Originally Posted by UkNuck
o multiply the wheel RPM by the tire circumference to get rolling speed in inches per minute or meters per minute

e.g. tire diameter 0.63m x pi => tire circumference 1.98m
Is there a way to calculate the tire diameter from the tire size, IE 225/45 17"?

The wheel itself has a diameter of 17" I know, but how much do you add for the sidewall using the 225 mm width and the 45 "profile"?

Thanks,
Ben

edit:

Quote:
Originally Posted by tirerack
Example: 185/60R14 85H or 185/60HR14

185mm x .60=111mm
111mm x 2=222mm
222mm + 355.6mm(14")= 577.6mm or 22.74"

The first number is the width of the tire in millimeters, measured from sidewall to sidewall. To convert to inches, divide by 25.4 In the example above, the width is 185mm or 7.28".

The second number is the aspect ratio. This is a ratio of sidewall height to width. In the example above, the tire is 7.28" wide, multiply that by the aspect ratio to find the height of one sidewall. In this case, 185x0.60=111mm or 7.28"x0.60=4.36".

The last number is the diameter of the wheel in inches.

To figure the outside diameter of a tire, take the sidewall height and multiply by 2,(remember that the diameter is made up of 2 sidewalls, the one above the wheel, and the one below the wheel) and add the diameter of the wheel to get your answer.
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Old 04-19-2006, 04:32 PM   #4
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So for my car it's:
Example: 225/45 17
225mm x .45=101.25mm
101.25 x 2=202.5mm
202.5mm + 431.8mm(17")= 634.3mm or 24.97244"
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Old 04-19-2006, 04:37 PM   #5
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TLAR (That Looks About Right) - although tire sizing is somewhat nominal I think - if you want to be more precise you can use the manufacturer's spec for your particular tire (they often give the revs/mile directly but certainly should give the diameter).
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Old 04-19-2006, 04:50 PM   #6
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http://forums.nasioc.com/forums/showthread.php?t=665334

Bottom of the first post...two links.
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Old 04-19-2006, 04:55 PM   #7
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jacobsen1, For comparison, I found an old Subaru brochure that contained a Car&Driver road test of a 2002 WRX sedan that had the same gears and final drive as yours: mph/1000rpm, mph @ 7000 rpm(max. test rpm)
1st gear=5.3, 37 2nd gear=9.5, 66 3rd gear=13.5, 94 4th gear=18.9,133
5th gear=24.9, 140(5600rpm)
tires size=205/55-16 rim=6.5x16 inch not 225/45-17 like yours.
C&D claimed top speed in 5th was drag limited
Hope the figures give you some fun calculating.

Last edited by 7th Subaru; 04-19-2006 at 05:16 PM.
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Old 04-19-2006, 10:52 PM   #8
Legba
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All I know is my speedo is off by 10mph at 80! It says 80 and I'm really doing ~70-71 according to my GPS which is accutate to .1mph.
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Old 04-20-2006, 10:01 AM   #9
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That's a lot of error. I would take it back to the dealer if you're on stock size tires. I've got 225/40/18's on my 2002 WRX with a 6MT and my GPS says that my speedo is spot on accurate. When I have my winter tires on it's off by about 1.5 mph.
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Old 04-20-2006, 03:08 PM   #10
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Drive + pencil?
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