What is the formula to go from axle RPM to MPH? or is there one?
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You need to know several items:
Tire diameter in inches (ex. 245/50-16 = 25.6")
Rear axle ratio (ex. 4th Gen M6 = 3.42)
Tranny gear ratio
Assuming you are in your direct drive gear (1:1):
( 336.1 X AXLE RATIO X MPH ) / TIRE DIAM = RPM
If you are not in a direct drive gear, multiply the RPM results above X TRANNY GEAR RATIO
Example:
( 336.1 X 3.42 X 70) / 25.6 =
( 80,462) / 25.6 = 3,143rpm
In a T56 with a 0.50 6th gear ratio:
3,143 X .50 = 1572rpm
With an automatic tranny, you have to factor in convertor "slip" unless you know you are locked up in the direct drive gear.
There are also some online "gear calculators" that can make it easier.Fred
381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC engine management - 8 LS1 coils - 58mm TB - 78# injectors - 300-shot dry nitrous - TH400 - Gear Vendor O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor -
i'm trying to help a friend, i know the axle RPM:
2500 @ engine
6.72 diff gear [2.5 ton rockwell]
54" diameter tire
1:1 gear in trans [who needs OD with those honking tires?]
2500/1 = 2500/6.72 = 372.023809523
tire circumfrence: 169.6464 inches
I want RPM to MPH, i think you posted MPH to RPM.
so, the tire does 373 rotations per mile
the axle is at 372 RPM's.Comment
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BTW, you divide by the gear ratio, not multiply. Thats why its called Overdrive, the driveshaft goes faster than the engine. i thinkComment
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If you're running 3500rpm at 70mph in 1:1, and you put it in 0.50:1 overdrive (6th gear in a T56), your engine rpm will drop by 1/2...... Your driveshaft rpm doesn't change when you shift gears, its directly connected to the wheels via the ring and pinion. Only the engine rpm changes... and it drops with an O/D gear ratio.
3,500 X 0.50 = 1750rpm.
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Going back to your specific problem, if you know your tire rpm (372), and you know your tire circumfrence (169.6"), its simply a matter of multiplying the two numbers together, and doing the units conversion:
372 revs/min X 169.6"/rev = 63,112 inches/min
63112 X 60 = 3786741 inches/hour
3786741 / 12 = 315562 ft/hour
315562 / 5280 = 59.77 miles/hour.
Or:
(Wheel rpm X tire circumfrence) / .000947 = MPHFred
381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC engine management - 8 LS1 coils - 58mm TB - 78# injectors - 300-shot dry nitrous - TH400 - Gear Vendor O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motorComment
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thanx, i'll write that down somewhere and keep it...like on a website.
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