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Manually calculate MPH

J

JoshRountree

NAXJA Forum User
Location
Boone, NC
I feel dumb, but I can't get the numbers to come out right. I know I can just plug it into one of the online calcualtors but I'm trying to understand it as opposed to just punching in the numbers.

For example, how would I calculate the MPH if the engine was running 2000 RPM with an AW4 in overdrive and 3.55 axles?

Would it be:

(EngineRPM X AW4Gear) / (DiffGear) = WheelRPM

WheelRPM X TireCircumference = DistancePerMinute

DistancePerMinute X 60 = MPH

----------------------------------------
2000 X 0.75 = 1500
1500 / 3.55 = 422.54
422.54 = WheelRPM

(3.14 X 29in) / 12 = 7.58ft
7.58ft = TireCircumference

422.54 X 7.58ft = 3202.8532 ft/min
3202.8532 ft/min * 60min = 192171.192 ft/hr
192171.192 / 5280 = 36.4 MPH

I'm doing something wrong, this is going to be easy.
 
If your "AW4 Gear" number (0.75) is supposed to represent overdrive, the result of your first calculation should be HIGHER than your engine RPM's (since overdrives result with higher output shaft RPM's than engine RPM's.

You should divide the engine RPM's by the AW4 gear ratio...then it should work out.
 
Flyfisher said:
If your "AW4 Gear" number (0.75) is supposed to represent overdrive, the result of your first calculation should be HIGHER than your engine RPM's (since overdrives result with higher output shaft RPM's than engine RPM's.

You should divide the engine RPM's by the AW4 gear ratio...then it should work out.
Click to expand...

See, I told you it would be easy. Thanks.
 
ok so according to this formula if I did it right 31's and 3.07's with a OD of .75
at 2000rpm I should be doing 80 mph
does this sound right
I know my speedo is off but I didnt think it was that far off
 
Your numbers are about right. Now remember that if you are running an auto and the torque convertor is not locked, the speed will be considerably less.
 
sounds right to me, i'm runnin the same setup, and i hit 2000 rpm while locked up around 72-73, and have calculated that i'm about 10% off

MPETESS10 said:
ok so according to this formula if I did it right 31's and 3.07's with a OD of .75
at 2000rpm I should be doing 80 mph
does this sound right
I know my speedo is off but I didnt think it was that far off
Click to expand...
 
Distance = rate x time

Hit the highway, with a stopwatch. Using the mile markers, drive ten miles, and time how long it takes, at an indicated 60 mph on your speedo. (1 mile will work, but 10 will smooth out fluctuations)

Do a little algebra rate = distance / time

For example: D = R x T 10mi = ? x 10min

Algebra solves the ? to be = 1. You're going 1 mile per minute, or 60 miles per hour. You'd have no speedo error.

So, let's say you time your flying 10 miles at 8 minutes...

10 = X (8) Divide both sides by 8, X = 1.25 That's your correction factor. Multiply 1.25(60) = 75. 10 miles in 8 minutes is 75 mph.

Seems a lot more complicated than it is. Give it a shot.

Steve
 
Last edited:
It seems to me that with an overdrive gear of .75, your tranny would take 1 engine rpm and turn it into 1.333 rpm ( divide 1 by .75 = 1.333)...so 2000 engine rpm become 2666 tranny output shaft rpm (2000x1.333=2666)...so then you divide that by gear ratio to get wheel rpm...and take it from there based on wheel size...

So... I calculate 2000 rpm in overdrive with 3.07 gears and 31" tires right at 80mph
 
That's what I like about my way. It looks kinda screwy, but you get rid if all the other variables (true tire diameter, actual mounted height with weight, tranny ratios, rear end ratios, etc...). The distance is a constant, time is a constant, and the rate is calculated based on those two constants.

Steve
 
Help full tip: I used a GPS hand Held unit to double check my math against the real world. (Mine has a speed indicator) It's a Lowrance I-Fish thats WAAS rated for nautical and aviation navigation. Accuracy is quite good.
 
X2 above
when I was waiting for my correct speedo gear I used the GPS to double check... works great
Garmin Rhino 120
 
for a little more accuracy, look up the revs per mile data for your tire on tirerack.com and plug that into the formula instead of using the tire diameter. Or, as an alternative, you can multiple your results by .95 to allow for your tires rolling radius vs published or calculated size.
 
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