Calculate your Horsepower

  CALCULATE MPH FROM RPM

RPM:  Tire Diameter:  
Diff Ratio:     Gear Ratio:  : 		 MPH: 
Obtaining MPH figures from your rev counter is more accurate than just reading the speedometer
find your cars gear ratios HERE

CALCULATE TYRE DIAMETER

Eg.185/65R13
/  R 		 Diameter
Needed to work out the MPH from revs
After recent trips to a rolling road I decided to  to create a 'DIY'  (cheaper) way of obtaining engine output by calculating horsepower from
the torque produced from a car in motion. This is different from  other horsepower calculators I have seen, as you have to actually test your car!
Have fun and let me know how you get on.

Weight (in pounds please)
MPH Upper
MPH Lower
Acceleration Time
Deceleration Time (in neutral)

Rear Wheel Horsepower

 

This is how you do it, (its easier if someone can assist you)
 
1. Find a flat bit of road accelerate to a  pre defined speed/rpm e.g. 50mph or 4000 rpm. start a stopwatch and at the same time accelerate at full
throttle to a pre-defined speed/rpm e.g. 70mph or 5000rpm. when the pre defined end speed/rpm is reached stop the stopwatch. you now have your Acceleration Time.
 
Note: the car must be in the same gear for the duration of the run.
 
Note: you must take down the lower and upper mph reading even if you are going to calculate your real mph from your rpm. this is so you have reference points for the next step.
 
2. Go back down the same bit of road in the same direction and accelerate to a little above the upper speed in the previous test.  put the car in neutral and coast.  When the speedometer reads the the upper speed of your last test start the stopwatch, carry on coasting in neutral. When the speedometer reads the lower speed of of your test stop the stopwatch, this is your Deceleration Time. if you put you foot on the clutch instead of going into neutral that may give a different result, try it and see.

3. Measure the weight of the car and the driver/passenger. That goes in the Weight bit

Example

Car in 3rd gear accelerates from 4000 to 5000 rpm, speedometer reads 58 and 70  acceleration takes 3.5 seconds
the car takes 8 seconds to slow down from 70 to 58 in neutral
using the rpm, tyre size and gear ratio  to get the real mph we find that the car mph was 53.3 and 66.7 mph
the car weighs in at 2000 pounds including driver and assistant;
 
Weight                                   2000
MPH Upper                                66.7
MPH Lower                               53.3
Acceleration Time                      3.5
Deceleration Time (in neutral)    8
Rear Wheel Horsepower            80.7 between 4000 and 5000 rpm
You can repeat the test at different RPM to see where the maximum power is, for higher RPM second gear may be best so that you stay within the speed limits.
 
 

 

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