CALCULATION OF MPH PER 1000 RPM or
How fast are we really going?
On SCCA drives I have been noticing how ABP 102 seems to be undergeared compared to other Spridgets.
Is it my over-developed sense of mechanical sympathy that protracted travel along the expressway makes me shudder in the same way that scraping fingernails down a blackboard does?
Or is the tacho wrong - apparently only possible by a large factor, and therefore obvious, if it's electric?
Just what speed would I be doing between 4000 and 6000 revs?
With a wavering speedo needle that requires the pilot to take an average of each arc to approximate the speed, it was time to see what impact tyre size has made to the engine speed/road speed equation. Was the 15.5 MPH per 1000 RPM quoted in the February 1968 Modern Motor MG Midget road test correct for ABP 102 with its wider, lower profile tyres?
Accuracy was the order of the day so I used the formula in Daniel Stapleton's book, `How to Power Tune your MG Midget and A-H Sprite' for calculating road speed based on final drive ratio and tyre size.
Formula
60 000
RAR x WRPM
RAR = rear axle ratio
WRPM = wheel revolutions per mile
Calculations
The RAR is 4.222, this being the original differential fitted to ABP 102.
To calculate wheel revolutions per mile, first find the rolling circumference of the tyre - the distance travelled in one revolution of the wheel with the weight of the car on it.
To do this I drew a chalk mark across the tyre tread and while fitted to the car, rolled the wheel - and the car - along a piece of kitchen paper placed on the ground leaving two impressions of the chalk line. The distance between these was 67¾ inches or 1722mm on 165/70 x 13 Falken FK 07U tyres.
Then stick this into the formula -
Therefore WRPM = 5280 x 12
67¾
= 63360
67¾
Therefore WRPM = 935.2
Compare this with the WRPM for different tyres from the Stapleton book - 145x13 = 936 and 155x13 = 914.
MPH per 1000 RPM using the Stapleton formula
= 60 000
4.222 x 935.2
= 60 000
3948.4
= 15.196
Rounded to 15.2 MPH per 1000RPM
For the following tacho readings, the road speeds should be as follows
RPM in 4th gear
|
MPH
|
KPH
|
2500
|
38
|
61
|
3000
|
46
|
73
|
3500
|
53
|
86
|
4000
|
61
|
98
|
4500
|
68
|
110
|
5000
|
76
|
122
|
5500
|
84
|
135
|
6000
|
91
|
147
|
6500
|
99
|
159
|
So, the conclusions are -
fitting bigger tyres like 155x13 without a lower profile slightly increases road speed - at 5000 RPM it is 78 MPH or 126 KPHfitting a 3.9 rear axle ratio would increase road speed to 16.5 MPH per 1000 RPM and at 5000 RPM would equate to 83 MPH or 133 KPH
without changing either, it could help not to have an operative tacho!
since all these speeds are illegal on NSW expressways, it's academic in any event!
Text and image copyright Paul Orton.
