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Engine speed to Vehicle speed

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Please suggest me the formula conversion from engine speed to vehicle speed. What are all the factors I need to consider while doing the conversion.
 

Les Jones

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You will need to know the ratio between engine RPM and the wheel RPM in each gear. You will also need to know the effective diameter of the driven road wheel so that you can can calculate its circumference (So you know how far the car travels for one revolution of the wheel.)

Les.
 

JimB

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If the vehicle has an automatic gearbox, the associated torque converter (used instead of a clutch) will make the calculation impossible.

JimB
 

cowboybob

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To expand on Les' post and assuming a manual gearbox:

Engine RPMs times transmission gear(s) ratio times (if used) differential ratio = effective Driven Wheel(s) RPM (DWRPM). DWRPM times wheel circumference times 60 = MPH or KPH (depending on your choice of English vs. metric system).

For example (English):
Engine RPMs = 2,000
Transmission Ratio (2nd gear) = 2:1 (0.5)
Differential Ratio = 3.11:1 (0.322)
Driven Wheel diameter = 26" (81.68" circumference, or 6.8 feet)

2,000 (0.5 X 0.322) = 322 DWRPM.

DWRPM X 6.8' = distance driven of 2,189.6' in 1 minute, or 131,376' in 1 hour, or 24.9 MPH. If the transmission were in a 4th gear (with a 1:1 ratio), then the car would be traveling 49.8 MPH.

If the vehicle has an automatic gearbox, the associated torque converter (used instead of a clutch) will make the calculation impossible.
True enough, but if it is assumed that the lock-up clutch is engaged, the math holds. And if not, the particular torque converter speed ratio values can be applied to give "close enough" DWRPMs.
 

Nigel Goodwin

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Of course this thread is particularly pointless, as this isn't how measure speed :D

As we all know you measure wheel rotation, either directly at one of the wheels, on the propshaft/drive shaft, or most common the output shaft in the gear box.
 

cowboybob

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Please suggest me the formula conversion from engine speed to vehicle speed. What are all the factors I need to consider while doing the conversion.
(My emphasis).
Of course this thread is particularly pointless, as this isn't how measure speed :D

As we all know you measure wheel rotation, either directly at one of the wheels, on the propshaft/drive shaft, or most common the output shaft in the gear box.
Nigel, you and I differ, apparently, on interpreting the OP's requested information :D.

And I'm not sure that defining an ETO member's query as "pointless" is in this forum's best interest.
 

MaxHeadRoom78

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Even measuring the wheel rotation can introduce error as used to advantage by low tire pressure detection systems.
Max.
 

JimB

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The MG PA, a small sports car from 1934/35 did not have a dedicated speedometer.

The tachometer was also calibrated with the road speed in 3rd and 4th gear.

JimB
 

crutschow

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Using engine speed is a valid method for determining engine speed for a car with manual transmission albeit more complicated.
It just requires a circuit to provide the proper conversion factor for each gear (and indicate zero or indeterminate speed when the car is not in gear and/or the clutch is not engaged, perhaps after a suitable pause to allow time for a shift).
 

Nigel Goodwin

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Using engine speed is a valid method for determining engine speed for a car with manual transmission albeit more complicated.
It just requires a circuit to provide the proper conversion factor for each gear (and indicate zero or indeterminate speed when the car is not in gear and/or the clutch is not engaged, perhaps after a suitable pause to allow time for a shift).
And what would be the point? - far easier to do it the way it's always been done.
 

MaxHeadRoom78

Well-Known Member
Since the advent of the electric automobile where now often there is a traction motor on each wheel, it has to be done at the wheel level in order to detect wheel slip or maintain synchronism, locomotives have done it for many decades.
Max.
 

Nigel Goodwin

Super Moderator
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Since the advent of the electric automobile where now often there is a traction motor on each wheel, it has to be done at the wheel level in order to detect wheel slip or maintain synchronism, locomotives have done it for many decades.
Max.
I believe some modern cars use the ABS sensors to detect the speed?.
 

Diver300

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I believe some modern cars use the ABS sensors to detect the speed?.
I think it is all, or nearly all, that do that now.

ABS has been a legal requirement since 2004 in the UK, and that needs a speed sensor on each wheel. With those there anyhow, there isn't much point in having any other speed measuring device.

Some cars mix all the wheel speeds to give what is shown to the driver, but some just show the speed of one wheel.

Many years ago, the air-cooled rear engined VWs had a speedo running from one front wheel, as that was closer to the driver than the final drive.
 
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