# the speedometer on an automobile measures the rotational speed of the axle and converts that to a linear speed of the car, assuming the car has 0.62-m-diameter tires.

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## The speedometer on an automobile measures the rotational spe

Find step-by-step Physics solutions and your answer to the following textbook question: The speedometer on an automobile measures the rotational speed of the axle and converts that to a linear speed of the car, assuming the car has $0.62-\mathrm{m}-$diameter tires.What is the rotational speed of the axle when the car is traveling at $20 \mathrm{m} / \mathrm{s}(45 \mathrm{mph}) ?$.

### Question

The speedometer on an automobile measures the rotational speed of the axle and converts that to a linear speed of the car, assuming the car has

0.62-\mathrm{m}-

0.62−m−diameter tires.What is the rotational speed of the axle when the car is traveling at

20 \mathrm{m} / \mathrm{s}(45 \mathrm{mph}) ?

20m/s(45mph)?

### Explanation

Verified

Given the diameter , the radius is given by:

r = \dfrac{d}{2}= \dfrac{0.62}{2} = 0.3 \, \mathrm{m}

r= 2 d = 2 0.62 =0.3m

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## The speedometer on an automobile measures the rotational speed of the axle and converts that to a linear speed of the car, assuming the car has 0.62

Answer: I can't say for European metric but in the USA the speedometer is: Speedometer cable rotates at a rotational speed of 1000 rpm to equal 60 Mph. You get that by: Tire circumference (in inches). TC = pi * diameter in inches, that is the ground to the top of the tire ( loaded weight) (loa...

The speedometer on an automobile measures the rotational speed of the axle and converts that to a linear speed of the car, assuming the car has 0.62-m-diameter tires. What is the rotational speed of the axle when the car is traveling at 26m/s?

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1 Answer Russell Stringfield

, former Ford Parts and Service at Car Dealerships (1969-1999)

Answered 1 year ago · Author has 112 answers and 39.3K answer views

I can't say for European metric but in the USA the speedometer is:

Speedometer cable rotates at a rotational speed of 1000 rpm to equal 60 Mph.

You get that by:

Tire circumference (in inches). TC = pi * diameter in inches, that is the ground to the top of the tire ( loaded weight) (loaded weight will be true to the vehicle).

Axle rotations per mile: AR = (5280 * 12) / TC

Driveshaft rotations /mile: DR = final drive ratio * AR

Speedometer driven gear rotations: DGR = DR * number of teeth on spedometer driven gear (range from 5 to 8)

Speedometer cable rotations: SCR = DGR/ number of teeth on driven gear.(range from 15 to 22) (this is a part of the output shaft in automatic transmissions requiring a complete transmission disassembly and rebuild while it is a simple gear replacement with a manual transmission.)

The goal is to change the number of teeth on the last one or two inorder to get as close to 1000 as possible.

So changing your tire size even a minute amount will amount into a sizeable error in your displayed speed in your speedometer.

I actually developed a computer program using a Cimodore 64 back in the 80’s to find correct gearing combinations because people changed tire sizes and gear ratios but the Ford books were only concerned with “factory specifications."

I hope this helps as the math doesn't change for the RPM speed just the calibration markings on the face of the spedometer itself.

306 views Related answers Related Answer Richard Muller

, Prof Physics, UCBerkeley, author of "Now—The Physics of Time" (2016)

Answered 6 years ago · Author has 2.3K answers and 193.4M answer views

How do speed bumps decrease the speed of a car?

They create an incentive for going slow. When most people go fast over a speed bump, they regret it. Of course, if you enjoy being thrown out of your seat, and like the sound of your car being put through a potentially damaging shaking, then that incentive doesn't exist.

I once found that even going well below the speed limit over a speed bump on a motorcycle almost caused me to lose control. Speed bumps are not designed for motorcycles, except to the extent that when you see one, you can often go around the edges.

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Related Answer Steve Baker

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Damon Resnick

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Arbaz Malik

, Ph.D. Physics, University of Illinois at Chicago (2025) · Author has 25.3K answers and 124.9M answer views

How do speedometers measure speed so accurately?

Old-school speedometers really didn’t measure speed very accurately at all.

They used a Bowden cable with one end connected to the back end of the gearbox (usually) - or perhaps to one of the wheels. As that cable rotated, it spun a magnetic contraption that moved the needle on the dial against a spring. Not very accurate at all.

These days, most cars have wheel rotation counters - that count the number of rotations of each wheel. This makes it easier to do things like traction control, cornering assist and tire pressure monitoring. So the car computer counts the number of revs each of the four

Related Answer Jamil Nassar

, Owner General Manager at Nassar Auto Inc (2010-present)

Answered Dec 24, 2021 · Upvoted by

Guy Geens

, M.Sc Civil Engineering & Physics, Ghent University (1995) · Author has 1K answers and 220K answer views

Can a speedometer tell the highest speed of a car?

No, it does not.

General Motors had a speedometer max out at 85 MPH for insurance purposes in the eights.

In some TRANS AM models, they not only have it at less than 85MPH, but they also underreported the engine's Horsepower.

This is an image of the 1982 TRANS AM speed meter

The car can do 160 MPH

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John Jay Related Answer Ralph Berger

, studied at University of California, Berkeley (1979)

Answered 8 months ago · Author has 5K answers and 2.7M answer views

What is the relationship between rpm, which is the rotation speed, and the vehicle's travel speed in a pmsm engine?

A pmsm engine is an electric motor. What connects the motor to the ground is the transmission and wheels. So to fully answer, one would need to know the gear ratio gr (turns of wheel/turns of motor) and wheel circumference L (tread length if scalped and rolled out into a straight line). Then:

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