Speedo Facts

Reasons for Speedometer Error Tolerance

There are sound technical reasons for a vehicle speedometer having a wide tolerance. The factors that influence the accuracy of the speedometer are:

	Diameter of the tyre
	Accuracy of the speedometer sensor
	Accuracy of the speedometer indication
	Ability to accurately determine the reading on the speedometer dial
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Basic geometry teaches that circumference is calculated as: Radius x 2 x Pi.

There are a number of factors that influence the radius of a tyre, hence its effective circumference and consequently vehicle speed. Note that is important to consider the effective radius of the tyre, that is the distance from the bottom of the wheel rim to the road. This distance can vary with tyre pressure and is used to calculate the effective circumference of the tyre.

Anyone who has seen a flat tyre can attest to the dramatic alteration to a tyres radius. The following factors can change the tyre radius by 4-5% on a 50 profile tyre, more on a high profile tyre. The factors are:

Tread depth

The 215/70 x 17” tyres fitted to my car have a radius of 313mm.

The tread depth of a new tyre is 10mm and the minimum legal depth is 1.5mm. The difference of 8.4mm tread depth from new to legal minimum alters the circumference of the tyre by 2.8%. This means that if the car travels at exactly 100 km/h on a fully worn tyre it will travel at 102.8 km/h on a new tyre.

This is a significant proportion of the tolerance being allowed on speed limits.

The calculations if you wish to verify them are:

313mm x 2 x (22/7) = 1967mm circumference

(313mm - 8.5mm) x 2 x (22/7) = 1915mm circumference

Difference = 52mm

52 ÷ 1915 = 2.79%

The change in circumference varies slightly between tyres sizes, but averages around 3%. e.g. a 205/55R15 with a radius of 280mm will have a difference of 3.1%.

Alternative Tyres

The Regulations are silent on alternative tyre sizes, the only requirement being to match the speed and load rating of the tyre.

There is nothing in the Victorian relevant State Legislation and Regulations that state you cannot fit an alternative tyre size. Some tyre manufacturers provide suggestions for alternative tyre sizes for vehicles. Bridgestone is one www.bridgestone.com.au

Referring to the example car, Subaru Liberty, with a recommended tyre size of P215/45R17 one of the suggestions is P235/45R17, slightly wider, same profile and rim size. Fitting this tyre would result in an increase in tyre circumference of 2.9%, which in turn would mean the car would travel at 2.9 km/h faster at an indicated 100 km/h. The calculations are:

Standard P215/45R17 tyre circumference = 313mm x 2 x (22/7) = 1,967mm

Alternative P235/45R17 tyre circumference = 322mm x 2 x (22/7) = 2,024mm

Difference = 57mm

57÷ 1967 = 2.89%

Add this 2.9% to the 3% difference due to tyre tread depth and we have 5.9% or 5.9 km/h @ 100 km/h speed difference, significant if a 5% tolerance is being applied.

Tyre Pressure

Anyone who has pumped up a flat tyre has witnessed a dramatic change in the radius of a tyre.

There is no legal requirement to maintain tyre inflation pressures at a particular pressure. The manufacturer recommends an inflation pressure for the vehicle, however tyre inflation pressures will alter due to:

• Driver may prefer a lower inflation pressure to improve ride quality.

• Driver may prefer a higher inflation pressure to provide improved grip when cornering

• Tyre inflation pressure will change with the changes of temperature of the air in the tyre. A type pressure checked at 5am in Melbourne at 15 ° C will increase if the car is then driven to Albury arriving late in the afternoon with high ambient temperature.

Detail of the rim height measurement of the tyre at 22psi.	Click on the image above for a full size version of the tyre at 22psi.

		Detail of the rim height measurement of the tyre at 40psi.	Click on the image above for a full size version of the tyre at 40psi.

Any change in air pressure in the tyres will alter its radius. Consequently when tyre pressure is increased the tyre circumference will alter and the car will be travelling at a higher speed for each revolution of the tyre than the lower pressure. The question is how significant is the change of tyre radius due to air pressure changes..

A change in tyre pressure from 22psi to 40psi on a 2005 Subaru Liberty Station Wagon fitted with P215/45R17 tyres (manufacturers standard) resulted in a change in radius of the tyre of 6.5mm and effective tyre circumference of 40mm. This calculates to a 2% variation in tyre circumference. The calculations if you wish to verify them are :

313mm x 2 x (22/7) = 1967mm circumference

(313mm - 6.5mm) x 2 x (22/7) = 1927mm circumference

Difference = 40mm

40÷ 1930 = 2.07%

This will result in the vehicle speed altering by 2km/h at 100 km/h

A car with a full tank of petrol, 4 extra passengers and baggage could be heaver by say 60 litres fuel (45kg), 4 x 70kg people + 60kg luggage, total 385kg. This weight will reduce the diameter of the type and hence the car will travel slower for each revolution of the tyre.

The question is; what did the manufacturer assume when they calibrated the speedometer?

It would be reasonable to assume that they had half of the normal load, i.e. half a tank of fuel, 2 people, and half a load of luggage. If this is the case, then a single person driving a car with an almost empty fuel tank and no luggage would have a higher tyre than the half laden assumption, in which case the type circumference would be larger and the car would be travelling faster than the indicated speed on the speedometer.