speedo vs sat-nav speed

[Brum1]

Just wondering,

If a car's speedo reads up to 10% higher than the actual speed, the odometer will also read 10% over.

So if your car's clock says 50,000 miles, it has really covered 45.000 miles.

Are we loosing out on resale values?

Brum

 

[VIP640]

and having the car serviced before it is needed...

 

[ProfJohnL]

Hi Brum & Nick

Simple answer is yes. Even if the odometrs do actually read correctly, it is likely that many of us are servicing more frequently than is strictly necessary.

The aviation, nuclear, marine and other high tec industries have for some time operated service schedules based on actual need established by measurement rather than on an arbitrary time or distance figure. The methods they use to establish if a service is necessary include listening for new or bigger vibrations when the equipment is running, others look at the fume or temperature content of emmisions. Oils can be checked for viscosity and particulate content. These are quite high tec solutions and rely on well know or controlled operating environments which is why they have not filtered down to the humble car in any great extent.

Obviously service downtime is very costly to them, and if parts are being replaced before they have been fully used then that is a waste of resource.

Some cars do now use techniques to monitor various items such as the quality of the lubricating oil, and it is likely that we will see more of these technologies being introduced, either by customer pressure, or Govt. legislation to reduce waste and emissions.

Until then, the car (and caravan) manufactures will suggest that service intervals should be governed primarily by distance travelled or time, and to take into account the people that do high mileages the intervals will be shorter than may be the case if measured wear was the arbiter.

 

[rob_jax]

Quote "Just wondering, If a car's speedo reads up to 10% higher than the actual speed, the odometer will also read 10% over."

Basically no.

Older generation cars use a cable drive and gearing to convert gearbox rotation into miles driven - modern cars use an electronic sensor to count the same fact.

As long as the tyres are within specification, then the mileometer should be acceptably accurate.

The car speedo converts the gearbox output into a speed display - and in an old speedo the "speed offset" was engineered into the spring that resisted the moving needle. In a new car, the speed offset is engineered into the computer.

Robert

 

[empee]

We use a Tom Tom 920.

In our just sold 3 yr old Grand Voyager - no difference in speed indication.

In our replacement new Grand Voyager - again no difference.

The speedo in my 1yr old (& 4000mls) Hyundai Tucson reads 4.5 mph high throughout the speed range.

I do wonder are manufacturers fuel consumption tests carried out using the cars' speedo - an over reading speedo would result in a better fuel consumption figure, although you wouldn't notice it unless you then used GPS to monitor speed!

 

[rob_jax]

Martin,

Official fuel consumption figures are determined by "driving" the car on a rolling road in a test laboratory. And these are highly reproducible. The driving is performed by a computer controlled profile - speed and distance are set by the rolling road.

But are these tests representative of any journey you'll ever make - No - but they do give a comparison between different makes.

For instance, the extra-urban part of the test involves the car accelerating to a fixed speed - holding that speed for about half of the test - then periods of speeding up and slowing down and finally then stopping - but does it exactly relate to any road near where I live - obviously not.

Robert

 

[JTQ]

Quoting rob_jax,

"But are these tests representative of any journey you'll ever make - No - but they do give a comparison between different makes."

Yes up to a point but this is I feel being too generous; certainly in the caravanning context.

The test takes no account of how the vehicle would respond to towing or even a solo journey of more than the few miles.

I suggest that a light weight small engined vehicle that would inherently do well in the test would very likely return relatively poor towing mpg/CO2 figures. Simply because its optimisation is set for the low powers required by the test.

I expect vehicles will now be optimised very much for the test and that might be fine for those in a city commuting just a few miles to work, with frequent cold starts. However those like us who tow, cover greater distances, or who otherwise need sustained engine power are going to suffer poorer fuel economy.

The test unfortunately will corrupt natural product development.

 

[rob_jax]

I believe those are two different aspects - which do interact slightly.

On the one side there is a discussion about how accurate the fuel consumption tests are, and how you could use them to compare two different cars on paper before you take any test drive.

Then there's the subject of car manufacturers "optimising" their car to perform well on these tests - because it seems that everyone is concerned about fuel consumption and excise duty. And of course manufacturers will (actually for many years) always try to sneak their car into a lower CO2 band or achieve some similar showroom advantage.

And as you say - there have been some unfortunate examples of overzealous "sneaking" - the Toyota Avensis and Lexus IS200 diesel models were just awful - the gearing was so over long (to improve CO2 figures) as to make acceleration something of a distant memory. The Sport versions were different though - and Toyota saw the light - and that's been the standard gearbox setting for sometime now...

Robert