Speedometer Vs Milometer

sale_2

hi all,

i was on an early morning run on M1 and was driving between 95 - 100mph. i wanted to clock all the reading and to my surprise after 1 hr the distance covered was indicated as 76miles. is my understanding is correct i should have cleared 95 miles in 1 hr on a 95 miles speed.

am i getting lost or is my speedometer or odometer is not working properly. i drive a P reg audi A4 1.8. also in many forums i have read where in to get good mpg one need to drive max at 3000rpm now in amy audi if i strict to 3000rmp i may be running at 60miles and at that speed i'll reach nowhere.

cheers.....

balsingh

On most cars the speedo's are optimistic so you were probably only just doing 90mph.

Chances are that your odometer is slighly pesemistic so shows slightly less miles than have actually been covered.

Also, did you include the time taken to accelerate up to speed??

By the way, you were doing 90mph on a 100mile stetch of PRIVATE road .... not on the M1. You are only allowed to 70 on the M1

Throbbe

Have you ever noticed the white squares in the middle of traffic lanes on some trunk roads and motorways? They are set at exact distances apart (multiples of 1/4 mile iirc) for the purpose of calibrating speedos (that's what the police do). You'd get the most accurate results from using these.

Also, never mind the fuel economy, travelling at 90-100mph will get expensive sooner or later! Otherwise, keep your revs down without labouring, and avoid heavy use of accelerator and brake (as it scrubs off speed you've 'paid' for) and avoid carrying a boot full of unecessary stuff (if you're like me, you're carrying an extra 50 kilos and burning extra fuel).

Hope that helps.

raymond

I drive an Audi A4, the speedo reads over by 10% according to my Garmin GPS. I checked a few of our other company cars and they read the same as mine.

vansboy

Don't worry - you can be certain the NIP you get in the post, will verify you're actual speed, in relation to the distance travelled.

Lets hope you're readings were more than the 10% variation generally expected!!

VB

tomstickland

Fuel consumption will rise with speed squared, so 90 uses (90/60)^2 more fuel than 60. That works out at about 2.25 times as much.

tonys_3

Apart from the speedometer under reading, another reason could be the state of your tyres. I know this is more of a problem with heavy goods vehicles
where their tyres are bigger and have more tread. However if your tyres were underinflated at the time and/or they were also pretty worn, then your car
would not be travelling quite as far as the odometer reckons. It might only be
a difference of 2 or 3 millimetres per wheel revolution but as a mile is 1609
metres and you probably did around 86 of them [95 minus 10% re speedometer] that extra 2mm could help to explain the difference.

tomstickland

Underflated tyres can also cause a distortion in the space-time field, so you need to do a relativity correction. Assuming you were travelling close to the speed of light though, I'm sure someone on the Audi forums can confirm that a 1.8 is capable of that sort of speed?

Cumbrian_Male

Throbbe wrote:

Have you ever noticed the white squares in the middle of traffic lanes on some trunk roads and motorways? They are set at exact distances apart (multiples of 1/4 mile iirc) for the purpose of calibrating speedos (that's what the police do). You'd get the most accurate results from using these.

VASCAR markings

Throbbe wrote:

Also, never mind the fuel economy, travelling at 90-100mph will get expensive sooner or later! Otherwise, keep your revs down without labouring, and avoid heavy use of accelerator and brake (as it scrubs off speed you've 'paid' for) and avoid carrying a boot full of unecessary stuff (if you're like me, you're carrying an extra 50 kilos and burning extra fuel).

Hope that helps.

You a traffic officer by any chance?

sale_2

hi all,

thanks for sharing the information. answers to some of your query..

- about tyres.....the tyres are brand new and yes i did check the pressure as i do the same every 10-15 days...i think the balancing need to be done as the vehicle moves towards left when stearing left loose.
- i do agree that there's around 10% difference between a navigator and the speedometer as i have expereienced this many times and thought navigator was wrong.

martindow

On a motorway you can get a fairly accurate mileage by using the boards giving how many miles to London and doing a bit of maths.