Speed v Distance

MikeJXE

They say curiosity killed the cat but if you don't ask you don't get to know.

Whenever I'm not sure of something I will ask google, I did but didn't get an answer so I will ask on here. 

I own a Jaguar XE Diesel 66 plate, I bought it almost 4 years ago, I have logged all my costs, I'll be posting the results on here for comparison at the end of next month (4 years) 

Whenever I pass a roadside speed check my speedometer is 10% over, the radar say 30 my speed says 33, always 

My question is does that make my distance travelled different, 10% more, less or make no difference ?

I have followed the AV discussion and I'm sure there are very knowledgeably people on here who might shed some light on my query

Thank you 

uknick

I'm not sure what you're asking as I can't believe you're asking if you're travelling slower do you go less distance. (I'm not sure what the AV discussion was)

Car_54

The short answer is yes. 

Whatever mechanism is used to drive the speedo, it's vanishingly unlikely that there will be a different one for the odometer.

MikeJXE

Car_54 said:

The short answer is yes. 

Whatever mechanism is used to drive the speedo, it's vanishingly unlikely that there will be a different one for the odometer.

Thats what I am wondering, if the speedo is 10% over then the milage could be 10% over 

That also makes working out MPG different 

Nobbie1967

I suppose the way to check would be to plot a route on google maps, see what distance they give and then see what you get following the route. I’ve always perceived my odometer as pretty accurate judging against road signs, so I do wonder if the manufacturers aim to have mileage correct, but speed over by around 5% to be on the safe side.

MikeJXE

Nobbie1967 said:

I suppose the way to check would be to plot a route on google maps, see what distance they give and then see what you get following the route. I’ve always perceived my odometer as pretty accurate judging against road signs, so I do wonder if the manufacturers aim to have mileage correct, but speed over by around 5% to be on the safe side.

Thanks I'll try postcode address to postcode address if there is a decimal point on my odometer 

B0bbyEwing

Nobbie1967 said:

I suppose the way to check would be to plot a route on google maps, see what distance they give and then see what you get following the route. I’ve always perceived my odometer as pretty accurate judging against road signs, so I do wonder if the manufacturers aim to have mileage correct, but speed over by around 5% to be on the safe side.

Have had this discussion recently regards cycling speed sensors & accuracy.

I was trying to find out what is the 'Gold Standard' of distance measurement and how do we come to know that whatever it is is 'RIGHT'? Reason being if I travel a route tracking via GPS, via speed sensor connected to one device and via SAME speed sensor connected to a different device (configured the same) then I get 3 different readings and not just a little bit different either - significantly (what I'd call significant) different.

Never really did get a solid answer other than Google Maps isn't it and that my speed sensor, if configured accurately with the wheel circumference, should have a higher accuracy.

MikeJXE

B0bbyEwing said:

Nobbie1967 said:

I suppose the way to check would be to plot a route on google maps, see what distance they give and then see what you get following the route. I’ve always perceived my odometer as pretty accurate judging against road signs, so I do wonder if the manufacturers aim to have mileage correct, but speed over by around 5% to be on the safe side.

Have had this discussion recently regards cycling speed sensors & accuracy.

I was trying to find out what is the 'Gold Standard' of distance measurement and how do we come to know that whatever it is is 'RIGHT'? Reason being if I travel a route tracking via GPS, via speed sensor connected to one device and via SAME speed sensor connected to a different device (configured the same) then I get 3 different readings and not just a little bit different either - significantly (what I'd call significant) different.

Never really did get a solid answer other than Google Maps isn't it and that my speed sensor, if configured accurately with the wheel circumference, should have a higher accuracy.

If I use a third party navigation system in my car the speed shown is 10% less than my odometer and 10% less than the car Satnav 

I will try that and find out distance differences 

forgotmyname

Roadside speed check  they are not calibrated and often unreliable.  Passed one today at 33mph and it said 24mph.

stuhse

As an engineer I can tell you it's pretty impossible to measure distance accurately, especially long distances, which is why the UK has been mapped using trig points, ie triangles .  Angles can be measured accurately.  As regarding your speed reading high, this is I believe a legislative requirement so that you cannot be exceeding the speed limit unwittingly. It stands to reason the same mechanism measures distance as measures speed.  You could use Google or GPS to check distance but they all have errors. My wife and I run together, just 5k, but our GPS devices never match at the end of the run.  Your calculated mpg may not be a true figure , but it is a relative figure you can use to compare journeys and cars as the errors will be reasonably constant.   To further complicate matters your cars speed will most likely work by measuring revolutions of your wheel; as your tyres wear out they will rotate more times for the same distance so when completing the same journey your car will show a higher speed and a greater distance as the tyres progressively wear out. You may remember from school days C= (pi)d.  Sorry haven't got a proper pi symbol on my key board 😁

daivid

On both my Honda Civics the speedo has reported 9 to 10 % over the satnav (and roadside checkers) but the odometer and trip computers have I believe been accurate. Certainly the trip computer has tied in with known distances and satnav and AA routefinder distances. I also used the trip computer to measure a frustrating average speed section on my commute, and from that work out the minimum time I could take (and from that set the cruise control appropriately). If the trip computer was 10% out I would have a lot of speeding tickets by now.

A simple way to check would be using 100m markers on a straightish section of motorway or dual carriageway. Reset the trip computer at post zero then count the posts for the duration of the straight, if your trip computer goes to tenths of a mile a couple of miles should show the discrepancy. If you can easily switch your speedo/odometer to kph/km then you would take out the need for conversion.