General Trip computer and high speed fuel economy.

[mzhussein]

Isn't the speed inaccuracy at higher speed just a percentage issue?

i.e. 10% inaccurate at 1mph = 176 yards.
10% inaccurate at 100mph = 10 miles.

exactly so.

my stilo is 7mph out at 80ph (as verified by various gps's)

and near makes no difference accurate at 20mph (lowest possible cruise speed)

 

[Shadeyman]

Like I said, the faster one goes the less accurate the speedo is.

If it is 10%

at 10mph your actually doing 9mph, 1mph out is not really a problem.

at 100mph your actually doing 90mph, personally I think + or - 10mph is a problem and may be the reason some CN+ display incorrect info?

The faster we go the less we can rely on its data.

 

[Shadeyman]

A car has slightly larger tyres fitted there by having a larger circumference than that of the standard tyre.

So at 2000rpm in 5th gear the car would actually be traveling faster than before when it had standard tyres fitted.

But the info displayed on the CN+ would still be the same as before the oversized tyres were fitted. (Or would it?)

So by fitting slightly oversized tyres the 10% error margin could be reduced to say 7% or 6%?

 

[gerhardj]

Two things that should be kept in mind here:
1. Factory fitted speedo's will always over read, by up to 10%. That is a given, that is just how they play safe.
2. If you now have different size tires than the OE, this figure will either go more or less, depending on if they are of smaller or bigger diameter.

 

[Shadeyman]

I do have slightly larger tyres on the car, 225 instead of 215. The increase in tyre circumference is very small though.

Edit.
By increasing the circumference I reduce the amount that the speedo is out, so 10% could be 9% or even 8%. Could this 1% or 2% difference be the reason my CN+ info is so far out?

I wonder if other members that have reported their's to be significantly out have also changed the wheels or tyres of the car from standard?

 

[cledan]

As I stated earlier, when I swapped ling long tyres (they came with the car ffs) to Firestone of exactly the same stated size it made a difference to the distance to work on the trip.

If you have wider tyres then the profile of the tyre will be greater, by a small margin, and obviously if you have a larger diameter wheel then that can make a difference too, although usually the profile, or aspect ratio is reduced.

The Profile of a tyre is expressed as a percentage of the width so 205/5516 will be smaller overall diameter than 215/55/16 this can make a huge difference to the number of revolutions per mile.

In saying all this the amount out you were, would I assume also be the percentage out your speedo is too? as I assume thats where the distance info is collected from?
Or is it the fuel metering side that holds the error? and how does the computer know accurately how much fuel is in the tank, do they still use the old ballcock method?

coming back two weeks ago I reset the trip b with the cruise on at 70 on the m54, and the average speed was shown as 65 ish, yet the needle was stuck like glue to 70????

 

[sussexa]

ther are a few rolling radius online calculators
http://www.club80-90syncro.co.uk/Syncro_website/TechnicalPages/TRC calculator.htm

 

[Percyhahn]

1. Doing a declaration like this on a public forum in South Africa will get you in serious trouble. Should you not be more careful with this info?
2. All the motor programs we get to see here, from the UK (Top Gear, Fifth Gear etc), all moan and bitch the whole time about the speed cameras all over the place. You not scared of them? Or do you do the same as here, remove your number plates if you know that you would be cruising at high speeds.
3. My first observation on the above was, but that is not that fast, 144kph, then the penny dropped, as you said in your last reply, the journey did not start at 90mph from the starting blocks.........................

haha! Removimg number plates lol. I would pay someone who removes there number plates and filmd and put on youtube a vid of them plateles seting off a speed camera

 

[Caravadossi]

I'm not quite sure what your trying to say ...

I wouldn’t get all Man United over it – the word I used was “confusing”

But it does raise some interesting points.

The outbound read-out clearly shows the distance to destination as 197.6 miles with 2:31 travel time (Start 09:19) & ETA 11:50 (78.5 av. mph) – with the prediction that the last 5 miles would take 7mins (42.9 av.mph).

In contrast your preferred view is the above is “WAY” out & you covered 194 miles in 2:14 (Start 09:36) & ETA 11:50 (86.9av.mph).

Stilo tyres roll at 813(±6) revs/mile - & 225/45R17 tyres (at 808 revs/ml) don’t fall outside that envelope. 215/45R17 have the most work to do (at 819 revs/ml) – but fitting 225/45R17 is effectively no different that running on 195/65R15 tyres (807 revs/ml).

If I’ve got the number crunching right - the difference in distance over the ground between 215/45R17 & 225/45R17 is less than the width of a football pitch for a 200 mile trip.

120117T21:34

 

[Percyhahn]

Probably wear a helmet or burka to

 

[Caravadossi]

If I’ve got the number crunching right - the difference in distance over the ground between 215/45R17 & 225/45R17 is less than the width of a football pitch for a 200 mile trip.

I don’t think the `football pitch’ calc. is at all right.

But doing 200 miles on 195/65R15 or 225/45R17 tyres requires the same wheel revolutions, & covers the same distance over the ground.

120118

 

[Shadeyman]

I wouldn’t get all Man United over it – the word I used was “confusing”

So your basing all this on the fact you think I took a photo of the CN+ the instant I pulled into the hotel carpark, hence the "197.6 miles with 2:31 travel time" quote? I can assure you after a rather speedy 200 mile journey the last thing on my mind was taking a photo of the CN+. That photo was take after I'd had breakfast and drove to a local cash point.

So as I've already said, my house to hotel took me 2 hours and 14 minutes, about 30 minutes of that was driving slowly on A and B roads so my motorway speed was in treble figures or are you calling me a liar?

 

[Caravadossi]

We know when the photo was taken – 11:43 (5ml/7min from your destination, either before or after reaching SGGH).

Whatever your max & min mph claims – the average is 80. It follows that you may have travelled part-way at 100+mph to be consistent with 80 av.mph. (It may be your average was largely 84+ when taking into account a 5ml/10min refuelling mission).

I made the presumption you hadn’t reached SGGH on the basis you claimed it was nearly 200 miles - & the photo clearly shows you short of that. So via instruments we have Home>SGGH as 187.6 (192.6 minus 5) in contrast to your 199 (194+5) to photo point.

I’d argue there’s a lot of room for error centred around generalisations such as “full” & “half a tank”

120118T14:00

 

[TangerineTractor]

The only way to measure mpg is brim-to-brim, and due to the innacuracies of the speedometer/odometer, some sort of GPS recording system...

Back onto the other bit though - generally, and my personal observations would support this, using 60mph as a base, fuel consumption increases by over 10% for every 10mph increase...
So some very simple maths, lets use 120 miles as the distance - that's a pretty decent UK motorway run - and is divisible by 60 and 80 for easy of calculation...

at 60mph, 120 miles would take 2 hours. at 80 it would take 1.5 hours.
let's be conservative on mpg - say 55 @60mph, and price it at £6.40 a gallon of diesel...
so at 60, you'd use 120/55 gals.. 2.18 gals, costing £13.94, let's call it 14
scale that to 80... mpg dorops to 49@70, then 44 @80 - so 120/44 is 2.72 gals, costing 17.40... let's say 17.50
So let's really round it up to make sensible, useable ball-park figures... going 60 instead of 80 if going to save you over 3 and a half quid every hundred miles... so it depend's whether you'd pay 3.50 to save half an hour? Same argument as using the M6 toll or not, I guess.
Logically, most people do long runs relatively infrequently, so the extra cost is less relevant - but it' you're doing 50 miles a day saving 1.75, that's forty quid a month - nearly 500 quid a year.
In my younger days, I had a nice 2.5 litre BMW and a company fuel card - I used to drive from Suffolk to Lancashire two or three times a month, and do it as quickly as I dared... but I never managed an average of more than 75mph for the 270 mile trip. I did get the car past 140 on the clock down the A11 once...

 

[Caravadossi]

I think GPS just brings another set of errors into play. I believe it measures by triangulation the ground track between two points (?) – that may not be the same as following the actual road surface. Then there’s elevation (or altitude) error. Any 14yo cadet force kid will tell you going up-hill-&-down-dale ain’t as simple as looking at the track on the map.

Brim2brim as a one-off will have error. It has to be done over a number of refuels to record enough data sets to reduce variation. Doing it once only ain’t good enough.

The 1.9 engine has an optimum`sweet spot’ that I believe is around 70mph. Going faster (or slower) & mpg will fall off. Anyone can test it for themselves. Just stick the flight recorder on inst. mpg & have a play around until you find it.

120119

 

[Shadeyman]

We know when the photo was taken – 11:43 (5ml/7min from your destination, either before or after reaching SGGH).

Whatever your max & min mph claims – the average is 80. It follows that you may have travelled part-way at 100+mph to be consistent with 80 av.mph. (It may be your average was largely 84+ when taking into account a 5ml/10min refuelling mission).

I made the presumption you hadn’t reached SGGH on the basis you claimed it was nearly 200 miles - & the photo clearly shows you short of that. So via instruments we have Home>SGGH as 187.6 (192.6 minus 5) in contrast to your 199 (194+5) to photo point.

I’d argue there’s a lot of room for error centred around generalisations such as “full” & “half a tank”

120118T14:00

Google Maps says driving direct from my house to the hotel is 194 miles. However my CN+ says I drove 193 miles total even though I drove to fill up at the nearest BP garage which is about 10 miles(5 there, 5 back) away in Wendnesbury near J9 M6. I drove straight to Gretna Green after leaving the motorway. Just going to the cash point from Gretna Green into Gretna added another 5 miles to my journey. Another 4 or 5 miles can be added trying to find a cafe to grab some breakfast then returning to the hotel. So the 193 miles the CN+ displays is about 20 miles short of the actual mileage.

So back to the point of the thread. My CN+ is no where near accurate, in fact its miles out. I'm unsure if thats due to the speed I was travelling at or the oversized tyres I have fitted to the car but it's no where near accurate. MPG, Distance travelled etc. are all wrong. I admit my calculations are not perfect but they are good enough to show how inaccurate my CN+ is.

I've checked the distance using Nokia Maps and Google Maps and my house to the Hotel is deffinately 194 miles, add the extra 20 miles I travelled(cash point, fuel and cafe) and my CN+ should have read about 214 miles but it indicated 193 miles so its very inaccurate IMO. The most accurate part was the time and that's not perfect.

Edit.
I've just realised something, your basing everything on the data you've seen in the photo of my CN+.
The whole point of this thread was to show how inaccurate my CN+ is ...

 

[richydraper]

i thought we'd already established that it wasnt an exact science and can be *upto* 10%out. So the best part of 200 miles for it to be 20ish miles short is acceptable right?

 

[Shadeyman]

i thought we'd already established that it wasnt an exact science and can be *upto* 10%out. So the best part of 200 miles for it to be 20ish miles short is acceptable right?

I thought the 10% thing only referred to my speedo not to the data my CN+ was displaying?

The CN+ reports only 403 miles total after I'd returned home. I'd driven around Gretna and Gretna Green while I was there. Must have clocked up at least another 20 or 30 miles before I drove home.

The real distance I traveled from start to finish was about 440 miles so may be the 10% doesn't just apply to my speedo?

Either way the faster and further you go the more it seems to be out. For short slow journey's, 20 or 30 miles the error margin is minimal. From now on I'll be ignoring 90% of what my CN+ tells me. How much I spend at the pump and a good watch is far more accurate.

 

[Davren]

i thought the 10% thing only referred to my speedo not to the data my cn+ was displaying?

the cn+ only duplicates the cluster display for trip information, so it's not a fault of the cn+.

.

 

[Caravadossi]

The problem with it, is the view that the car read-outs stink & the alternative views are fragrant. 193 & 214 maybe some 10% out . But then so is 90% `going fast’ & 22% (30/134mins) `going slow’ – that’s argued to prove the read-outs are “WAY” out. What other errors are in these fragrant `measures'?

We already know that read-outs have error by design - given that indicated speed over-reads actual speed. The tolerance limits will be designed to allow for replacement tyres – such that whatever (within spec) tyre option is fitted, indicated speed will exceed actual speed.

120120