[Phil №❶]

Ffoxy

I agree, CC is reactive in it's approach to car speed. That is it senses a slope & increases throttle to compensate. It is proven that to increase the car speed before encountering a slope and allowing the speed to bleed off whilst climbing, uses less fuel. Probably Mercedes or someone of that calibre have this built in but not for the I30.,,,, yet. 

[rustynutz]

I bought cruise control so I could give my foot a rest.... 

[ElleB]

I am with you Russ.... the aged ankles do need a rest and a bit of  regular movement and the CC sure assists there..

[Phil №❶]

On a good day, I can travel 27 Ks on our freeway and just let the diesel do it's thing. 

[mjt57]

So for some reason, my economy hasn't improved over the past 14000 km.  yet fuel use when commuting to work and back averages at 4.8L/100 km in 70kph traffic, yet 800km with an average speed of 92kph gives 5.0.

Are you relying on the instrument trip computer's figures? If so, set them aside and log actual kays done between fills and how much is put in each time, then calculate off of those figures.

My Android phone app, "Acar" tells me different figures. While the TC told me for the last fill that it's doing 5.9l/100km, Acar calculated 6.3l.

And for consistancy's sake I'm filling to the first click of the nozzle, usually at the same bowser each time.

Roughly, I'm getting about 740km per fill for 42l.

[Ultralights]

When I refill, I fill it to the brim, I think it's a good way to be sure of the same amount of fill level with each tank, if you can actually see the fuel level.   I use an app called road trip to track fuel usage, and it's usually 0.2 to 0.5 L/100 more than the cars trip computer, so I won't get a real usage figure until the next refill when I head home,  with the tank brimmed every time I average 950 to 1000km to a tank, and get an average of 53 ltrs in it.

[Lakes]

When I refill, I fill it to the brim, I think it's a good way to be sure of the same amount of fill level with each tank, if you can actually see the fuel level.   I use an app called road trip to track fuel usage, and it's usually 0.2 to 0.5 L/100 more than the cars trip computer, so I won't get a real usage figure until the next refill when I head home,  with the tank brimmed every time I average 950 to 1000km to a tank, and get an average of 53 ltrs in it.

That's interesting, i have not checked my trip computers accuracey, but i have discovered the oddometer is out it's slow , i compare it to the distance my Garmin GPS say's i've gone and the Garmin says around 7k per 100k travelled more than the oddo reads out.
so if you can check yours against a GPS. the speedo is close to what the Garmin GPS speedo reads out so puzzling.

[Phil №❶]

I wonder whether there are inaccuracies in the gps at civilian level. I know at military level the accuracy is much better, how much better is probably a secret. Maybe Fatboy knows more about this. 

[AlanHo]

A few months ago I detailed the errors in my odometer in this thread :-

https://www.i30ownersclub.com/forum/index.php?topic=9323.msg107972#msg107972

[Ultralights]

The speedo and odometre are out on mine as well,  even compared to the ultraguage I have fitted.
100kph on the Ultraguage and the speedo is reading 105.
Same applies to the odometre,  but using the roadside calibration signs on the freeways, 5km of roadsign markings = about 4.8 km.

Funny thing is, the Ultraguage speed reading is pretty close or a identical speed reading to my iPhone and iPad gps related apps, trapster in particular.

This is only a theory, but I think the fuel use trip computer uses the info that drives the odometre and speedo needle, and this might explain the difference between the trip computer figure, and the actual figures on fuel use,  though this would suggest real world figures would be lower than the trip computer.   Or it could be the opposite, fuel use data for the trip computer might be derived from the obd computer, and we are basing our real world figures on the erroneous data provided to the odo and speedo. 

As for the speedo error, it's legislated in oz, that the speedo reading must be the same or higher than the actual speed so to ensure speedo error can not be used as a defence agains speeding or other fines and infringements,  or as the government states ,to ensure drivers don't get booked by speed cameras when using cruise control or driving at the posted speed limits.

[FatBoy]

I wonder whether there are inaccuracies in the gps at civilian level. I know at military level the accuracy is much better, how much better is probably a secret. Maybe Fatboy knows more about this. 

Not a secret at all.  I haven't got my books with me at the moment (they are in a container), but from memory the restrictions on accuracy were released by the US Government some years ago (with the ability to withdraw them when they see fit).  However, most GPS units (e.g. car, handheld, etc) that don't require the accuracy generally will be "within" 10m (30ft) of the actual position.  The more accurate GPS are used for mining, survey work, etc.  With the actual military codes (ie. secret squirrel codes) the accuracy reduces to less than 1m!

It also depends on how many satellites are within a certain area of the sky as there are many errors with GPS (again, I don't have my books so I can't list them all).  The more satellites, the better the accuracy.

[Phil №❶]

When you say accurate to 10m is that each time the calc is performed and if so, could that add up to the variation quoted above. 

[FatBoy]

My understanding is that if you travelled exactly say, 100km, on a FLAT road, then the GPS could say anywhere between 99.980km or 100.020km (the 10m accuracy at either end).  It also depends on the algorithm in the GPS (most average out the speeds, they know if the car was going 100km/h it will keep that in memory).  Saying that, GPS is a 3D navigation device (it would still read 100km/h if you travelled straight up at that speed).

Also remember that the GPS tells you how fast you were going, not how fast you are going.

[Lakes]

Thanks Fat Boy, interesting, so if your excellerating the GPS speedo has to catch up & speed it's reading out is slower than actual?

[Lakes]

i was just thinking, if you are useing guid posts as an indication of distance to compare against, you would have to travel in perfect straight line. if you wavered off a perfect line you would travel further, than the marker was indicateing.

[FatBoy]

The GPS works out speed the old fashioned way, distance (which it knows) divided by time (which it also knows). It has to have both before it gives you speed. There will always be some lag, it all depends on the refresh rate of the position. The time is very accurate (normally taken from the satellite which is second only to atomic clocks in accuracy).

So in short, the GPS will give you your speed at the last refresh.

[Dazzler]

Thanks Fat Boy, interesting, so if your excellerating the GPS speedo has to catch up & speed it's reading out is slower than actual?

I have actually noticed that small amount of lag and try to allow for it 

[Phil №❶]

Yes, this is why all the GPS devices say not to use the speed as a reliable tool in preference to your vehicles speedo. However, some vehicles speedo's are way out.

Lakes

When you say using guide posts on a curved section of road, my understanding is that the distance given would be accurate for a vehicle following that course. For example, 5 k's on a winding road would in fact be close to 5k's linearly but not 5 k's between co ordinates on a map. Fatboy  can you explain

[FatBoy]

I think what Lakes was referring to was keeping in the centre of the road, if you cut the corner or weave a bit over the road, then the distance will be different to the signposted distance.

[Lakes]

Yes what FatBoy said, i learnt how hard it is to drive or ride in a perfect straight line, you only have to waver off the line to go further, or even change lanes to overtake. might only be a small different .
I like to use the GPS speedo in the I30 as, how i have it possitioned i don't have to take eye off road for long to check speed. i don't want to get a speed ticket, we have so many speed camera's & safty camera's in & around Sydney. but i've never been booked yet. i do a lot of K's per day too.

[mjt57]

I've used GPSes to calibrate or to check vehicle speedos in the past. I've used a road near my place. It's about 6 km long, flat and straight. And best of all, rarely used. So it allowed me to do a few runs at constant speed to check odometer and speed readings.

I understand that these conditions are ideal for attaining GPS accuracy. And if this is scientifically valid, then my i30's speedo is optimistic by a couple of percent, at best.

ie. 100 km/h indicated is a true 98 km/h. 60 kays is about 58-59 km/h.

I haven't done odometer as the current GPS doesn't have a trip function. (TomTom VIA180). In the past I used a Garmin Etrex Legend which has full trip computer functionality. I also used it to calibrate and to check a Sigma Sport cycle computer which I have on my motorcycle (Honda speedos are notoriously inaccurate, mine being no exception). The bike's odo is fairly good, speedo up to 10 percent optimistic.

Put it this way, when I got pinged by a speed camera once, the cycle computer was indicating 70 km/h when I glanced at it after I saw the camera. The fine was for 67 km/h alleged, 70 km/h detected (minus 3 km/h legislative tolerance.) So, that was good enough for me...

And it also proved that those overhead speed signs are as accurate as most footy tipsters...

[Phil №❶]

I'd be very interested to see the comparison between GPS & Speed over a fixed distance.
The only truly accurate way is to have a surveyor use electric distance measuring to mark out exactly 1km on your road. Then, without breaking any laws, you should cover that distance in 1 minute @ 60 Ks & 30 seconde @ 120 Ks,,,,,, and even less seconds @ higher speeds!

I still feel that the indicated gps speed indicates slow due to it's refresh rate & your speedo may in fact, be accurate. 

[Lakes]

I think, if you ever got booked for say 1kph over limit, all this information we have could be useful. to prove nothing we have at our disposal is 100% perfect so the speed camera's should allow 10% error.
My Harleys speedo seems close, but old harleys i have owned in past were not very accurate & were cable driven so not always steady but the newer ones are computerized and drive off sensor in gearbox, as long as you keep stock final drive ratio you are right.
i'm not sure about the Hyundai or how speedo is driven? i just drive it.
have a nice weekend all.

[rustynutz]

My GPS confirms pretty much what Martin had to say re speedo error.....

[rustynutz]

I think, if you ever got booked for say 1kph over limit, all this information we have could be useful. to prove nothing we have at our disposal is 100% perfect so the speed camera's should allow 10% error.

They already allow a percentage for error......on the part of the camera..lol

Mobile speed cameras are permitted a tolerance of 3kmh or 3% which is why, if you receive a ticket in the mail, there will be 2 speeds.....one is the detected speed and the other is the alleged speed.

[Phil №❶]

My GPS confirms pretty much what Martin had to say re speedo error.....

I'll go out on a limb and forecast that nearly all owners would report the same type of error and the similarity only proves what I said above, not that your speedo is inaccurate. It's a GPS data update issue. 

[rustynutz]

Phil, I think you'll find the GPS is the more accurate of the two as car manufacturers have to comply with the Australian Design Rules relating to Speedometers which stipulate they must not show less than the actual speed of the vehicle.
There is an allowance for error of 10% so manufacturers err on the side of caution rather than run the risk of failing the ADR.

[Dazzler]

Phil, I think you'll find the GPS is the more accurate of the two as car manufacturers have to comply with the Australian Design Rules relating to Speedometers which stipulate they must not show less than the actual speed of the vehicle.
There is an allowance for error of 10% so manufacturers err on the side of caution rather than run the risk of failing the ADR.

I am of the same thinking Russ 

[mjt57]

I'll go out on a limb and forecast that nearly all owners would report the same type of error and the similarity only proves what I said above, not that your speedo is inaccurate. It's a GPS data update issue.

When I checked and calibrated my speedo and cycle computers I would sit on a constant speed, or as near as possible as I could for nearly 5 km. Now, during those runs if both speedo and GPS readings are constant and steady and no discernable change in vehicle speed occurs, then surely the GPS has to be pretty accurate.

As far as I'm aware the only time that a GPS's current speed indication would be wrong is during speed changes where the unit has to recalculate the new speeds, or the lag of the instrument which occurs.

The same would have to be said of the speedo, too, particularly if it's electronic and is reading pulses from a spinning bero or magnets on an axle or tailshaft.

And as far as speedos with analog dials go, at the end of the day it comes down to being a mechanical device. ie. it's a needle connected to a spring loaded spindle which is reacting to some sort of electro/mechanical actuator, much like a multi-meter, I would assume. Motorcycles with digital speedos (and most are these days), have that lag, too. Accelerate and you watch the speed jump up in varying increments. It might read say, 40 km/h one second, next update of the instrument has it displaying say, 86 km/h, then 103 km/h and so on.

[Dazzler]

I use our Garmin GPS pretty much all the time as an eye level speedo when in our home state (11 months of the year) Much as Lakes does...

There is a lag but it is fairly subtle and can be compensated for. In the Hybrid the speedo is reading about 106kph when the GPS says 100kph and this seems consistent in most conditions so I am comfortable going by the GPS (haven't been booked for speeding since I started relying on a GPS (about the last 3 years...) Touch wood