speedo true speed

[rutter123]

i had the sat nav in my 92 tgd for the first time and found that the true speed of the car was considerably slower than indicated on the speedo, bearing in mind that this tgd has been changed from an automatic to a 5 speed manual.
At a true 10mph speedo read 12mph @20/23 30/35 40/45 50/56 60/67 70/77 80/88 90/100 the increased difference seems consistent so my question is, does the gearing inside the speedo head of an auto differ from that of a manual? i have done the same test in my tzd and the speedo was fairly accurate on the true speed of the sat nav within 1 or 2 mph. the tgd had a donor box from a 1990 tgd 1.9 n/a standard be3 and is running 165/70/14 tyres.

[Tim Leech]

Its not uncommon for speedos to read under, up to 10%, wear and tollerances are a factor, ive got 4 BX's, the Mk1 GT is pretty spot on where as the DTR (with its drum speedo) is miles out, saying that the DTR has done 127000 more miles than the GT.

The 16TRS reads 2/3 mph over and the TZI like the GT is quite accurate.

[Vanny]

It's also not uncommon for SAT NAV's to give false or manipulated speed readings, so make sure you check your speed against more than one brand of SAT NAV! I know that the old Tom Tom One sat nav only picks up 11 satellites at best and reads FAST compared to the newer TomTom which can lock onto 16 satellites.

Mechanical speedo's are normally quite accurate, but they aren't calibrated, and they don't know if you've changed your tyres, if the tyres are under inflated etc etc and so can end up reading out.

Modern digital speedo's normally have an error built in so that at 70mph they will show 73mph. You can normally see this difference if you can access the CAN network traffic and read the road speed from there.

The biggest influence i've seen from day to day on any vehicle is the tyres/inflation pressure (and this is on a highly precise rolling road with new vehicle!).

PS my valver reads out by 10mph at 80, but then it starts at 8mph when the engine is off!

[rutter123]

i know the sat nav is accurate as a hgv driver is usually stuck at 56mph so i dont doubt the true speed, tyres are running at correct pressure 30psi as according to the handbook, i have a spare binnacle so will swap it over to see if it makes any difference.

[JayW]

Mine is exactly 10% out. My old TGD was spot on on brand new tyres but dropped to 66 @ 70 on the satnav when the tyres were on their last legs, so tyre wear isalso a consideration.

[Vanny]

i have a spare binnacle so will swap it over to see if it makes any difference.

I could be wrong, but i think all the binacles are the same and it is the gearing in the gearbox take off that unifies gearbox speed to binacle result. It would probably be easier (if not dirtier) to check the take off unit on the back of the gearbox is correct.

[MULLEY]

When my speedo is reading 70, i'm doing 63mph according to the Satnav. I'm quite happy that the speedo is fibbing as i'd be tempted to drive at an actual 70mph & then my fuel economy would suffer along with my eardrums due to the extra noise the car makes. It is quite a difference at a few miles an hour faster than 63

[kiwi]

Forget Satnavs I went for a dynoroad test on one my BXs after a speeding ticket

Anyways regarding satnavs I also have noticed peculier readings on them like the one where I was going up hill and sudenly found myself going from 90kph to 120kph yet my speed did not move then suddenly down to 30kph. Often notice it on corners and steep inclines but that was the weirdest by far.

Ok this Dynoroad on the BX 19TZS correct tyre size
30kph speedo 28kph Dyno
50kph 50kph
80kph 82kph
100kph 105kph
120kph 125kph

So on we go this time trying to calibrate with satnav

The BX19TXD was reading 10kph high across the range (speedos in miles btw) then I replaced it when some weird noise so now just 5kph across the range.

Now it gets more interesting the latest addition to the fleet the BXTZS Auto! Oh man 119kph on the speedo is 100kph GPS. The solution I thought was to replace the speedo! First I tried the other 19TZS speedo this was the one giving excess speed issues above 50kph and dyno tested. Surprised that No differance the speedo read exactly the same as the original, so I tried the one from our TRS same thing.

Irony of this is of 4 cars the only one spot on is the 19TRS!

Now I was told it is to do with the speedo head located in the top of the gearbox. Tried swaping them and no differance either! so gearbox set up might well be the issues! Except in the case of the TXDs miles per hour speedo that was simply wear it seems.

Personally in the TXD I am happy trundling along at 60mph which comes up on the GPS as 90kph! this translates to 56mph so therfore economical speed. Unfortunately the average Kiwi driver in the Jap crapper with a speedo already out by 10kph thinks this speed is to slow and has to pass.

So reality is what do you trust? satnav or Speedo? My experiance says satnav combined with knowing the differance on your speedo and the rev counter. Three things to confuse the modern motorist only instinct and experiance gets the right answer.

Sorry long winded reply.

[Defender110]

I always understood that GPS worked out your average speed betwween 2 set points so wasn't pinpoint accurate.

[mat_fenwick]

There's too much error in the position the GPS thinks you're in for that method to work with any accuracy (±2 to 5 metres depending on how many satellites it can lock onto), so it uses the doppler effect to calculate the speed. I.e. the sound of a car changes as it's coming towards you to going away from you, and so does the signal from the satellites. So long as it knows how fast the satellites are moving (I think they transmit this data to the GPS unit) then it can calculate how fast it is going.

I don't know how accurate they usually are but would guesstimate within a couple of mph. But probably more with a poor signal, and/or quickly changing speed.

[Vanny]

Sorry long winded reply.

Shorter response,

How many satellites can you lock onto? If your only betting 8 or 9 altitude locking is poor, giving your issues.

Based on some knowledge, some guess work and some Googling i'd propose the following as part of your problem, Kiwi;
GPS does not provide uniform global coverage, and the coverage of the satellites dramatically drops off as you get away from major land mass. I suspect that NZ piggies off the satellites covering Australia for the large part, and hence you can't lock into as many reference points.

[kiwi]

Nice try Vanny! GPS satellites are in geosynchronous orbit around the equator if anything the problem would be less being located nearer to the Equator. What your also forgetting is GPS has long been used by Mariners and aircraft so the positioning of satellites is spread evenly across the globe not focused on land masses.

Mat was closer to the answer! Altitude and angler changees can affect the signal!

However probably the fact the software was modified by the australians designed by the americans and the unuits built by the chinese may have a more reasonable explanation!

[Vanny]

You might want to have a look at the wiki, the 'constellation' looks like this;



And that is the ideal model, and not the final pattern, which prefers land mass to perfect pattern.

They're obviously not in geosynchronous orbit around the equator!

As for marine use, if your 10ft out of position in deep sea, it doesn't matter, so you can live with one less satellite. I've done rather a lot of reading on the subject, specifically on the use of different algorithms to adjust for error in GPS tracking systems, and while I wish i knew more, i'd like to think I've got some of the basics covered.

[Paul296]

Blummin' 'eck! It all looks a bit complicated to me. You want to get yourself a slice of 17TGD action - they only have three speeds; slow, very slow and stop!

[JayW]

30kph speedo 28kph Dyno
50kph 50kph
80kph 82kph
100kph 105kph
120kph 125kph

There must be something wrong with somee of your equipment?

Despite speedos not being accurate, the results are always linear (if the error was 10% for example).

But, your readings look more like this:


Now, your results could be achieved by taking the readings as you go through the gears, but if that's the case then it's useless as referencing for calibration (even mentally) as there's no way you'll always be in the same gear at the same speed.