Speedo Inaccurate

[Topend]

I have 3 mobile GPS units and they are all within 0.3km/h of each other. I also have had various boats over the years fitted with GPS plotters. They are also within 0.3km/h of my original mobile GPS unit.

Based on my experience my money is on the GPS being quite accurate.

[gregozedobe]

Based on my experience my money is on the GPS being quite accurate.

That is the general consensus, ie the analogue speedo is usually much less accurate than most GPS'. The OEM digital speed reading (if you can access it on your particular car) is usually pretty good, as is the MFD average speed.

[Corey_R]

That is the general consensus, ie the analogue speedo is usually much less accurate than most GPS'. The OEM digital speed reading (if you can access it on your particular car) is usually pretty good, as is the MFD average speed.

Yeah - the digital speedo is usually accurate. I'm surprised the OP is saying that it's only 2km more accurate - which means it's still 5km too slow... that's strange.

[mtaoz3]

There are of course only two things required to measure speed: distance and time. We can assume that the clocks in our cars are accurate to within a few seconds a year, and the distance is from the odometer. The odo in my Mk VI is very accurate; 100.6km indicated = 100 km actual, which is of course a 0.6% error. Why the speedo error then? VW take the accurate figure, then deliberately multiply it by 1.06 and then display it. How do I know that? BMW do exactly the same. I once drove down the Hume to Melbourne and just before hitting suburbia I noticed that the distance computer said 300.0km, the elapsed time was 3h00m, and the average speed shown was...106km/h. Made me laugh that I had bought a $70k calculator that couldn't do a simple division accurately.

[prise]

Same problem with Mazdas I've owned. However the local product these days (ford, commodore) have speedos that are only optimistic by 1 km/h which catches a few people out when they rent them.

It would be very simple for manufacturers to have speedo calibration available on a menu in the MFD - its all SW controlled these days.

[GTi Crow]

With my Mk5 GTi the speedo at 110kph was reading 119kph. I approached the dealer to adjust - gave them all the info of the websites on how to do it. They told me that they would lose their VW license if they did it - they have strict instructions from VW Head Office (can't remember whether that was in Aussie or Germany Head Office) that they are not allowed to do it.

So I got in touch with a fellow GTi driver with a vagcom and did the changes myself. After changes at 110kph speedo read 112kph - I was a lot happier with that.

With the test run of the Mk 6 GTi, I noticed a variance between digital speedo and normal speedo - so will probably have to do it all again when my MK6 arrives.

[logger]

Should be able to be corrected slightly with VAG-Com..

Except that any "correction" that you make by this method (distance multiplier) will equally effect your analogue and digital Speedos AND induce an error into your previously accurate odometer. For this reason I have left mine alone with VagCom.

My car displays 106kph on both speedos when GPS and VagCom CanBus speed from ABS indicate 100kph. Interesting thing here is that my cars CanBus "knows" its true speed to the kilometer and can display it via VagCom to me. But VW have set it up to display this intentional overread error on the speedos. I can live with this.

Ross Tech who are the makers of VagCom are of the opinion that there is no known way to adjust the relationship between the two speedos and the ODO via diagnostics (ie VagCom). This relationship is fixed. So change one and you change the other two. They believe the distance multiplier "exists primarily to accommodate different transmission types with different gear ratios between the the driveshafts and vehicle speed sensor."

So if you want to tweak your speedo via VagCom and make it slightly closer to true speed, do so in the knowledge that you will be inducing and error into you odometer. I suppose this ODO error will be in your favour in any case.

[MurphyTheElf]

Except that any "correction" that you make by this method (distance multiplier) will equally effect your analogue and digital Speedos AND induce an error into your previously accurate odometer. For this reason I have left mine alone with VagCom.

My car displays 106kph on both speedos when GPS and VagCom CanBus speed from ABS indicate 100kph. Interesting thing here is that my cars CanBus "knows" its true speed to the kilometer and can display it via VagCom to me. But VW have set it up to display this intentional overread error on the speedos. I can live with this.

Ross Tech who are the makers of VagCom are of the opinion that there is no known way to adjust the relationship between the two speedos and the ODO via diagnostics (ie VagCom). This relationship is fixed. So change one and you change the other two. They believe the distance multiplier "exists primarily to accommodate different transmission types with different gear ratios between the the driveshafts and vehicle speed sensor."

So if you want to tweak your speedo via VagCom and make it slightly closer to true speed, do so in the knowledge that you will be inducing and error into you odometer. I suppose this ODO error will be in your favour in any case.

Interesting. So could one set their speedo extremely low, thereby affecting the odo to the point that it reads a fraction of the true distance travelled?Can major changes to the odo be made otherwise?

[Topend]

The was a time when drivers would check their speedo accuracy by resetting the odometer and driving a known distance. As a kid I remember distance signs on the side of the road as we would travel north out of Brisbane.

Gone are the days where the odometer and speedo match. Regardless I'm going to check the odometer anyway.

[G-rig]

Adjusted it with VAGCOM but still seems to read 120 on the dials vs 110 on the Aircon speed etc.

Can live with it as the overrated speedo would probably save you a few times.

[logger]

Interesting. So could one set their speedo extremely low, thereby affecting the odo to the point that it reads a fraction of the true distance travelled?Can major changes to the odo be made otherwise?

No, because from memory there are only about 4 valid choices for the distance multiplier. ie you alter a number from 1 to 4. Changing from one end of the range to the other is only going tweak the distance measured by several percent. If you particular car and transmission set up necessitates the correct setting is already in the middle of the range ex factory, at say 2 or 3, then the change possibility is even less. This is also the reason that doing this tweak invariably never gets ones speedo spot on because the range you can play with via coding is less than the intentional speedo error.
As an aside, my car indicates 215 on the speedo when it is really only doing 203kph

[Corey_R]

Adjusted it with VAGCOM but still seems to read 120 on the dials vs 110 on the Aircon speed etc.

Can live with it as the overrated speedo would probably save you a few times.

yeah G-rig, as logger is saying above, the difference will still exist. It's just that now your aircon/odo/trip metres are all wrong and only your speedo is correct.

[STV4SYT]

I know what the error is and drive to that, always have done. as a general rule, i know that my car reads 100 @ 93 also. in VIC you cant get booked for going 3km over so i set cruise on 110 which equals 103km/h likewise at 110 i set the cruise just a touch over the 120 line which equates to 113

I have verified my speed with 2 or 3 different GPS units, overhead speed checks and also the climatronic display.

[G-rig]

yeah G-rig, as logger is saying above, the difference will still exist. It's just that now your aircon/odo/trip metres are all wrong and only your speedo is correct.

Well now i don't know which is correct. I thought the aircon readout never changes?

[Corey_R]

From what Logger and the makers of Vagcom are saying, when you adjust it in Vagcom your changing the speed the car thinks it is doing, not the analogue speedo directly. Therefore all you're doing is introducing error into the ODO/trip metres/aircon unit - not correcting the difference between the analogue speedo and digital speed.