Speedo reading accuracy

[Hillbilly]

Hi, my Polo has the standard Lakeside 15" wheels with 195x55 R15 rubber and when my Garmin GPS reads 100 km/h the speedo reads ~107 km/h. When the GPS reads 110 km/h, the speedo reads ~118 km/h. When the GPS reads 120 km/h, the speedo reads ~122 km/h. It seems then that the rate at which the speedo overestimates the car's true speed is inconsistent and so I think that changing wheels/tyres to make the speedo accurate will actually do the opposite, and make it much less accurate. Just my opinion.

If you dont speed you will be ok cos its ok up to 110 same as mine LOL They are usually linear that is the difference gets larger the faster you go. Havent checked mine at 120 as I cant afford the fine ROFL

[alexaescht]

If you dont speed you will be ok cos its ok up to 110 same as mine LOL They are usually linear that is the difference gets larger the faster you go. Havent checked mine at 120 as I cant afford the fine ROFL

Haha I wasn't the one driving, backseat passenger I was! Mum was driving and inadvertently sped while I was looking at the GPS so I quickly glanced to the speedo to check the difference. When I get my licence I won't speed. Promise

[Hillbilly]

Haha I wasn't the one driving, backseat passenger I was! Mum was driving and inadvertently sped while I was looking at the GPS so I quickly glanced to the speedo to check the difference. When I get my licence I won't speed. Promise

I think you misread it at 120k It was more likely to read 132 not 122

EG 80 = +5 or 6 100 = +7or 8 110 = +9 or10 120 = +11 or 12

[alexaescht]

I think you misread it at 120k It was more likely to read 132 not 122

EG 80 = +5 or 6 100 = +7or 8 110 = +9 or10 120 = +11 or 12

That's perfectly possible, I'm well known in my family to be slightly dyslexic when it comes to remembering numbers. Your formula seems right to me so we'll run with that.

[Diesel_vert]

As I already posted, the answer for which I asked a tyre company in USA

mine give an incorrect speedo reading of 7kph over.

They are 195/ 55/ 15

To correct it I would have to go to 195/65/15 which are 6.7% taller or 40mm

Although my reply was more aimed at Lemonskin and his car (Polo GTI with 215/40 R17 tyres), you are correct in regards to the 15 mm limit for allowable increases in the overall tyre diameter (at least for QLD, NSW & VIC).

Would fit but unfortunately are illegal

Not illegal, but unroadworthy.

Will also affect take off as it will up the gearing somewhat and put some extra strain on drive components

Would be the motoring equivalent of putting the car on stilts!

Just wondering why would you bother going to such effort to correct it. If you know it's out, go 7km/h faster or just do the limit and never ever get busted by the police...it's what i do.

I definitely wouldn't bother spending the money to get the speedo 100% correct (which you never will anyway, due to 100 factors including the fact that a speedos accuracy varies from 0-240km/h) and especially wouldn't do it at the cost of fatter worse performing tires.

Just my two cents.

I agree.

To depart significantly from the manufacturer's recommended tyre fitment for the sole and only purpose of altering the speedometer's readout, is perhaps a bit short-sighted.

[Lemonskin]

When I get my licence I won't speed. Promise

Famous last words

[brad]

also if you are speeding a bit you don't get snapped by the camera

For me, this is akin to those people that set there watch 5 minutes fast so they aren't late for appointments.

If you know your speedo is accurate, you tend not to speed (that's how it works for me). I can't nelieve the amount of times the fast lane is held up by somebody in a VAG group vehicle who is religously sticking to the speed limit (good on them) but their speedo is deceiving them.

[brad]

So hearing that the speedo is linked to the size of the wheels, if I was to get bigger wheels, that would stuff the speedo up would it not? Or is there a setting somewhere to specify your wheels size?

Wheels are the metal bits. Tyres are the rubber bits.

[AC74]

For me, this is akin to those people that set there watch 5 minutes fast so they aren't late for appointments.

If you know your speedo is accurate, you tend not to speed (that's how it works for me). I can't nelieve the amount of times the fast lane is held up by somebody in a VAG group vehicle who is religously sticking to the speed limit (good on them) but their speedo is deceiving them.

Haha! Subarus are just as bad - my MY07 Outback consistently over-reads by ~10% (i.e. speedo says 100, buy you're actually doing 90). So next time you're stuck behind a seemingly dozy Forester, Outback or Liberty remember that!

[wai]

For me, this is akin to those people that set there watch 5 minutes fast so they aren't late for appointments.

If you know your speedo is accurate, you tend not to speed (that's how it works for me). I can't nelieve the amount of times the fast lane is held up by somebody in a VAG group vehicle who is religously sticking to the speed limit (good on them) but their speedo is deceiving them.

Unfortunately it is the litigous world we live in where we look out for someone else to blame. This is why vehicle manufacturers overstate the speed so that they can never be accused of understating the speed.

What amazes me is that with all the electronics and computing power in modern cars that there is not a simple GPS module that is periodically (like say every 10 minutes) re-calibrating the speedometer so that an accurate speed is shown to the driver. Heck, I would give up an electric driver's window (after all, there is always a driver in the car...I hope) and have that invested in something that would give me the true speed.

For the record I have an AlerteGPS and an ANS610 that give me the correct (well more correct than the speedo readout) speed.