Speedo reading accuracy

[Hillbilly]

The 77TSI has 195/55r15 as standard just to correct you.

With the sports pack they are 215/40r17 same as the GTI.

Not to sure about the bottom of the range Trendline...

Correct thats what mine has and they give an incorrect speedo reading of 7kph over.

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

Would fit but unfortunately are illegal

[brad]

The 77TSI has 195/55r15 as standard just to correct you.

With the sports pack they are 215/40r17 same as the GTI.

Not to sure about the bottom of the range Trendline...

Thanks, I must have been looking at an overseas spec sheet.

i think if I owned a 77tsi I'd be sneaking some 195/60x15 or 205/55x15 on there to take up part of the inaccuracy.

[nomadx38]

I think you will find the variance in speed is not completely related to the wheels but just the way they calibrate the computer.

Out of the 3 combos of wheels offered standard, doing 100km/h on the GPS I would be getting this on the speedo:

195/55r15 (596mm Radius) (108km/h)

215/40r17 (604mm Radius) (106km/h)

So for the 1% difference in side wall height I have about 1% difference in speed.

At the end of the day though the speedo is out on average 5km/h, (disregarding the variance from the wheels) which I would say is good because it allows for variance in tyre pressure and the height of the remaining tread and also if you are speeding a bit you don't get snapped by the camera

[Lemonskin]

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?

[Umai Naa!!]

Judging by the findings in this thread, a larger wheel/tyre combination may improve the situation.

[Diesel_vert]

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?

Not strictly.

The bigger the rolling circumference of the tyre, the less the speedometer will underestimate the vehicle's speed.

Think of the tyre as just another gear - the bigger it is, the more "slowly" the needle in the speedo will rise.

Fitting bigger tyres (for example 205/50 R17 or 215/50 R17) will reduce the discrepancy, but will also affect the vehicle's dynamics, which you may or may not approve of.

[nomadx38]

No the speedo is not linked to the size of the wheels it's a mathematical calculation on how speed is determined. The speedo is configured to accommodate for all situations so the speedo is always faster through all models, specs, options and ranges.

You have to do that math on it. If you wanted to go to 18" you would need a 35 (will display 103 when doing GPS 100) or 30 (will display 107 when doing GPS 100) tyre profile (so 225/35r18 or 225/30r18 ) wether it will even fit on the car or not is another story

[Hillbilly]

Not strictly.

The bigger the rolling circumference of the tyre, the less the speedometer will underestimate the vehicle's speed.

Think of the tyre as just another gear - the bigger it is, the more "slowly" the needle in the speedo will rise.

Fitting bigger tyres (for example 205/50 R17 or 215/50 R17) will reduce the discrepancy, but will also affect the vehicle's dynamics, which you may or may not approve of.

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

Would fit but unfortunately are illegal

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

[nomadx38]

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 wouldnt do it at the cost of fatter worse performing tires.

Just my two cents.

[alexaescht]

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.