Speedometer readings

**logger** · 07-11-2009, 04:55 PM

Mines GPS 100 km/h=CanBus/ABS 100 = digital speedo 106 = analogue 106. So digital & Analogue match for me which is fortunate.

**oileoi** · 23-11-2009, 11:57 AM

I'm planning to change the wheels of my car to 18" once it's deliver and I was wondering if the speedometer reading would change? How can this be adjusted on the VAGCOM

**logger** · 23-11-2009, 12:18 PM

Originally Posted by oileoi

I'm planning to change the wheels of my car to 18" once it's deliver and I was wondering if the speedometer reading would change?

Most probably not. Depends on the tyres you fit with the new wheels. Generally when you fit lower profile tyres to the larger wheels, the rolling diameter will stay much the same and no change to speedo. So specify tyres on your 18" wheels that match the circumference of your delivered tyres and it wont be an issue.

Originally Posted by oileoi

How can this be adjusted on the VAGCOM

It can be fiddled with, but not fixed with VAGCOM. But as mentioned you wont have an issue if you keep to your original rolling diameter.

**oileoi** · 24-11-2009, 08:27 AM

Logger, thanks for your answer the tires im planning to fit into my car are 225/40/R18 Y KUHMO 31 SPT.

Do you reckon the speedo reading will change at all?

**logger** · 24-11-2009, 09:23 AM

Originally Posted by oileoi

Logger, thanks for your answer the tires im planning to fit into my car are 225/40/R18 Y KUHMO 31 SPT.

Do you reckon the speedo reading will change at all?

What sort of car are you getting and what sort of tyres will it come with?

If for example you got a stock TSI118 it will come with 205/55/R16 and changing them to 225/40/R18 will alter your speedo by 0.8km at 100kmh which is trivial. Way less than the ~6% over read error that the car intentionally comes with. In fact this change should reduce the 6% over read to about 5% and is arguably an improvement.

Once you know what tyres your are replacing, use a tyre size calculator and it will let you compare the two sizes.

**alexmk6gti** · 24-11-2009, 10:52 AM

I've found the dead reckoning using the RNS510 is accurate
analog = 109
digital = 106
rns510 dead reckoning = 100 kmph

**Mutley251** · 24-11-2009, 10:54 AM

Another way of checking is sit on the freeway. Set your cruise control, reset your computer so the average speed resets. Keep a constant speed and there you will have the difference. The average speed will be the true speed.

So as it goes for most 100 real is 106 / 107 displayed.

**oileoi** · 25-11-2009, 09:22 AM

Originally Posted by logger

What sort of car are you getting and what sort of tyres will it come with?

I'll be getting a 90 TSI but I'm not entirely sure about the specs of the tires that it comes with. I called Barloworld and the sales person couldn't tell me either. The only thing they told me was that it could come with Continental tires or Michellin but they didn't tell me the specs. That really didn't help me at all.

I had a look at the specs PDF on the VW website and I couldn't find the specs for tires that comes with the (Wellington) 15 x 6½” Wheels either, I'm guessing they will be 195/65R15 tires.

I used the tire calculator (thanks logger) and got the following results:

Differences between Wheel/Tyre 1 ( 195/65R15 ) & 2 ( 225/40R18 )
Rolling Radius 1.35mm
Circumference 8.48mm
Diameter 2.7mm
When your speedo reads 100km/hr you're actually at 100.42 km/hr

So basically minimal difference, however I will be using a 8.5 x 18" rims so my tires would be stretch a little but not much at all so I guess the speedo reading should be fairly accurate?