Reducing rim size on vRS

[Mysticality]

If you have a modern phone, pull up the sat nav and compare GPS speed with speedo speed.

I've found that when my tablet (connected via BT to the OBD-II port and running Torque Pro) shows GPS speed at 99.6-100.5km/h (0.1 intervals), it also shows OBD-II speed at 100.0km/h (1 intervals). However, the speedo and MFD show roughly 106-107km/h.
Seems as though OBD-II speed is 'corrected' by the RNS510. When I'm in a long tunnel (cross city, harbour, airport, etc) the OBD-II speed drops to be inline with the MFD/Speedo.

It makes sense.
The sat-nav is linked in to the speed, so the map keeps going when the GPS has disconnected, so obviously its possible.

Just a bit of food for thought.

[brad]

I think you have your speeds back to front.

[Mysticality]

I think you have your speeds back to front.

Cheers - corrected.

[gldgti]

ahhh... if you guys are wondering about the actual speed vs speedo speed etc, its pretty simple - Mysticality has got it almost right, but not quite.

The ECU receives data from the vehicle speed sensor which is directly connected to the output shaft in the gearbox.

Its simple for the ecu to calculate actual vehicle speed and milage from that, if you have standard tyres on the car (every octavia will calculate this the same, nomatter what size rims it came out with, because the stock tyre sizes are all close enough to eachother in rolling diameter to make no difference as far as VW is concerned).

The output of the speedometer needle is adjusted to read faster than actual speed. All production cars do this. This is to maintain some buffer for you, so that you dont get speeding tickets all the time.

The ECU uses the actual vehicle speed (as calculated based on standard overall wheel/tyre diameter) for all the odometer and fuel milage calculations.

OEM satnav may utilise the vehicle acutal speed to do funky things if the GPS signal is lost, but the GPS reveiver certainly does not provide any input for the vehicle ecu (in this case - I don't want anyone citing examples of the nissan GTR satnav only letting you go fast when you are actually at a race track )

If you have some device that allows you to read off the actual vehicle speed through the OBDII port or the can-bus, then you can plot the discrepancy between the speedo reading and actual speed. Then, you can compare it to your GPS, and you will see that if you have stock sized tyres, the actual speed is damn close to the indicated speed as calculated by GPS (likely within 1%).


hope that helps.

[Mysticality]

More like .2%, really. >.<
At least for me.

[DunlopRS]

Guys,
Checking in after a long quiet stint.
As previously noted I spend a lot of time heavily loaded at highway speeds on quite atrocious roads. Just turned over 106000 km and will not be going up again from 16” rims. Whereas on the 18s normal speeds could not be maintained on many country roads for risk of breaking the car (with many Camrys flying by.....) with the 16s road surface condition is seldom a concern.
Front struts were shot at 100k km. Replaced with Koni sports. Fantastic. Now saving to do the back ones as it is floating a bit when pushed and loaded…..
Currently on 215 55s which need to be replaced soon. Would like to try getting onto 225s, but I guess I would need at least 7” rims for those. Anyone with experience of 16x7 or 16x7.5 on a MKII rs? Any clearance problems?
Thanks

[brad]

I've run 225/55r16 on my Mk2 with Eibach springs (lower than an RS I'd think). Rims were 16x7.5 ET50. There were no issues with clearance.

You can run 225/55r16 on a 6"-8" rim but Audi 16x7.5 (entry level A4) rims are as cheap as chips. I sold my set for about $180 and they were factory fresh.

I actually prefer 17" rims as a good compromise between ride & handling.

The rims I sold were like these - you almost can't give them away.
SET OF 4 Genuine Factory Audi A4 Alloy Wheels 225 55 16 Tyres Caps Nuts | eBay

Go to 17" and fit 235/45r17
Genuine Skoda Wheel SPA 17 X 7" ET54 5 112 Silver Octavia AND Others CCA600002 | eBay

[DunlopRS]

Thanks Brad,
I am tempted by the 17s for the better visual appeal, and the strength of my comment is to stop me going there, as my head tells me that I personally will not be as happy with them on the road.

I may well follow up on the 16s you have provided the link to - not too far away either.

Looks like I will need spigot rings? - I guess most tyre shops could / would supply those?

Re running 225 55r16 on your mkII, I assume that would have made your speedo slightly slow (or perhaps spot on)? on 215 55r16s mine is about 2% fast.

[brad]

Yes, I had spigot rings. They were $15/set for plastic ones. eBay will have them.

225/55r16 were 100kph speedo = 96kph actual.

I currently run 225/50r17. 100kph speedo = 99kph actual. With the Eibachs & 17x7.5 ET45 there are minor clearance issues when encountering a mid-corner bump. I think if they were ET50 it wouldn't be an issue. The distance from the centre of the axle to the edge of the guard is 355mm.

[DunlopRS]

Well, ended up with the 7.5x16 ET45 Audi ones that Brad pointed out + 225 50 Bridgestone RE003s.

Did not get around to weighing the rims but the are significantly lighter than the 6.5x16 skoda ones that came off...

Lets see how it goes. Feels good at the moment (don't mind the road grime....).

[brad]

Did not get around to weighing the rims but the are significantly lighter than the 6.5x16 skoda ones that came off...

I don't think so - there's a lot of decorative metal there.

My OEM 16x6.5 were 8.7kg and the 205/55r16 CSC2 (a very light tyre) was 8.6kg = 17.3kg
The steel 16x6.5 space saver spare with tyre is only 16kg so alloy being lighter isn't always the case.

The 16x7.5 Audi A4 rims were 10kg & the 225/55r16 Primacy HP tyre is 10kg = 20kg (maybe your tyres are lighter?)

17x8 BBS RC (7.5kg) with 225/45r17 Kumho (10.5kg) are 18kg

17x7.5 Audi A4 (10.1kg) with 225/50r17 RE050A (10.9kg)are 21kg

18x7.5 Audi S3 (10.6kg) with 225/40r18 RE050A (11.4kg) are 22kg