Reducing rim size on vRS

[DunlopRS]

Wow mate - they look HEAPS better on a white wagon than they did on my black one.

I reckon it looks tough

Thanks gldgti – yes, the looks are growing on me – the red calipers still are reasonably visible thru the wheel too. Especially now its covered in (almost) mud from road works in the rain it looks very “purposeful” rather than a “show pony” (please don’t anyone take offence at that remark – I really love the look of a nicely set up car such as the “dark arrow” – superb! - but operationally they do not work for me… - each to his/her own needs) - all mine needs now is the spotlights…….

Basically, because the 16" tyres are more flexible radially and tortionally, they will conform much better to the surface they are resting on and employ the tread much better. Very low profile tyres are very stiff torsionally, and under all but ideal circumstances will have much less success gripping the road surface.

Extreme examples being drag racing tyres.

Also thanks to gldgti for the physics/engineering lesson – could visualise this but not ‘verbalise’ it. FWIW one of the few Top Gear episodes I saw demonstrated this with I think a HSV Commodore on a track. They went up a rim size by I think 1” and over repeated runs the taller rims were seconds slower than the smaller ones. On the 18” I had learnt to come off the throttle significantly as the turbo lag dissipated to try & reduce the incidence of spin (my first turbocharged car - do not need to now with the 16"s) – it was always interesting watching the other half who does not drive the RS often having the wheels spinning at almost every significant acceleration around town. I would not have got 25k km out of those (expensive) tyres.

Covered 475 km this afternoon with an indicated fuel consumption of 5.2 L/100 km at the speed limit (gen 110 – normally notice a significant increase in consumption from 100 to 110) with the kids and luggage. Have the kids working on some note taking comparing mileage markers with the odometer and satnav – the odo seems much closer to the ‘actual’ with my setup than I expected – will report when I have covered about another 800 km tomorrow. With this setup the vRS really eats the miles on any sealed / semi sealed surface.

[gldgti]

Great result mate. Sounds like perhaps you really have jumped into a subvariant "GT" class, reminicent of the 90's european sold VW's of the same badging - Golf Mk2, 3, and 4 all came out in diesel GT variants - diesel economy and torque mixed with GTI-esque suspension but they usually had more "normal" sized wheels and they were a lot harder to pick out on looks than a GTI. You could even get syncro and later 4motion ones.... wish they came here

[kaanage]

Tyre (and indeed suspension) compliance are highly underrated by many on this forum. Really glad to hear the outcome has been so good.

[DunlopRS]

Tyre (and indeed suspension) compliance are highly underrated by many on this forum. Really glad to hear the outcome has been so good.

Thanks – my main reasons for going with 205/55-16 on the 6.5” rims were
1. Skoda plated/recommendation for the rim size.
2. Should be no problems putting snow chains on.
3. Common fitment – have not that long ago spent most of Sat on a 2 day weekend away scouring used tyre heaps in a major regional centre to get a suitable tyre to get me home following staking a sidewall on an Aus built car – no new ones the right size in town (population 20000+…..).
4. Was most likely to get the result that I was after – and it has.

You realise that the smaller diameter of the 205/55x16 will make your fuel economy look better even though it's not? Also, 15,000km on the odo will now be more like an actual 13,500km (I think that's what I worked out mine was).

Have had a go at determining odo accuracy with this set up (new rubber - could not say that for the 18s I had as the 2900km trip would have been more than 10% of the tyres life…).

Firstly, comparing distance travelled by satnav to odo (trip meter) distance. Criteria of >40 km but <99 (to get reasonable distance but keep satnav to 0.1 km intervals). Ten observations, odo 0.3% less than satnav (range 0 to 0.6% less).

Secondly, comparing distance travelled by roadside marker to trip meter. Criteria > 40km. 14 observations, over distances of 40-130 kms. Odo 0.4% less than mileage markers (range 0.3 to 0.7% less).

0.4% or 400m in 100 km is likely close to the errors associated with the measurements – I am happy to say the odo on these tyres represents reality (and the odo seems to be showing slightly less (if anything) that the actual distance travelled).

As for the speedo – didn’t think to get the kids going with a stopwatch over some long stretches – will leave that lesson in 2D motion for the kids to the next trip out west. A number of cameras did this for me – I hope I will not get any surprises in the mail in the next couple of weeks……..

So, over 2900 km in the week the indicated fuel consumption was 5.5 L/100 km or 51.4 mpg in the old language. Happy to call that it – not sure how accurate the fuel metering is but it’s likely as good or better than estimating based on fuel bowser measurements and degree of tank fill.

Moving away from any OCD tendency (or is it just my Germanic heritage…. ) to real life & work now…….

[brad]

As for the speedo – didn’t think to get the kids going with a stopwatch over some long stretches – will leave that lesson in 2D motion for the kids to the next trip out west. A number of cameras did this for me – I hope I will not get any surprises in the mail in the next couple of weeks……..

You shouldn't unless you were deliberately speeding. 100km on the speedo should be around 93kmh actual with those tyres.

[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.