Wheel Change

mjs4x4

A2OC Donor
I have just changed the wheels on my Sport from the 17" with 205/45 tyres on to 16" with 185/50 tyres on.

I figured that the tyres will have a different circumference and so the speedo/milage recorder will now be a little inaccurate.

I measured the circumference of the two tyres (because I was not clever enough to calculate it). The 17" wheel/tyre was 74" and the 16" wheel/tyre was 72.5".

If the speedo/mileage recorder reads from wheel revolutions (as I assume it must) the readings will be 2% inaccurate - doing 98mph when reading 100mph (I wish)!

Right or wrong?

Can you use the VAGCOM to correct this - just set the computer to calculate figures on the 16" wheel/tyre combination?


Mike
 
Hi Mike
I'm no expert on vagcom but I don't think it can be used to correct speedo readings.
I agree with your speedo error calculations.

Out of interest, has the ride improved and have you fitted the Sachs dampers yet?


Cheers Spike
 
No, VAG-COM cannot correct speedo readings. It can be used to correct GALA settings for the radio by inputting your tyre cirumferance in mm, but that's it.

Cheers,

Mike
 
Presumably, if you can input (change) the tyre circumference in mm then by implication you ARE changing the data given by the speedo / mileage recorder.


Mike
 
Spike,

I was so sick of the ride quality on the Sport that two days ago I got the Sachs dampers fitted AND changed the wheels to 16" SE alloys (ex eBAY). I drove it home from Richmond and it seemed a lot less uncomfortable (no back ache after 20 miles). I toyed with the idea of changing the springs as well - but concluded that I am spending enough money sorting out this particular car.

I'll get to use it a bit over the next week or so and can form a proper judgement.

You recall that I had the 17" wheel/ABS sensor wire problem when I got it - well for good measure the other side has gone now!!

Set of 17" Sport wheels and near new Michelin tyres appearing on eBay soon methinks!!


Mike
 
Hi I don't think 2% extra inaccuracy on your speedo is anything to worry about, speed cameras assume a 10% discrepancy from what I recall (ie only prosecute 33 or more in a 30) so 2% is OK. If you google this topic there are plenty of sites that give you tyre dimensions but I wouldn't worry. Just drive along at a steady indicated 30, 50, 70 whatever and get someone to follow you and see what their speedo says.
 
lyndonbuck said:
Hi I don't think 2% extra inaccuracy on your speedo is anything to worry about, speed cameras assume a 10% discrepancy from what I recall (ie only prosecute 33 or more in a 30) so 2% is OK. If you google this topic there are plenty of sites that give you tyre dimensions but I wouldn't worry. Just drive along at a steady indicated 30, 50, 70 whatever and get someone to follow you and see what their speedo says.

I was told speedcams assume a 10% inaccuaracy aswell but in my experience they only tend to go off if you're 10mph over the limit. I.e. cams in a 30mph zone only fire if you're going 40 and above.
 
Hmm, my mother in law got busted for 34 in a 30, I do not know what her speedo was reading though.
 
You will probably find your speedo it over reading anyway if Eggberts was anything to go by, if you know anyone with a GPS unit see if you can borrow it, I know when I swaped from the 205/40/17 to 205/50/16 the speedo was then showing correctly! I believe speedos can legally read 10% high but not low, so anyone done for speeding isn't really in a position to complain as their speedo would have read either the correct speed or a higher speed!
Strangley my A4 reads about the same out as the A2 did, yet my old Honda accord was spot on as certified by Blue i gps camers detector.
 
I Have a Garmin Etrex legend GPS , and according to my GPS im travelling about 2mph faster than what my speedos telling me ......
 
You have got me thinking now, but I am fairly sure that with the speedo reading 70mph, my Tom Tom Go 710 is reading 67mph.

I have changed from the standard 15" rims to 17" wheels though, but the speedo is still over reading which is a positive. That said I guess at 30mph or so the speedo would be accurate or possibly under reading!!:confused:

Possibly check this out this weekend!!
 
Back
Top