How to get speedo & trip computer recalibrated for bigger alloys - definitive answer?

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Michael O'Connor

Guest
Hi,

So I upgraded from 16" alloys and 255/55 tyres to 18" with 225/40 tyres. Not a huge difference in size really but my trip computer is being pretty weird, telling me I have 325 miles in my tank when it only ever used to say 310 on a full tank with the old alloys on. Also I've noticed that I set off '30 - slow down' signs and general 30 speed limit electric signs when the speedo needle is just under 30, so more like 29. Would like to get this sorted as it's getting on my nerves not being quite right.

I've searched around for this all over the net and no one seems to be able to give a straight answer, a lot of the threads seem to evolve into tangents etc. I've heard lots about speedos generally being optimistic in terms of speed, but this seems the other way round and it's getting on my nerves a tad. I would imagine it's also affecting the mileage on the car too?

Is this something that can be done with VCDS? Cheers :)
 
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Michael O'Connor

Guest
How do you mean tyre circumference? It's a bit late now that I've bought them and had them fitted haha
 

fangio

Registered User
You will do more miles as the wheels travel slower than they did, so less turns per mile.
Also I've noticed that I set off '30 - slow down' signs and general 30 speed limit electric signs when the speedo needle is just under 30, so more like 29.
So you won't get done for speeding, then? :thumbs up:
 

samuelh_888

Registered User

StuartG

Registered User
Assuming you were on 255/55/16 7" wide wheels with an ET 0f 50, and you went to 225/40/18 7.5" ET54 then you're rolling circumference has reduced by 7.8%.
Essentially, when you're doing 30mph on the speedo you're actual is closer to 32.5mph...
https://www.willtheyfit.com/index.p...2=40&wheel_size=18&wheel_width=7-5&offset2=54

Nope - other way round as the new tyres are smaller rolling circumference - at any indicated speed you are going 7.8% slower on the new tyres then you were on the old tyres but that doesn't fit with your comment of 30mph flash-up signs.
 

samuelh_888

Registered User
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Michael O'Connor

Guest
You will do more miles as the wheels travel slower than they did, so less turns per mile.
Also I've noticed that I set off '30 - slow down' signs and general 30 speed limit electric signs when the speedo needle is just under 30, so more like 29.
So you won't get done for speeding, then? :thumbs up:

Nah, what I meant is because the speedo is out of calibration for the size of the new wheels, it is reading 29mph when I'm technically doing 30. Now a huge difference but small things like that wind me up. When I used a tyre upgrade calculator, the result was that the speedo would be 0.8% too slow, which seems to be about right.
 
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Michael O'Connor

Guest
Nope - other way round as the new tyres are smaller rolling circumference - at any indicated speed you are going 7.8% slower on the new tyres then you were on the old tyres but that doesn't fit with your comment of 30mph flash-up signs.

Whether the rolling circumference is more or less, the speedo is out of calibration.

Surely the rolling circumference would be bigger... Bigger wheel = more circumference no? I thought it would be kind of like running round the outside a race track is actually a slightly longer distance/ bigger, wider circle than if you were to run around the inside, which is a smaller, narrower curve and less distance.
 
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Michael O'Connor

Guest
Assuming you were on 255/55/16 7" wide wheels with an ET 0f 50, and you went to 225/40/18 7.5" ET54 then you're rolling circumference has reduced by 7.8%.

Essentially, when you're doing 30mph on the speedo you're actual is closer to 32.5mph...

https://www.willtheyfit.com/index.p...2=40&wheel_size=18&wheel_width=7-5&offset2=54

I couldn't say to be honest. But the offset of the old wheels was 57, the new ones are 54. Not sure how much difference that makes haha
 
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Michael O'Connor

Guest
I want to get it re-checked anyway. Does anyone actually know where this could be done?

I was under the impression that if my tank was reading '310 miles' left on a full tank on the original 16" wheels that the speedo was calibrated for, then the new wheels must have measured whatever calibration device the full tank reads from.

Either way it needs to be re-calibrated because I want the car to be reading everything accurately. I can't see how it would be that hard to do, you could spec my car with the new alloys from the factory according the 2006 brochure, along with about 4 other wheels options, so it must be something they can change fairly easy in the factory depending on what the customer specs as their wheels?
 
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Michael O'Connor

Guest
1mph out at 30 is pretty accurate........

Yeah but... most people's speedos: doing less speed than it says they're doing; e.g. it says they're doing 30, it's more like 28. Most speedos are optimistic for obvious reasons.

Mine: doing more speed and the speedos says I'm doing less, which judging by the speed limit signs, isn't true.
 
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Michael O'Connor

Guest
Are you sure you started off with 255/55/16 tyres ???. Thats no where near a standard size for an 8P A3.
The normal 16 inch size is 205/55/16 . The 18 inch size is normally 225/40/18 or 205/45/18 .

Yeah my bad, I was pretty tired when I wrote that. It was indeed 205 haha
 
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Michael O'Connor

Guest
Yeah but... most people's speedos: doing less speed than it says they're doing; e.g. it says they're doing 30, it's more like 28. Most speedos are optimistic for obvious reasons.

Mine: doing more speed and the speedos says I'm doing less, which judging by the speed limit signs, isn't true.

Sorry, meant judging by the speed limit signs, it IS true. It's been a long day...
 
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Michael O'Connor

Guest
There is less than 1% difference between a 205/55/16 and a 225/40/18 tyre so will hardly make any difference to the speedo calibration. The 225/40/18 tyre is fractionally bigger than the 205/55/16 by about 0.8% so speedo will read 0.8% lower than it did before . Not even 1mph at 90mph.

Yeah that's like I said before, the difference is 0.8% and that's how much slower the speedo would be. It was the full tank measurement in the middle display that I noticed first, that's what made me think it was out first, not the speedo signs.

But according the calculation site I used, it would be reading wrong, so at 30 it would be measuring more like 30.5 or slightly over. Not a huge different I know, but I'd rather it be reading more than I'm really doing than less.

I'm also assuming it would affect the trip computer's measurement of how much miles I'm putting on the car, which I'd also rather not be incorrect personally.
 

Paul Johnston

Registered User
Michael, did you ever get this issue sorted out? I’m considering changing wheels and tyres as well, and while I understand that the differences in speed are marginal, I’d rather everything was kept legitimate. I would think that mechanics garages or Audi dealerships would be able to recalibrate as cars can be spec’d with different size wheels?
 

desertstorm

Moderator
Staff member
Moderator
VCDS Map User
If it's doable then you would tweak it in VCDS. with the tyre sizes above plugging them into a tyre size calculator you get
upload_2019-3-1_12-41-24.png

So assuming these are correct sizes, and different tyre brands / makes will be slightly different as they will also be different with varying tyre pressure.
With 6mm difference in the tyre diameter that works out at 0.8%.
If a new tyre has 8mm of tread and a used tyre has 3mm of tread that is a 10mm reduction in diameter which is a lot more of a variation than the 6mm difference in diameter that the tyres start off with.
So are you constantly going to alter your speedo calibration when you change tyres or as the tyres wear?. It's not really worth looking to tweak the speedo calibration unless the variation is something like 2% plus.
 
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