Downgrading from 19" to 18"

[fizagaren]

Hi

Thinking of downgrading from 19" AMG Line alloys to 18" sport

Is there any downsides to doing this / things I should look for ?

Is there anyone who can exchange / sell / buy alloys ?

 

[Petrol Pete]

Not sure that is a 'downgrade' on the UK's potholed roads these days. 18 will give you a little leeway on taller sidewalls.

 

[fizagaren]

Ha, I mean downgrade in terms of the size not benefits .

I want to go down for that extra protection, efficiency , ride quality and price.

 

[Petrol Pete]

Ha, I mean downgrade in terms of the size not benefits .

I want to go down for that extra protection, efficiency , ride quality and price.

My point exactly.

 

[fizagaren]

I was told that changing the size of the wheel can affect the speedometer reading ? Is that true

 

[whitenemesis]

I was told that changing the size of the wheel can affect the speedometer reading ? Is that true

Have a look in the owner's handbook for suggestions for 18" or winter wheel sizes, all of which would not adversely affect the speedometer accuracy

 

[ALFAitalia]

Yes......but not if you stay within the factory range of tyre sizes. Most speedos overread by a few mph from the factory...this is done deliberately as its illegal to sell a car in Europe with an under reading speedo.. So usually going to a smaller circumference will make the speedo under read more...whereas going bigger will just make it more accurate!!!

 

[DSM10000]

I was told that changing the size of the wheel can affect the speedometer reading ? Is that true

Of course it is true unless you ensure that the new wheel and tyre combination maintains the same rolling radius or as close as possible to the existing set up.

 

[Chrishazle]

To sell the 19's, advertise first on here - with photos and full spec (MB part numbers,, ET etc.) then on Ebay, although I'd be surprised if they do not sell on here.

 

[OneForTheRoad]

GB wheels has a few sets in for a glc .

Security measure

www.ebay.co.uk

Not sure why you'd go down to 18's though , the 19's have a 55 profile sidewall anyways . The 18's are a 60 profile. Tyres are not that much more expensive,if at all .

 

[Smiley]

I was told that changing the size of the wheel can affect the speedometer reading ? Is that true

No. It is the diameter of the tyre that matters. The smaller wheel will have a tyre with a deeper sidewall which makes the overall diameter of the 18" (wheel size) tyre the approx same as the tyre for the 19 " wheel. As already said you need to get the manufacturer recommended tyre size for the smaller wheel.

 

[MikeInWimbledon]

It’s an upgrade to smaller wheels: better ride quality, more compliance, reduced tram lining, greater suitability to non-German roads.

In my BMW and Porsche history I’ve often seen people choose the bigger size and then spend an age whining about tramlining and a harsh ride. Personally I moved to almost new premium tyres on newer smaller wheels on both the 335i and Boxster at zero cost.

.

 

[Petrol Pete]

Quick way to measure the real rolling circumference of your present set up .

With tyre pressure corrected use chalk to draw a vertical line on the sidewall nearest the road , continue the chalk line on to the road surface .

Roll the car forward until the chalkline on the sidewall has done a full 360' then mark the road surface at that point .

Measure between the two marks on the road surface and- Robert is your mothers brother - the rolling radius of your wheel has been discovered !

 

[ALFAitalia]

Hmm.....a long as its a very narrow chalk line......and the tyre pressure and ambient temps are spot on. Otherwise just use one of the tyre size calculator online!!

 

[Petrol Pete]

Hmm.....a long as its a very narrow chalk line......and the tyre pressure and ambient temps are spot on. Otherwise just use one of the tyre size calculator online!!

It will be a narrow chalk line , how thick is a piece of chalk ? I have French chalks used to mark steel that can be ground down to a 1mm thick chisel point if needed.

I mentioned tyre pressure and quite frankly having the actual car fitted with the wheel right in front of me in real time (I have also used this method with motorcycles before 'online' calculators existed) is a good way of checking that the online calculator is not incorrect before spending thousands on new wheels and tyres.

 

[OneForTheRoad]

The difference in tyre wall is 5.6 over 5.1 inches.

19's are 235 55 19.

18's are 235 60 18.

No chalk was harmed during this measurement. I own a glc and know the sizes lol.

 

[Gazwould]

The difference in tyre wall is 5.6 over 5.1 inches.

19's are 235 55 19.

18's are 235 60 18.

No chalk was harmed during this measurement. I own a glc and know the sizes lol.

Math or real world ?

 

[OneForTheRoad]

Math or real world ?

Real world . But if you've still got that photo . Feel free to paste it

 

[jdrrco]

I've done this on several cars and the difference in ride quality, quietness and fuel economy is significant.

Those who say it will affect the speedo reading don't know what they're talking about, as the increased side profile of the tyres (which is exactly what creates the difference in ride quality, quietness and fuel economy) compensates for the reduced wheel diameter.

No manufacturer has the time, money or inclination to calibrate the speedo of every individual car on the production line depending on the buyer's choice of wheel/tyre size. The overall diameter of the wheel plu the tyre is what matters, hence higher profile tyres on smaller wheels.

Hope that makes sense!