first impressions - up from 17" to the option 18" wheels

Botus

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on the 221 long wheel base...

I meant to do this years ago and got all the bits (genuine OEM 18" 8.5" wheels, longer wheel bolts, another set of TPMS sensors and decent tyres) but never got round to fitting at the right time, so by October that year I thought I'd leave the 17" with winter tyres on the car. And its been like that for 3 years !!!

The idea was to keep the slightly battered 17"s swap the worn summer tyres for winter ones and play musical chairs with the 18" as the weather improved.

But I was lazy and never did it.

So I'd gone from a set of worn Michelins 235 / 55 / R17 99 W on the std. 8.0" wheels with summer tyres. Which did the job competently. Then went to low rent Nokian winter stuff in the same size. The change was pretty clear. The car was more vague on the steering and wandered about more on motorway work, it squirmed in a lively manner on hard acceleration and you could get the back to move more easily than I expected. The ride comfort dropped and the noise levels increased quite a lot too. Luckily fuel consumption didn't seem to nose dive, but the noise and wander were a little wearing.

But now I'm shocked how it behaves on the 18" wheels with 255 / 45 / 18 R 99 Y Dunlop MO (MO for Merc original spec I believe). Had all 4 wheels set up to point the right way when I put the tyres on.

So what's it like...

Ride comfort change is extraordinary. Its now like I have the sport button on when in Comfort. Brittle and lumpier and not really right for the car. Acceptable, but I expected a gain taking off the winter stuff I had. I've only tried Sport for a short while, its bumpy thumpy enough now I don't feel the to need to press a button to encourage it more. And on the motorway it felt odd (don't get why).

Noise levels, I guess lower??? but there are times where you clearly hear the tyres where I didn't notice any intrusiveness at those points with the winter ones.

Steering weight and feel is better, the steering being slightly lighter and more consistent even though the tyres are 20mm wider !

Handling, not the transformation I expected, steering is a bit crisper, turn in improved, but grip on summer tyres in warm dry conditions isn't better ! Back lane scratching, its better but nowhere near the gains I expected. Accelerating hard through a bend feels better. But not sure about stability slowing for bends, OK ish but no real gains.

And now the concerning stuff. The car moves in conditions where the old stuff didn't. First shock was a cattle grid that's placed on a bend, which is always some thing interesting... Been over this particular one lots with the old set up and it ALWAYS stuck where it shouldn't. Drove over with this set up more gingerly than usual and got a massive wobble on, moving more than I'd have anticipated if it was icy ! Another place where it should be better but isn't, turn on to a dual carriageway in a long arc from a place near my parents and I had to be sensible waiting for the old tank to try and negotiate the corner, runs wide on old set up and have to wait an eon to apply power... but now there's far less grip and it was scruffier and moved when going slower !!!

Stability, this thing has never done the straight line stability I expected. It got noticeably worse with the winter tyres, but has always been hard work on motorways. Wandering in its lane and you have to fight to get it to behave (which is at odds with the cars standing and irritating when "relaxing" letting active cruise mange the gap). And with the new set up its no different at all. Equally rubbish until you up the speed and then its actually worse ! There some odd goings on at the back with it "talking" its own game. When I tried Sport this taking seemed to make things even more lively.

Not exactly dangerous??? and not enough for me to be excited to go see what wrong. But clearly strange, and enough I'd expect other drivers to wonder what I'm doing ! I can see the pressures on board and they match what I saw as I leveled them out before I set out (with it at 2.6 bar on the motorway). But its not good, its more like a high mile, light weight car being thrashed in strong blustery sidewinds than a high value luxo barge just upgraded to the set up the dev drivers would have had.

all in all an expensive way to not really improve anything !!!
 
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Blobcat

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How old are the tyres you’ve just fitted?
 
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Botus

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….you may have a point. the fronts are nearly new, the rears are older SP9000s with 6 mm each

I was in cost co and asked about tyres the other day, they had been £210 each but were now down to £140 each the other day ! but I had to refurb the wheels and they did a big money fairly average job and someone somehow has stolen my longer wheel bolts so thrown 500 quid on stuff I didn't want to do, so the rear's will have to do at the moment
 
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