Implications of increasing tyre rolling radius

[simonej]

It's new tyre time for my motorway hack - an MG ZT.

The standard size is 225 45 18 and for a set of four Pirelli P Zeros I'm looking at a cost of about £480. The same tyres can be bought in 225 50 18 for £400 so I was just wondering what the implications of doing so would be?

I understand the increased rolling radius (+3.5%) will knock the speedo accuracy out but it already reads about 10% too fast anyway. So surely this shouldn't be an issue?

I don't care about the effect on acceleration but surely higher profile tyres should increase the comfort level a little? That would be a bonus. Shouldn't it also increase the mpg a little as well due to more distance travelled per revolution? Again, a bonus. The only downsides I can see is that the insurance company might not allow them and that the tracking and geometry might be slightly out.

I realise a lot of people will tell me to stop being a tight Yorkshireman - and they have a point, but for arguments sake is stepping up 5% in profile likely to cause any real problems?

[saaby93]

It'll slightly reduce the number of miles you do per year too.

You can guess which way your premium will go when you let your insurer know about the modification

[simonej]

I know, the increase in insurance premium would undoubtedly negate any savings made by buying the cheaper tyre.

[randomwalk]

Higher sidewall may reduce responsiveness slightly to steering inputs, but it will reduce mechanical wear & tear on drivetrain, plus as has been mentioned improve mpg.

[eltax91]

Do you really have to notify the insurance company if you change size of tyres?

[frosted]

Insurance premium , that's quite funny !

Don't think they will even notice as long they have thread left

[AndrewW-G]

Did the ZT come with other wheel options, i.e. 17inch instead of 18, if so head to ebay and swap over to the smaller and cheaper wheel size

[simonej]

Unfortunately the 17" wheels don't fit over the brakes on the 190 version of the ZT otherwise it's something I would consider.

I think I might give the 50 profile tyres a go, mainly due to the potential for improved mpg and comfort.

[AndrewW-G]

eltax91 said:

Do you really have to notify the insurance company if you change size of tyres?

You should inform them of any deviation from standard spec.

[planetdave]

Comfort is slightly better, turn in is slightly worse.

Getting up hills can be tiresome (ho ho).




I enjoyed the £40/tyre saving at the time but was overjoyed to get the proper size back on.

[Kinky]

Pastie Bloater said:

I know I'm being an arse but just what level of detail do they want to know about?

The best way to think of it is quite simply any reasonable excuse to avoid making a payout.

Having a vehicle on a road which deviates from standard manufacturers spec, and they have not been informed is a good excuse.

It even goes so far that some companies alledgedly consider stickers to deviate from said spec (according to a thread on PH)

[saaby93]

Pastie Bloater said:

Just seems like a whole big grey area to me.
Cheers
Dan

Not grey at all
You can imagine someone changing the cars handling by changing the tyres
Is that more likely by fitting remoulds or michelins rather changing from 45 to 50 profile?
Just about anything is a modification either let them know or let them decide at payout time
Wheres R1Loon today?