different profile tyres front and rear

BeenThroughItAll

I'm sure I had something with 205/40/17s on a few years ago, but 205/45/17s were about 40% cheaper, so I bought them when the fronts were replaced, then did the rears later. No issues with handling etc and as pointed out the actual size difference is tiny.

Iceweasel

I can't agree with the several posters who say the difference is tiny and it will make little or no odds.

The car will NOT handle the way the manufacturer designed it to.

If all 4 tyres were oversized that just might retain the correct balance between front and rear.

The 45 profile tyres have higher sidewalls than 40 profile tyres therefore have more flexibility and the rear of the car will have more lateral movement.

For 95% of drivers for 95% of the time there will be no discernible difference but ....

I don't want to begin a big debate concerning Nuns with baskets of kittens, but it's in an unexpected swift/emergency manoeuvre that the discrepancy will show itself.

Taking a corner at high speed in the wet (which would normally be perfectly safe) could well turn out to be 'interesting'.

I stand by what I said in my earlier post.

I would be changing the tyres for the correct size.

Mis-matched tyres would not pass the equivalent of the MOT in many countries - it would be an immediate TÜV fail for example.

Spicy_McHaggis

Iceweasel wrote: »

I can't agree with the several posters who say the difference is tiny and it will make little or no odds.

The car will NOT handle the way the manufacturer designed it to.

If all 4 tyres were oversized that just might retain the correct balance between front and rear.

The 45 profile tyres have higher sidewalls than 40 profile tyres therefore have more flexibility and the rear of the car will have more lateral movement.

For 95% of drivers for 95% of the time there will be no discernible difference but ....

I don't want to begin a big debate concerning Nuns with baskets of kittens, but it's in an unexpected swift/emergency manoeuvre that the discrepancy will show itself.

Taking a corner at high speed in the wet (which would normally be perfectly safe) could well turn out to be 'interesting'.

I stand by what I said in my earlier post.

I would be changing the tyres for the correct size.

Mis-matched tyres would not pass the equivalent of the MOT in many countries - it would be an immediate TÜV fail for example.

Not relevant unless it's a fail here or he takes it out of the country.

Iceweasel

Spicy_McHaggis wrote: »

Not relevant unless it's a fail here or he takes it out of the country.

You're right of course Spicy - but I mentioned it simply to indicate that it isn't the 'no problem' that several posters seem to think.

Spicy_McHaggis

Iceweasel wrote: »

You're right of course Spicy - but I mentioned it simply to indicate that it isn't the 'no problem' that several posters seem to think.

I'm still not convinced the insurance wouldn't have an issue with it.

BeenThroughItAll

Iceweasel wrote: »

You're right of course Spicy - but I mentioned it simply to indicate that it isn't the 'no problem' that several posters seem to think.

I'm one of those posters, having done it. I don't drive like a nun and never experienced any issues.

Sidewall difference is 10mm. Total of 20mm difference in diameter.

New tyre has 8mm tread depth, so there's a potential 16mm of diametric difference between new and bald tyres.

Different tyres have different sidewall stiffnesses anyway depending on manufacturer and bias/speed rating.

If a manufacturer was making a road car that was that marginal on tyre choice, they'd sell their own brand tyres for it.


Equally, if it were THAT much of a danger to all road users, we'd have legislation preventing people putting different sized wheels on cars.


The 5 Series I've been using the last week has winter tyres on with sidewalls approximately 35mm taller than the usual 'summer' tyres. It's not killed me yet.


It's use on the M1 we're talking about here, not use in F1.

Iceweasel

Spicy_McHaggis wrote: »

I'm still not convinced the insurance wouldn't have an issue with it.

I would be concerned that in the event of a claim they might deem having non-recommended tyre sizes as a 'modification' that had not been disclosed.

I wouldn't give then the opportunity to give me hassle.

colino

They may feel fine but as there is more than the recommended + or - 2.5% difference, they are wrong and you should change them asap.
Where else on the car has the previous owner shaved off a few bob?

Iceweasel

BeenThroughItAll wrote: »

I'm one of those posters, having done it. I don't drive like a nun and never experienced any issues.

Yet.

Sidewall difference is 10mm. Total of 20mm difference in diameter.

I agree.

New tyre has 8mm tread depth, so there's a potential 16mm of diametric difference between new and bald tyres.

I agree.



Different tyres have different sidewall stiffnesses anyway depending on manufacturer and bias/speed rating.

That is my point - the OP's tyres are mis-matched - but much more than if they were the correct size.

If a manufacturer was making a road car that was that marginal on tyre choice, they'd sell their own brand tyres for it.

BMW have tyre manufacturers making tyres with the *star rating for that very reason (not marginal but optimal in their case though) M-Benz have MO, Porsche have N etc


Equally, if it were THAT much of a danger to all road users, we'd have legislation preventing people putting different sized wheels on cars.

That's why I made reference to the TÜV - the EU is moving towards common vehicle inspections - so whether we like it or not, we will get the same - eventually - as there is a danger.


The 5 Series I've been using the last week has winter tyres on with sidewalls approximately 35mm taller than the usual 'summer' tyres. It's not killed me yet.

But they are likely to be of a size listed on the driver's door-post.


It's use on the M1 we're talking about here, not use in F1.

I was thinking more of a twisty A or B road where the rear tyres being slightly more flexible will tend to create oversteer.

We live in a 'free country' where we can take risks as we wish.

The OP can now make an informed decision on what to do.

neilmcl

Iceweasel wrote: »

I can't agree with the several posters who say the difference is tiny and it will make little or no odds.

The car will NOT handle the way the manufacturer designed it to.

If all 4 tyres were oversized that just might retain the correct balance between front and rear.

The 45 profile tyres have higher sidewalls than 40 profile tyres therefore have more flexibility and the rear of the car will have more lateral movement.

For 95% of drivers for 95% of the time there will be no discernible difference but ....

I don't want to begin a big debate concerning Nuns with baskets of kittens, but it's in an unexpected swift/emergency manoeuvre that the discrepancy will show itself.

Taking a corner at high speed in the wet (which would normally be perfectly safe) could well turn out to be 'interesting'.

I stand by what I said in my earlier post.

I would be changing the tyres for the correct size.

Mis-matched tyres would not pass the equivalent of the MOT in many countries - it would be an immediate TÜV fail for example.

Well my manufacturer tyre ratio recommendations are 40 (front) and 35 (rear) on 18s but they also state I can fit a 40 ratio 18" tyre all round or between a 45 and 50 ratio on 17s.

So again, it doesn't make that much of a difference nor will it make the car unsafe or handle abnormally.