Car tyres wrong size, problem?

[spacein_vader]

Hi all, I'm not an expert so thought I'd ask here:

6 months ago I bought a used Honda Jazz, and the front tyres are now worn to the point of needing replacement. My issue is that the current tyres (all 4,) are 185/50R 16 while the handbook and manufacturers website say they should be 185/55R 16. How much of a difference is that likely to make? Is it illegal? Will it have a performance or economy impact?

Essentially I need to figure out if I'm buying a new pair of wrong sized tyres or 4 right sized ones.

[kalniel]

You have smaller rolling radius basically, so the handbook radius is 609.9mm while yours are 591.1mm. This means your speedo will over-read by 3.13%, so while travelling at an indicated 60 you'll be doing what would be indicated at 58.18 on the original tyres.

That probably isn't enough to be illegal, as long as the speedo wasn't already calibrated to over-read too much on the original tyres. If you have a smart phone and/or satnav, check.

In terms of performance/economy, you're effectively running a very slightly lower gear, so for the same real speed you have higher revs and all other things being equal, more fuel use. But in practise, if you keep to indicated speeds you'll have a slight fuel saving because you're travelling a touch slower and you're 9.25mm closer to the ground too. That is also nearly a cm of squishyness so if you were running over potholes all day you might prefer the 55s, but I would think in reality none of these effects will be noticeable.

[philehidiot]

The other thing you may wish to check is with your insurance company. There may be a couple of potential issues such as it being classed as a modification. I'd ring and ask but make it a hypothetical question ("I'm looking to fit 185/50, would it be a problem?") rather than saying that you have.

There is something from the ombudsman which says they would have to prove that any modification contributed to an accident but in my experience it's worth avoiding even getting into that kind of argument no matter how obvious it is that you'd win.

Myself I have fitted different tyre sizes before and the insurer just made a note and said it was fine but I've heard stories of insurance companies... haven't we all.

[Zak33]

no it's not ilegal, so you're ok there, because it went DOWN in size and you're looking at going back up to spec. (But going further beyond spec up would be inadvisable -not that you're talking about that)

Moreover, many cars turn up on UK shores, brand new, with slightly different tyres to the handbook, because sometimes they are specced with slightly different tyres for other countries and the factory just tyres them up like that. Many handbooks aren't right.. . or cover two different sizes!

Don't fret and if you choose to change tyre sizes, you're ok to swap one axle at a time - ie change the fronts together, or the rears, or both. But do NOT change the left and right front at different time as the differential won't enjoy that.

[DanceswithUnix]

If the handbook says 185/55 then I would be tempted to go with that. It isn't a sportscar, a softer ride is nice.

[Zak33]

If the handbook says 185/55 then I would be tempted to go with that. It isn't a sportscar, a softer ride is nice.

and if all tyre had the same tyre wall that would work ...but they dont.

however, the point is valid. i agree on the whole

[spacein_vader]

Thanks for the advice all, going to get 4 new ones at the proper size as the rears don't have that much left and it's for commuting so comfort and MPG are more important.

Annoyingly there are no sensibly priced all seasons in that size though.

[kalniel]

Thanks for the advice all, going to get 4 new ones at the proper size as the rears don't have that much left and it's for commuting so comfort and MPG are more important.

Annoyingly there are no sensibly priced all seasons in that size though.

Vredestein Quatrac 5, £81 a corner.
https://www.camskill.co.uk/m139b0s90..._Noise%3A_68dB

[Zak33]

what Kal says...they're good tyres and you will benefit from great all season use.

Although they're C rated for rain and C for economy - theyre good all rounders

[spacein_vader]

what Kal says...they're good tyres and you will benefit from great all season use.

Although they're C rated for rain and C for economy - theyre good all rounders

It was those I was looking for but couldn't find them for under a tonne so bought some others now.

[philehidiot]

I think you're making the right call. Personally, tyres are one area I won't cheap out on.

Reminds me, I need to check the missus's tyres and my own are approaching 2mm... very tempted to get something that works better in snow... For touring bikes that's something of a challenge.