A question about tyre sizes...

[mattcanuck]

So, I've made a bit of a mistake about tyre sizes it would seem - but I'm not sure why...

As you may or may not know - the Humber is in at the MOT place today, awaiting its test...

As it had some rather tired (no pun intended) looking crossplies on when I picked her up - I decided that while it was at the garage - I would treat her to some new radials...

After reading the markings on the wall of one of the tyres - I found that it wore 6.40 H15 crossplies... So off to google for some conversion charts to try and make sense of this strange (to me at least) number...

I found that 6.40H15 was comparable to 205 70 15 in new money - I was aware that the sidewall might be a little shorter - but I want to lower the car at a later date anyway, so I wasn't worried about that too much...

Anyway, just had a call from the garage, and he's told me the tyres are way too wide to fit the car!! They rub on the wishbones at the front, so he hasn't bothered to try the rears as they look like they won't fit...

I've checked and re-checked, and I'm still coming to the same conversion... So what's going on?

Can anybody enlighten me?

Thanks in advance,

Matt

[dodgealpine]

As far as i know the 6.4 is inches, the 205 is mm, so the conversion should be 162.56. the nearest size in metric is 165. 70 would be the wall height ( 70 per cent of the width) and the wheel diameter is 15".
Hence the tyre size = 165 x 70 x15.
If you were looking to lower 165x60(or 50) x15
like you say 205 is TOO wide

[lemansvk]

Dodgealpine is pretty much on the money as far as the width goes. 6.40 (which indicates a cross ply tyre) means 6.4 inches wide, at 25.4 mm per inch that is about 165 mm. The thing to remember though is that these tyres were 100% aspect ratio - as tall as they were wide. And this means that if you fit a more modern radial with (for example) a 70% aspect ratio you will reduce the gearing of the car. A 165/70 15 tyre will reduce your gearing meaning that when you were previously doing 70 mph, you are now doing 60 mph for the same revs.

The way to overcome this is to go for a wider tyre and try to find a higher profile. If you can fit a 185/80 tyre you will get pretty close to the original gearing. As you have 15 inch wheels I assume your Humber is a Super Snipe. I have a mate here in Canberra who has found that 185/80 radials (as specified for a new Toyota HiLux) work very well on his Snipe Series V

If you want the full story on wheels and tyres, spend some time at the following site - there are 4 pages that explain just about everything, and there are even some calculators to help you decide tyre sizes etc.

www.carbibles.com/tyre_bible.html

[mattcanuck]

Hey guys, thanks for the advice...

I've got the car back now, and it would seem that tyres are the least of my problems!! I've got a full-on weld-a-thon about to start at the weekend!

But the guy did manage to fit the 205's on the rear, and they feel much better on the road - so it looks as though I'll be looking for a pair of 185 70 15's for the front...

Thanks again!

[lemansvk]

Matt

A rim that is too narrow in relation to the tyre width will allow the tyre to distort excessively sideways under fast cornering. On the other hand, unduly wide rims on an ordinary car tend to give rather a harsh ride because the sidewalls have not got enough curvature to make them flex over bumps and potholes.

How wide are your rims? 205 tyres are best suited to rims between 5.5 and 7 inches wide. 185 tyres suit between 5 and 6.5 inch wide rims. You might want to talk to the tyre supplier about this while the tyres are still new.

Cheers, Vic