Need help with a tyre size

  WhiteTruckMan 17:51 16 Jul 2011

I've been pricing up some tyres and getting some frankly ridiculous prices so I am contemplating fitting a slightly different size for a lot cheaper price. the difference is the aspect ratio, which I know is the ratio of width to height. However although I know the width of the tyre its the definition of height that is stumping me. Is it the total height of the tyre - the maximum diameter - or the distance from the bead to the tread? or some different measurement. I want to calculate the difference in rolling radius to find out how much it would effect the speedo reading, and the maximum diameter to see if it will clear the bodywork ok.

Anybody know these things?


(oh, the expensive size currently fitted is 146/60 R13, and I am thinking of a 145/70 R13 at about 1/3 the cost)

  woodchip 17:59 16 Jul 2011

height is the rim size, you should not fit smaller tyres on a car or van than the original ones. you can go slightly over size on width.

If it is under testing like it as to go for MOT then he will or should check the size of the tyres with what it should have on it, if these do not correspond it will fail the MOT

  mark2 19:14 16 Jul 2011

A long time since I had to think about height / width tyre ratios, but memory tells me about the above 145/60 x13, rim size = 13inch 145mm wide 60= percentage of tyre width as height therefore diameter of the tyre at tread will be 13" + 60% of 145mm x 2 so 13" = 330mm, 60% of 145 mm = 87mm, 70% 145 = 101mm. I'll let you calculate the rolling diameter.

Mot aspect of tyres see and following pages.

By the way the current govt are looking into introducing testing on a 4-2-2 schedule even though previous research for the last govt concluded accidents and fatalities would increase. see

  john 52 19:25 16 Jul 2011

Just to be a bit of a killjoy do not forget to tell your insurance company as the specification is slightly different from the standard specification they may load your premium ??

  WhiteTruckMan 22:07 16 Jul 2011

I think that a 5.75% increase in the circumference might be a bit too much for my liking. I have seen in another forum dedicated to these vehicles that someone was using 145/65's. I havent tried pricing this size yet, but a 2.9% increase sounds acceptable, and in fact will bring the speedometer reading more in line with a true figure as given by the gps speed readout on a satnav.

on another note, this is the first I've heard of the 4-2-2 scheme. I'm still thinking about the implications of it, but I personally dont think it would be a good idea to have 2 yearly inspections past 10 years of age. But its fair to say that standards of vehicle design, construction and materials have improved beyond all recognition since the days when the yearly inspection was introduced (whenever that was). With the proviso that any vehicle that is used for hire or reward, from a motorcycle courier to a juggernaut should retain the 1 year test because of the way they wear out quicker.


  WhiteTruckMan 22:10 16 Jul 2011


I was disappointed to realise the scientific calculator included with windows doesnt have a pi button.


  Quickbeam 08:30 17 Jul 2011

The start button on a microwave does a bit of a too soggy pi for me.

  Quickbeam 08:34 17 Jul 2011

I once bought a Granada that has had non-standard tyres on when I got it. For the first time in years I unwittingly found that on motorways at 75mph, I was getting long tailbacks building up behind me. When I eventually replaced them with the right sizes, I found that the tyre combo on my speedo reading was about 10mph too low!

  WhiteTruckMan 09:02 17 Jul 2011

I've come across this site:

which seems to answer my query. It gives comparable sizes and shows the differences in speedo readings etc. It's also shown up a surprising possible alternative in the 155 range too, which I am currently looking into.


  bremner 10:48 17 Jul 2011


My MS Scientific Calulator does.

Bottom Row near the middle - called Pi

  BT 17:29 17 Jul 2011

Left hand side on 2nd row up on mine (Win 7)

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