Vintage Commercial Tyre Sizing

[Rangie]

Hi Folks, I appear to have stumbled into a minefield while looking for tyres for my Bedford!

 

The manual says 32x6 size.

 

Now, my interpretation of this is a tyre of 32 inches diameter of 6 inches width.

It is 100% aspect ratio, therefore a sidewall height of 6 inches also.

I have 20 inch rims so thats 20"+6"+6"=32" diameter : Bingo!

 

Now then, 32" equates to 813mm

 

So does that mean a nominal new tyre size of 800 or 825-20?

Or does the 80% aspect ratio factor in........ Grr!

 

Or I may just fit 750-20s to help the torque of the engine.

 

Sigh....... Any thoughts anyone?

 

Alec.

[N.O.S.]

Take the GMC example - 34 x 7 is 20" rim and 2 x 7" as your maths above.

 

Now these were replaced by 7.50 x 20 being pretty much identical o.d., so 7.50 x 20 must be about 90% aspect ratio.

 

800 x 20 means 8" x 20", and 825 x 20means 8 1/4 " x 20".

 

If similar 90% aspect ratio for modernish tyres then you need approx. 6.50 x 20?

 

7.50 x 20 would be a big increase in diameter (approx 2"?).

 

Or maybe not :red: I'm still willing to learn :blush:

Edited February 20, 2012 by N.O.S.

[N.O.S.]

So does that mean a nominal new tyre size of 800 or 825-20?

Alec.

 

Isn't this a hang-up from the changeover from defining width by inches to mm?

 

825x20 would be 8.25" x 20". The new metric designation equivalent would be 205 x 20

 

825 x 20 where 825 is in mm would be a tyre 33" wide :shocked:

 

Crazy that they metricated the width but not the rim size :laugh:

Edited February 20, 2012 by N.O.S.

[radiomike7]

Take the GMC example - 34 x 7 is 20" rim and 2 x 7" as your maths above.

 

Now these were replaced by 7.50 x 20 being pretty much identical o.d., so 7.50 x 20 must be about 90% aspect ratio.

 

800 x 20 means 8" x 20", and 825 x 20means 8 1/4 " x 20".

 

If similar 90% aspect ratio for modernish tyres then you need approx. 6.50 x 20?

 

7.50 x 20 would be a big increase in diameter (approx 2"?).

 

Or maybe not :red: I'm still willing to learn :blush:

 

You are correct about the 90%, from my chart a 10.00x20 is 38" overall diameter (10x0.9x2)+20=38"

 

A 6.50x20 would be (6.5x0.9x2)+20=31.7" so almost spot on.

 

A 7.50x20 would be (7.5x0.9x2)+20=33.5"

 

Bear in mind there will be a small variation between actual and theoretical tyre size and that metric width truck tyres are 80% aspect ratio or less i.e. 295/80 R 22.5.

[Rangie]

Very succinctly put folks, thanks!

 

Alec