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Difference in tyre size 10.50 x 20


Lauren Child

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Hi,

 

I've picked up some tyres to replace the ones on my CMP (which are cracking). The old ones are 10.50 x 20 tubed, and the new ones are 10.5 R20 tubeless. I understand that the tubed to tubeless shouldnt be a problem (as long as I do still put tubes in) but comparing them there's a substantial difference in size now that I've got them home. The old tyres are about 2 inches larger overall diameter.

 

Have I just got the wrong tyres?

 

(keeps fingers crossed)

 

Ta

Lauren

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It's a profile thing, that percentage that people go on about when talking about tyres.

 

Older tyres are normally 100% profile basically the cross section of the tyre is the same height as it is wide. Modern radial tyres tend to be lower profile, from 100% right down to about 50% profile.

 

Your problem is that instead of being 10.50 100 20 ( with the 100 being the profile ) you have bought 10.50 (x) 20 with the (x) being something less than 100.

 

They should fit, and work, but of course the two implications are that the truck will sit lower, and the speedo calibration will be wrong because the lower profile will have a smaller rolling radius anyway. There are tyre charts online that will tell you the difference in rolling radius between 10.50 100 20 and whatever you bought.

 

Fit them as a set, with tubes, but don't be tempted to inflate them beyond the maximum pressure stated on the sidewall, and don't mix old and new tyres as it is illegal to have (old ) crossply on the rear of a vehicle and new (radial ) on the front - stuff like that.

 

If you haven't fitted them, have a word with your tyre supplier and see if they can supply the same tyre in a higher profile for a restock / reorder charge maybe?

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Hi Lauren,

 

While Gordon is right about the aspect ratio, also for the same size there are always differences in diameter between the various brands.

 

Furthermore I have seen a tyre sidewall marked "11.00-20 replaces 10.50-20". I have since learned this was a "new alternative size marking" for the 10.50-20 tyres listed in the wartime Data Book of Wheeled Vehicles, so it must have been a wartime change. (Interestingly, while this tyre originally had Dunlop TrakGrip tread, it was recapped at least twice, the last time with NDCC bar tread.)

 

If I were you I would try to exchange your 10.50's for 11.00-20 tyres.

 

Hanno

Edited by mcspool
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Thanks for the advice chaps.

 

I've not fitted them yet. Before I see if I can exchange them, have you got any ideas if 1100x20 are still made? I got no reply from the vintage tyre sellers I found on the internet, so when I saw the 1050s I jumped at the chance to get a full set. If it's a case of hanging on to see what crops up on ebay/milweb I may hang onto them as not-far-off.

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11.00-20 (not 11.00x20!) is a current size, fitted to many US military trucks, so they are abundant in bar grip style.

 

I have seen 11.00-20 Trak Grip tyres on Green Goddess fire trucks, those would be an ideal pattern tyre for your CMP truck.

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  • 9 months later...
Older tyres are normally 100% profile basically the cross section of the tyre is the same height as it is wide. Modern radial tyres tend to be lower profile, from 100% right down to about 50% profile.

 

Your problem is that instead of being 10.50 100 20 ( with the 100 being the profile ) you have bought 10.50 (x) 20 with the (x) being something less than 100.

....

don't mix old and new tyres as it is illegal to have (old ) crossply on the rear of a vehicle and new (radial ) on the front - stuff like that.

 

 

The Radial 10.50 20 will have a sidewall height of 80 or 82 % of the width.

There is a way, and one way only, to mix radial and cross ply or bias ply tyres. That is to have a pair of radial at the rear, and a pair of cross ply or bias ply at the front. That is both legal and perfectly fine to do. As said, any other combination is illegal and potentially dangerous.

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The Radial 10.50 20 will have a sidewall height of 80 or 82 % of the width.

There is a way, and one way only, to mix radial and cross ply or bias ply tyres. That is to have a pair of radial at the rear, and a pair of cross ply or bias ply at the front. That is both legal and perfectly fine to do. As said, any other combination is illegal and potentially dangerous.

 

That'll make the transmission wince in 4WD! :D

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indeed it would, but you can get a radial to be the same height, it just has to be wider too in order for the 80% sidewall height to match the 100% of the front.

 

Anyway, that's all in theory. In my experience tyres very often bear no resemblance after physical measurement to what they should do after mathematically working out their size. Tape measures are best!

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