Lauren Child Posted February 14, 2012 Share Posted February 14, 2012 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 Quote Link to comment Share on other sites More sharing options...
Gordon_M Posted February 14, 2012 Share Posted February 14, 2012 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? Quote Link to comment Share on other sites More sharing options...
mcspool Posted February 19, 2012 Share Posted February 19, 2012 (edited) 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 February 19, 2012 by mcspool Quote Link to comment Share on other sites More sharing options...
unionjack Posted February 19, 2012 Share Posted February 19, 2012 The standard aspect ratio for any "r" tyre is 80% or 82% So, a 10.5x20 will be 10.5" high at the sidewall, and the 10.5 r20 will be 18-20% lower. Anything less than the standard "r" 80% will be marked as such, i.e 10.5/70r20 where the ratio is 70%. Quote Link to comment Share on other sites More sharing options...
Gordon_M Posted February 20, 2012 Share Posted February 20, 2012 OK, so approximately 2" less on the radius and 4" on the diameter then, compared to the old crossplys. That's a lot. 11.00 x 20 may be a better option, unless you have already fitted them? Quote Link to comment Share on other sites More sharing options...
Lauren Child Posted February 20, 2012 Author Share Posted February 20, 2012 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. Quote Link to comment Share on other sites More sharing options...
mcspool Posted February 21, 2012 Share Posted February 21, 2012 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. Quote Link to comment Share on other sites More sharing options...
ferret Posted February 21, 2012 Share Posted February 21, 2012 Hi Lauren, Hope "Dolly" is still running well! I woudn't worry too much about the smaller diameter tyres your F22 is fitted with larger wheels and tyres than standard CMP's anyway. Regards Chris Quote Link to comment Share on other sites More sharing options...
mcspool Posted February 21, 2012 Share Posted February 21, 2012 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. Like so: Quote Link to comment Share on other sites More sharing options...
Lauren Child Posted February 26, 2012 Author Share Posted February 26, 2012 Thanks for the help guys, this is my first vehicle at home so I'm still learning lots. Yes Chris, Dolly is still fine - just got a bit of work to do on the battery cables and the tyres (as and when). I still get a broad grin every time I look out of the window Quote Link to comment Share on other sites More sharing options...
unionjack Posted November 30, 2012 Share Posted November 30, 2012 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. Quote Link to comment Share on other sites More sharing options...
N.O.S. Posted November 30, 2012 Share Posted November 30, 2012 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! Quote Link to comment Share on other sites More sharing options...
unionjack Posted December 1, 2012 Share Posted December 1, 2012 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! Quote Link to comment Share on other sites More sharing options...
mogmaner Posted December 2, 2012 Share Posted December 2, 2012 You may find that the smaller tyres improve the road manner and mpg as the engine is not worked so hard ,this is what i found on my unimog 404 ,holding top gear in general running rather than screaming in 5th or lower at the smallest off hills . Quote Link to comment Share on other sites More sharing options...
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