750R16 tyre size calculation

Thanks David. Seems their site doesn't handle tyres defined in the 750R16 style. Very useful for comparing my 265/75R16 to the 285/75R16s I want though.

Thanks Willem. Have looked through several manufacturers sites, and all they give (if at all) is the actual diameter of their tyre. This is where the confusion comes in, as none seem to be the same even though they are supposed to be the same size!

That's exactly what I thought it was - but couldn't be sure as I'd thought a 750R16 was a bigger (in diameter) tyre than the 275/70R16.

I guess the difference between the 'official' 787mm and the 810mm I've seen on manufacturers sites for 750R16 tyres is simply the 3% variation allowed. Oh well, looks like I'll need that engineering certificate after all..... :-(

The nominal tyre size can be quite different to the measured tyre size. Not only does tyre pressure change the measured diamter, but so does tyre speed.

The same tyre at the same pressure will have a slightly larger diameter when rotating. The faster speed, the larger the diameter increase. And different tyres will have a different change depending on tyre carcass strength, wall thickness etc.. Some manufacturers quote tyre diameter at a certain speed (eg BFG quote revs/mile at 45MPH).

Anotherwords, the nominal tyre size varies considerably for each manufacturer and even tyre type from the same manufacturer. But the important one is that the size rating of your tyres is the same as the tyre placard on your vehicle (to be "legally" correct).

While a lot of attention is paid to the legality of "oversize" tyres, few bother to look at the load rating of the tyre. IMHO, this is probably more important than a 1" or 2" increase in tyre diameter.

Cheers

Mark

Well, if the 750R16 is the same as the 235/85R16, then the diameter is 805.9mm.

Given that, and the details provided in the 'Letter of the Month' in Overlander magazine, there is a 3% variance allowed under Australian Design Rules - meaning the 750R16 can be between 781.7 and 830mm. It is apparently on top of this figure that the 15mm increase in size can then be added.

If this is the case then the largest size that can be fitted to a car with a placard for 750R16s is 830mm + 15mm = 845mm. Thus I can legally fit 285/75R16s (834mm) and the 255/85R16s you have are also legal as they are 839.9mm.

Sorry Steve L, but thats wishful thinking. Once the calculator comes out the "exact" size is used, not the "is nearly the same as..."

750R16 = 787.4 +3% = 811
add 15mm = 826mm

265/70R16 = 777.4 +3% = 800.7
add 15mm = 816 max

275/70R16 = 791.4 +3% = 815
add 15mm = 830 max

285/75R16 = 833.9

So, for the sake of a few lousy mm I am deemed to be fitting illegal tyres. But, a 285/75 Cooper ST has 16mm tread depth (32mm diameter) so is a part worn Cooper legal !!!!!!!!!!

The GU TI Nissan had 270/70R16 as standard so this is deemed the largest size for this vehicle. I have 285/75 so miss out by 3.9mm.

Would like to see an insurance company argue that 3.9mm (1.95mm tread depth) would have made the difference in an accident. Perhaps this is why there is a lot of talk about insurance issues with oversize tyres, but no-one who has had insurance claim cancelled because of it.

Cheers

Mark

Oops, a typo. The Nissan GU TI tyre is 275/70R16, not a 270/70R16

Mark

Mark,

Going by Eric's response below, they are legal. Even using the smaller of the 2 sizes:

750R16 = 804mm +3% = 828mm

Add 15mm = 843mm.

285/75R16 = 834mm

255/85R16 = 840mm

By my maths that makes them (both sizes) legal if your tyre placard states 750R16s, and is no different to what I calculated above. Where was the 'nearly the same as' coming from? You used the exact same 3% I did, just a different first number......

If we're talking offroad tyres, then we're allowed even more;

750R16 = 812mm +3% = 836mm

Add 15mm = 851mm

Steve,
I really hope that you are right, as I have 285/75's on my GU. I wonder what the "official" size of a 285/75 is? You may find that its larger than the calculated 834mm.

Cheers

Mark

So if that's the 'official bible' that roadworthiness is determined on, the 285/75R16 is a legal tyre if your placard says you can run 750R16. Correct?

That also means that 315/70R16 is legal if it's an offroad ('traction tread') tyre, as the 315/70R16 is 849.4mm, and the 812mm of the 750R16 plus 3% plus 15mm = 851mm.

Guess that's finally settled! Thanks everyone for your help - it's obviously not just me that finds all this a bit confusing (and sometimes hard to actually gind figures to work with!).