750R16 tyre size calculation

Submitted: Tuesday, Jan 27, 2004 at 12:07
ThreadID: 10049 Views:32092 Replies:8 FollowUps:10
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Just a quick question. I'm trying to find out how to calculate what the 'official' diameter of a 750R16 tyre should be, and haven't had any luck. I know how to calculate the diameter for tyres where their specifications are stated 'metrically' - i.e. 285/75R16, etc.

Can anyone explain how to work out the 'official' diameter for the 750R16 tyre? I've seen a diameter quoted by one manufacturer as 810mm, but don't know if this is larger or smaller than the official size (most manufacturers don't seem to get their tyres too close to what they're meant to be!). I'm hoping the size is larger than a 275/70R16, and that the 810 is close to what it should be, so I can put 285/75R16s on without requiring engineering.

Regards,

Steve
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Reply By: ExplorOz Team - David - Tuesday, Jan 27, 2004 at 12:25

Tuesday, Jan 27, 2004 at 12:25
Steve,

try this calculator - I have found this to be excellent:

http://www.miata.net/garage/tireCALC.html

Or click here for the embeded link:

Tyre Calc.Regards
ExplorOz Team - David
--------------------------
Always working, not enough travelling ;-)
AnswerID: 44455

Follow Up By: Steve L - Tuesday, Jan 27, 2004 at 12:37

Tuesday, Jan 27, 2004 at 12:37
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.
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FollowupID: 306651

Reply By: Willem - Tuesday, Jan 27, 2004 at 12:29

Tuesday, Jan 27, 2004 at 12:29
Interesting question...so I went out and measured my spare which is a Bridgestone Desert Dueller 750x16 and its diameter is 790mm. It has been on the truck once for around 10,000km so there would not have been more than 2mm wear. There is nothing that states any other sizes on the tyre as you have probably found. Maybe if you searched a tyre manufacturer website you may be lucky.

Cheers,

Willem

Googs Lake camp
AnswerID: 44456

Follow Up By: Steve L - Tuesday, Jan 27, 2004 at 12:39

Tuesday, Jan 27, 2004 at 12:39
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!
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FollowupID: 306652

Reply By: Mark - Tuesday, Jan 27, 2004 at 12:59

Tuesday, Jan 27, 2004 at 12:59
A 7.5R16 is 7.5" high sidewall at 100% aspect ratio. So the diameter is 7.5" + 7.5" (top and bottom) plus diameter of 16" all equals 31". Convert this to mm by multiplying by 25.4 and it equals 787mm.

A 275/70R16 has a 192.5mm sidewall height (275 x 0.70 = 192.5) so its size is (2 x 192.5) + (25.4x16) = 791.4.

Cheers

Mark
AnswerID: 44460

Follow Up By: Steve L - Tuesday, Jan 27, 2004 at 13:05

Tuesday, Jan 27, 2004 at 13:05
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..... :-(
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FollowupID: 306653

Reply By: Diamond(due to duck season) - Tuesday, Jan 27, 2004 at 13:29

Tuesday, Jan 27, 2004 at 13:29
gday steve.
easiest way is to pop into a tyre place or road worthy place.
there is a book called rim and tyres standards.
it is what i use for road worthy checks on tyres.
a 750x16 is a 32 inch roughly 285/75/16 is 33 inch.
im in the same boat my self and do need an engineers reprt.due to duck season coming
ive decided to hide out
love jemima puddle duck
back after the season
AnswerID: 44464

Reply By: Ridgy - Tuesday, Jan 27, 2004 at 13:36

Tuesday, Jan 27, 2004 at 13:36
Scuse my lack of knowledge but does the inflated versus flat factor make a difference ?
My thinking is that the tyre while loaded and rolling has a lower effective diameter.
Which is the one we have to pay attention to ?Drag me away from this keyboard.
AnswerID: 44469

Follow Up By: Mark - Tuesday, Jan 27, 2004 at 14:13

Tuesday, Jan 27, 2004 at 14:13
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
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FollowupID: 306659

Reply By: Steve from Armidale - Tuesday, Jan 27, 2004 at 15:27

Tuesday, Jan 27, 2004 at 15:27
Had the same problem when getting tyres for the Troopy. At the RTA (NSW) they said a 750R16 is the same diameter as a 235/85R16. (Roughly a 32inch tyre). So unfortunately my 255/85R16 is probably illegal (Roughly a 33 inch tyre).

Such is life :(
AnswerID: 44474

Follow Up By: Steve L - Tuesday, Jan 27, 2004 at 15:40

Tuesday, Jan 27, 2004 at 15:40
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.
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FollowupID: 306669

Follow Up By: Mark - Tuesday, Jan 27, 2004 at 16:30

Tuesday, Jan 27, 2004 at 16:30
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
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FollowupID: 306673

Follow Up By: Mark - Tuesday, Jan 27, 2004 at 16:38

Tuesday, Jan 27, 2004 at 16:38
Oops, a typo. The Nissan GU TI tyre is 275/70R16, not a 270/70R16

Mark
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FollowupID: 306674

Follow Up By: Steve L - Wednesday, Jan 28, 2004 at 07:35

Wednesday, Jan 28, 2004 at 07:35
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

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FollowupID: 306743

Follow Up By: Mark - Wednesday, Jan 28, 2004 at 11:38

Wednesday, Jan 28, 2004 at 11:38
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
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FollowupID: 306759

Reply By: Member - Toonfish - Tuesday, Jan 27, 2004 at 15:58

Tuesday, Jan 27, 2004 at 15:58
try this link davis used some caps

http://www.miata.net/garage/tirecalc.html

1999 NISSAN NAVARA DUALCAB
DIESEL 3.2 & SPRINGY CARLTON TOY
2 awestruck kids (dads driving!)
AnswerID: 44479

Reply By: Eric Experience. - Tuesday, Jan 27, 2004 at 22:18

Tuesday, Jan 27, 2004 at 22:18
Steve.
Just pulled out my copy of the tyre and rim standards and it shows 804 mm for highway tread and 812 mm for traction tread. Eric.
AnswerID: 44556

Follow Up By: Steve L - Wednesday, Jan 28, 2004 at 07:43

Wednesday, Jan 28, 2004 at 07:43
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!).
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FollowupID: 306744

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