What metric size are my 33" X 12.5" tyres?

[3 Sisters]

Pardon my ignorance, but I'm looking for some thinner tyres but the same height or higher than my 33" X 12.5" ones I have on there. Everything seems to be in metric and without the diagonal measurement that gives me the 33" tyres I have now.
I'm rolling on 15" rims, and while I have anyones attention that may have tyre knowledge, is there any reason not to run 16" rims? I'm thinking I'd get less tyre deflection(roll)on sealed roads, and with such large tyres, I think they'd still have enough side wall height.
Is the main reason to run 15s over 16s still just cheaper available 15 compatable tyres, or has that balanced out now days?
So in short, I'm after some 10-10.5" wide tyers that will be as tall or taller than my 33" Mickey Thompsons, and can I get away with 16" rims? and what about ally rims over steel, are they much lighter, and much weaker?
Also, any reccomendations for some good dirt tyres that'll match the Mickeys, but still last as long and handle on road use as well?
Again, pardon my ignorance.

[Psimpson7]

255/85 R16 are a fraction over 33" tall, and about as narrow as you are going to get.

Rgds
Pete

[Tombie]

or 285-75-16s about 33 x 10.5 x 16"

And will allow you to run Mickeys...

Dont forget, 285s need 8" wide rims.

[Benny_IIA]

I think the metric size they would around a 305,75,15.

[Bush65]

The 1st part of metric size is the tyre width, 315 mm is the nearest equivalent to 12.5".

The 2nd part of metric size is the ratio of tyre section height to width as a percentage. So the height of 33 x 12.5 x 15 tyres is (33" - 15") / 2 = 9" and the ratio of height to width is 9" / 12" = 0.75. Expressed as a % is 0.75 x 100 = 75.

So the equivalent metric tyre size would be 315 R 75 15 (R is radial ply construction).

[Slunnie]

305/70-16 => 33x12.5-15
285/75-16 => 33x11.5-15
255/85-16 => 33x10.5-15

[3 Sisters]

so roughly speaking, a
31 X 10.5
32 X 11.5
33 X 12.5
will all have roughly the same diameter overall?
Edit:- I worked out that the 33 would still be the tallest, the 32 possibly 3/4 of an inch shorter and so on for the 31.

[Sleepy]

so roughly speaking, a
31 X 10.5
32 X 11.5
33 X 12.5
will all have roughly the same diameter overall?

Wouldn't they be 1inch increment diameters - or am I missing something?

[3 Sisters]

Wouldn't they be 1inch increment diameters - or am I missing something?
No and they wouldn't be exactly the same either.
The 33" measurement is from the outside bottom edge of the tyre tread, to the inside top edge of the tyre, so measured diagonally through the rim and tyre. The yanks must have thought it up.
I'm after a larger diameter but narrower tyre than my 33" X 12.5"s.
So I think I'm after a
33"x10.5-11.5", and in metric on a 15"x8" rim, I think that's about 275-330 x 85+.

[Sleepy]

No and they wouldn't be exactly the same either.
The 33" measurement is from the outside bottom edge of the tyre tread, to the inside top edge of the tyre, so measured diagonally through the rim and tyre. The yanks must have thought it up.


Well there you go - learn something new every day. It only took me 20 years to work out the metric version:confused:

No wonder I run 7.50 x 16s:angel:

[Slunnie]

so roughly speaking, a
31 X 10.5
32 X 11.5
33 X 12.5
will all have roughly the same diameter overall?
Edit:- I worked out that the 33 would still be the tallest, the 32 possibly 3/4 of an inch shorter and so on for the 31.
Not quite.

The first number is the outside diameter, not a diagonal measurement. The tyres you have listed increase in size progressively by a nominal 1" per tyre.
The second number is the width through the bag in the tyre. Each tyre you have listed gets wider through the bag of the tyre (widest part of the sidewall) nominally by 1" with each tyre listed.

[Sleepy]

watching with interest :angel:

[3 Sisters]

Not quite.

The first number is the outside diameter, not a diagonal measurement. The tyres you have listed increase in size progressively by a nominal 1" per tyre.
The second number is the width through the bag in the tyre. Each tyre you have listed gets wider through the bag of the tyre (widest part of the sidewall) nominally by 1" with each tyre listed.

Awe god I'm confused now. I even looked in a book at the tyre shop. Where did I get the concept of the diagonal measurement? I'm just going to measure my tyres in the morning to confirm what the 33" is, then I'll edit my posts to save others from absorbing my confusion.

[Tombie]

I can assure you Slunnie is correct..

I've worked in my mates tyre shop helping him out..

Its a *nominal* outside diameter...

[3 Sisters]

Forget all I've said for now. Here is what all the tyre numbers mean.
Tire Width
The 235 is the width of the tire in millimeters (mm), measured from sidewall to sidewall. Since this measure is affected by the width of the rim, the measurement is for the tire when it is on its intended rim size.

Aspect Ratio
This number tells you the height of the tire, from the bead to the top of the tread. This is described as a percentage of the tire width. In our example, the aspect ratio is 75, so the tire's height is 75 percent of its width, or 176.25 mm ( .75 x 235 = 176.25 mm, or 6.94 in). The smaller the aspect ratio, the wider the tire in relation to its height.

Calculating the Tire Diameter
Now that we know what these numbers mean, we can calculate the overall diameter of a tire. We multiply the tire width by the aspect ratio to get the height of the tire.

Tire height = 235 x 75 percent = 176.25 mm (6.94 in)
Then we add twice the tire height to the rim diameter.

2 x 6.94 in + 15 inches = 28.9 in (733.8 mm)
This is the unloaded diameter; as soon as any weight is put on the tire, the diameter will decrease.

I'll try to find where I got the diagonal measurement confusion from.

[3 Sisters]

Sorry to derail my own thread a bit with this slightly off topic question.
I'm after bigger wheels so they roll over stuff better, as I'm changing my diff gearing to suit the 33"s and it'd suit the 35"s better anyway, but I'd rather thinner than 12.5" tyres for better rolling resistance, and this brings me to my next two questions.
Am I dreaming, would 10.5 or 11.5 tyres have much less rolling resistance(and traction also, while I have your knowledge base)?
Could I get 35" tyres for 15 or 16" rims that are 10.5 or 11.5 wide, or do they not make tyres with a height that are less than a certain % of the tyres width?

[Slunnie]

Sorry to derail my own thread a bit with this slightly off topic question.
I'm after bigger wheels so they roll over stuff better, as I'm changing my diff gearing to suit the 33"s and it'd suit the 35"s better anyway, but I'd rather thinner than 12.5" tyres for better rolling resistance, and this brings me to my next two questions.
Am I dreaming, would 10.5 or 11.5 tyres have much less rolling resistance(and traction also, while I have your knowledge base)?
Could I get 35" tyres for 15 or 16" rims that are 10.5 or 11.5 wide, or do they not make tyres with a height that are less than a certain % of the tyres width?
Narrower tyres do have less rolling resistance than a matching wider tyre. Traction depends on a lot of things such as tread pattern and surface in the same comparison, but I find that a broader tyre gives more traction on a hard smooth surface.

35's can be had in a marked 10.5" width and a 11.5" width in both rim sizes, though Simex may be one of the few in that size. In the Simex a marked size doesn't necessarily mean that this is actually the width or diameter.

[TonyC]

Narrower tyres do have less rolling resistance than a matching wider tyre. Traction depends on a lot of things such as tread pattern and surface in the same comparison, but I find that a broader tyre gives more traction on a hard smooth surface.

35's can be had in a marked 10.5" width and a 11.5" width in both rim sizes, though Simex may be one of the few in that size. In the Simex a marked size doesn't necessarily mean that this is actually the width or diameter.

If you are after a low rolling resistance the Simex is not what you are after.

In a 33inch a 255/85R16 is 33X10.5, as far as I know you can get a BFG Mud Terrain or Maxxis Big Horn in this size in Aus.

You can get a Mud Terrain in a 33 10.5 R 15 and you used at be able to get a 33 9.5 R 15, I've never see these sizes in another brand.

Tony

[3 Sisters]

If you are after a low rolling resistance the Simex is not what you are after.

In a 33inch a 255/85R16 is 33X10.5, as far as I know you can get a BFG Mud Terrain or Maxxis Big Horn in this size in Aus.

You can get a Mud Terrain in a 33 10.5 R 15 and you used at be able to get a 33 9.5 R 15, I've never see these sizes in another brand.

Tony

What about in larger than 33"? On 26 or 15" wheels.

[TonyC]

What about in larger than 33"? On 26 or 15" wheels.

Have a look here, BFG go to a 42 inch others go bigger.

http://www.bfgoodrich.com.au/config/downloads/MudTerrainTA_KM2_DimensionData.pdf

The 15 inch stuff you are after is listed under flotation down the bottom.

You say that you are after low rolling resistance, so I'll assume you want a radial, but what sort of tread pattern do you want? The more aggressive you go the higher it will be.
Why are you chasing low rolling resistance?

Tony

[3 Sisters]

Have a look here, BFG go to a 42 inch others go bigger.

http://www.bfgoodrich.com.au/config/downloads/MudTerrainTA_KM2_DimensionData.pdf

The 15 inch stuff you are after is listed under flotation down the bottom.

You say that you are after low rolling resistance, so I'll assume you want a radial, but what sort of tread pattern do you want? The more aggressive you go the higher it will be.
Why are you chasing low rolling resistance?

Tony
I plan on doing a lot of KMs, want to save peterol, better for the environment(I'm only 70kg, driving a 2.3 ton disco seems greedy, my Willys jeep is less than half the weight at 1 ton).
I still want traction, but I also wonder if fatter tyres really help that much in a lot of (most)situations, any knowledge here would be appreciated.
I'm happy to get 16" wheels, as it seems to make more sense with such big tyres, I just thought generally 15" tyres were cheaper, are they?

[3 Sisters]

What reasons are there not to run 35 x 12.5" on 18" Ally rims for around oz trip.
Is there much need/benefit for a big bag(high side walls)?

[GuyG]

Couple of ideas for not changing for long distance trips:
- gearing unless you've changed the gear ratios to suit the larger tyre
- legality is your vehicle engineered to run the larger tyres
- availability of tyres should one be required elsewhere in the country
- Where abouts is your spare mounted, and possibly a 2nd spare if you decide to carry it
- has your speedo been recalibrated or you have a gps
- has the driveline been strengthened to handle running 35's

Just a few thoughts

[3 Sisters]

Couple of ideas for not changing for long distance trips:
- gearing unless you've changed the gear ratios to suit the larger tyre I'm currently running 33"s on standard gearing, I'd like to change the gearing at the diffs when I get out of debt, and get a rear locker. I have some fine spline diffs ready as a donor.
I think changing the gearing for the 33"s will go too low,hence why I'm partially interested in running 35"s.
- legality is your vehicle engineered to run the larger tyres I have my Disco engineered to run 35"s, currently she rolls on 33"s though.
- availability of tyres should one be required elsewhere in the countryCouldn't anything be couriered? Would 35"x18" tyres be much harder to get than 33/35"x15/16" tyres?
- Where abouts is your spare mounted, and possibly a 2nd spare if you decide to carry itI'll be towing a No5 trailer with one spare on it, it has 33" tyres on it(although I may put smaller tyres on it so it's not as top heavy/high),so if I flatted the spare, I could always leave the trailer and take both it's wheels.
- has your speedo been recalibrated or you have a gpsSpeedo is out on 33"s anyway, so I'd take a GPS, and also just work out what % out my speedo was, and possibly mark it on the speedo with marker.
- has the driveline been strengthened to handle running 35'sSome parts have been, what needs to be done in total?

Just a few thoughts Thank's for your thoughts, keep them comming please, very helpful to reassure my ideas, or make me realise something I've overlooked.
I think 33x10.5s on 16"s will suit me most, but still uncertain.
Are 16" tyres as easy to get as 15"s? Are they more expensive? What about for 18" wheels?
Why do people not like Ally wheels? Wouldn't they be lighter? Meaning lower fuel cost, but less flywheel type affect for keeping momentum in the rough?
Remember I'll have 3 spares, so destroying a rim isn't such a drama.
Would running a smaller wheel/tyre do much/any damage to the drive train if I had to drive a few hundred KMS on it?