True Tyre Size Comparisson

[lay80n]

As tyre posts come up daily, im going to try to streamline it a bit. If everyone could post up what size tyres they are running, and give a true measurement as well as the nominated size. If this gets a good response i will add it into the bible. With any luck this will help with people deciding on what size of different brands will be suitable for their use.

Some tyres i have lying round -

TSL/SX swampers - Marked 32x11.5, mounted on 15x8 -28 offset. True size 31.2x11.4 (at road pressure ~20psi) Good Tread depth
Bogger - Marked 33x12.5, mounted on 15x8 -28 offset. True size 32.8x12.5 (at road pressure ~20psi) Good Tread depth
MT/R - Marked 31.5x10.5, mounted on 15x8 -23 offset. True size 30x10 (at road pressure ~ 25psi) Average tread depth

(will measure up some more tyres tonight, i have way to many sets of rims and tyres )

Layto....

[Gwagensteve]

Good Idea lay80n - I'll get some measurements on the Weekend.

Quick Q though - are your measurements loaded or unloaded - i.e I assume your measuring diameter from 3-9 O'clock or on a tyre off a car.

Will make a bit of difference I think.

Steve.

[lay80n]

Good Idea lay80n - I'll get some measurements on the Weekend.

Quick Q though - are your measurements loaded or unloaded - i.e I assume your measuring diameter from 3-9 O'clock or on a tyre off a car.

Will make a bit of difference I think.

Steve.

Ah, good thinking Steve, i forgot to add that. Measurements are taken off car. If people are lazy, indicate that your measurements are taken loaded.


Layto....

[ScrawnC]

Diameter measurements will vary with tread depth though, maybe adding a tread depth measurement too?

[lay80n]

Diameter measurements will vary with tread depth though, maybe adding a tread depth measurement too?

I forgot to measure the tread depth on my tyres, so i just added in good or average

Layto....

[Moph]

Goodyear Wrangler MTR's - 235/75R15 (full tread) mounted on 15x7 -12 offset. True size 28.6" @ 25psi measured vertically while loaded.

[ofr57]

road 31 x 10.5 mtr's around 29.5 high unsure on actual width will measure tomorrow these are worn out about 50% tread -poor run these at 30 psi

33x10.5 silverstone extremes about 31.7 high unsure on width , these are brand new (still got hair) there on a 15 x 8 -25 king rim i believe

[alien]

Cooper STT Armortek - 31x10.5r15 on 15x7's measures 30.7" tread to tread. Tread is 15mm tall.

[CairnsZook]

Federal Couragia M/T 235/75R15 on 15x7 rim, 26psi, unloaded (spare), minimal tread wear (near new).

Measures 28.5"

[lockdup]

TSL Super swampers, 34/10.5/15 really measure at 33/10. i would say 10mm tread depth in center and deep lugs on side. (loaded)

[GRPABT1]

I'll let you know how big my 31" pedes are in a week or so

[david123]

Wouldn't it be better to get the rolling distance ie, circumference rather than diameter, most every tyre maker has both of these on their sites, and a rolling distance is more accurate.

[CairnsZook]

[david123]

Exactly, measuring a full rotation will be say 1 meter, where as measuring from one side to the other, you have to be very accurate, or when multiplying this out to circumference, big mistakes can be made.

Far better to get circumference then use maths to get diameter, rather than the other way around.

[islandvitara]

maxxis Trepadore 31"x10.5" mounted on 15"x8" rims really measure 30.5"x10.5"

brand new 99% tread

[brendan_h]

BF muddys

32x11.5x15

realy are at 30PSI (i think)

31x10 closest to the half inch

[suzyted]

There is many way to measure tyres.
Static load radius is the radius of the tyre when working
this measurement is used for 4wd tractors big earth movers etc with a lead in measurement to take into consideration. the tyre manufacturer supplies a tech sheet on all tyres they make and this varies from tyre maker to tyre maker

od is worked out this way eg 235/75R15

Side wall heights , you x by 2 because of two sidewalls either side of the centre line eg (235x 75%) x 2 = 352.5mm

rim diameter is 15 inch thus 15 X25.4 = 381.00mm to turn into mm
Total 733.50 mm
or 30/ 9.5 R15 means Approx 30 Inchs or 762 mm tall 9.50 inchs wide or 241.3mm wide with a 15 inch hole in the middle

I repeat the most critical measurement when operating a 4wd is the static loaded radius

[just cruizin']

I repeat the most critical measurement when operating a 4wd is the static loaded radius

Which comes down to pressures

[CairnsZook]

Exactly.

So the most "comparable" way to measure tyre diameter, is to measure them unloaded, with some sort of indication of tread wear and pressure. So you don't have variations of different vehicles (Sierra / Vitara / LWB / SWB), and extra weight (pax, fuel, cargo, etc...)

If you really want to get technical, then rolling diameter will change on a day to day basis due to tyre pressure, temperature, wheel alignment, surface friction, level ground, etc...

But who can be bothered measuring that?
And really - how accurate do we have to be? It's not as if we are trying to shave milliseconds of a Formula 1 car laptime.

It's way easier just to run a tape measure across the spare that's hanging off your tailgate.

[lay80n]

Wouldn't it be better to get the rolling distance ie, circumference rather than diameter, most every tyre maker has both of these on their sites, and a rolling distance is more accurate.

Just a shame that those figures on the websites are oh so often wrong.
Hence if we get some real world figures, including pressure and rim size, a bit of SIMPLE maths will get your the rest.
Layto....

[Pyrotech]

Simex Cetipedes- 32x10.5 low tread mounted on 15x5.5 -47 offset. True size 32" @ 20psi measued loaded

[GRPABT1]

How did you fit a 10.5 inch tyre on a 5.5 inch rim?

[lay80n]

Bwahahahaha i have been corrected

Layto....

[Pyrotech]

hey Layto.. hows that crack pipe going?

and yes the are one a standard sierra rim that has been cut and flipped

[GRPABT1]

Fark me swinging they must be on there tight Did you fit them yourself of get a tyre shop to do it? Cause I reckon a tyre should would send me away if I asked lol.

[Pyrotech]

I get them done at a tyre place, they are tight yes, but they were not an issue to get on the rim.
I have run them on rock at about 3 or 4 PSI, never rolled a beadm and never spun a rim.

are u thinking about doing it?

[Spike_Sierra]

33x12.5 BFG Mud Terrains

mounted on 15x8 sunnies.

actual 32.6x11.8 @ 25psi.
80% tread depth.
on LWB suzuki, fark all weight as its being sprayed.

[GRPABT1]

I get them done at a tyre place, they are tight yes, but they were not an issue to get on the rim.
I have run them on rock at about 3 or 4 PSI, never rolled a beadm and never spun a rim.

are u thinking about doing it?

Umm no lol, I recently bought 15X8's and 31X10.5 pedes. Will look into some staun bead lockers eventually though.

[dank]

Thought I may as well bump this up to the top again seeing the current interest.

Super Swamper LTBs 34x10.5 fitting on -30 15x8 rims

Measure 33.6"x 9.5 tread width/10.2-3" total tyre width @ 30psi.

this tyre is wider than the tread width about 1/2 way down the sidewall. Its not really that noticable but would probably be more pronounced on a narrower rim.

[mach.25]

31x10.5x15 Maxxis Bighorns
Brand New 30.5x10.2 on 15x7 Rims @ 25psi
16mm tread depth.