difference in tyre sizes

**2_door** · 5th April 2008, 08:29 PM

this is puzzling me, hopefully someone can explain it

BFG 245/75/16 MT's have a diameter of 722mm
BFGoodrich - Size lineup

Cooper 245/75/16 STT's have a diameter of 780mm Cooper STT

now, BFG claim their diameter is a loaded size and Cooper dont specify, but the difference in diameter is 58mm or 29mm difference in ride height/ground clearance. That seems a bit much to my mind for a loaded difference???

when I do my calculations, a 245/75/16 tyre should have a diameter of 774mm and this is close to Coopers specs of 780mm.

the difference across most sizes (ie 33", 35" and even 235/85/16's) is similar with huge differences where the Coopers are much larger than the BFG's( in 235/85/16's there is a whopping 62mm difference)

overall tyre diameter is important when off roading as it is factored into ground clearance and I cant see why there is such a big difference in essentially the same tyre sizes....

as Pauline says, please explain

**Discopug** · 5th April 2008, 08:50 PM

I can't explain the technical difference but I have just gone from the Cooper AT 235/70/16 to BFG AT 245/70/16.
The official difference in diameter was 14 mm but I am sure there is more than that.
Also when I lined them up side by side the BFG was about 50mm higher than the Cooper AT.
Having said all that I like the BFG better than the Cooper espcially on the dirt.
See quite a bit of that here !

**2_door** · 5th April 2008, 09:09 PM

thanks for that. even when you compare the BFG AT to the BFG MT there is a difference in size! here must be slight differences in the carcass size and tread depth, thats all I can put it down to

for the record, my S1 Disco had 235/70/16 street tyres on it when I bought it and I now have Cooper STT 245/75/16 and while they are great in the mud (I have had a variety of mud tyres in the past from BFG, Kumho and others) I am thinking of changing to a larger diameter for better ground clearance, maybe 235/85 or something similar, or go to a 33 or 35 inch 15

but I dont want a 12.5 inch wide tyre really as they will poke out past the guards...ahh so many choices

**justinc** · 5th April 2008, 09:18 PM

G'day Mark,

Apparently, some manufacturers measure from the outside of the tread, and some from the actual carcass...maybe that could explain the huge difference? My BFG 255/85/16's are slightly bigger than 33" when new, but the tread block is very deep, so when almost worn out I would say theyll end up closer to 32" or less...

BTW, R you planning on coming to the west coast on the 18th etc?

Recent reports are that parts of the Ring River track are very muddy and the Montezuma falls are speccy after the rain...

JC

**2_door** · 5th April 2008, 09:21 PM

yeah mate, I am hoping to make it and bring the family and camper trailer

**JDNSW** · 6th April 2008, 05:55 AM

I think that tyre manufacturers have no agreed part of the tyre to take the measurements from, except for the rim diameter, and this is what leads to these quite large differences.

Several years ago I had to replace a tyre on my (rather elderly) tractor - I ended up having to buy a pair of new tyres since the ones I could get were nearly six inches different in diameter from the ones I had, although the same nominal size and tread, but different brand. I was able to sell the good one of the pair I had to the retailer, who had the same tractor with a pair of mismatched tyres, and my good one was the same as the better of his two.

John

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