GC tires - 225/75 vs 245/70 (R16)

Yes, the 70 or 75 is the Aspect Ratio, and the aspect ratio is relative to the width, 225 or 235, etc. Multiply the width by the aspect ratio to find the size of one sidewall, there are two sidewalls -- one at the top and one at the bottom -- that must be added together, then added to the rim diameter to find the overall diameter of the tire.

A 225/75 and a 235/70 (same rim size) are essentially identical in diameter, but one is wider. A 225/70 and a 235/75 are completely different. The 235 is both taller and wider because 75% of 235 is a much larger number than 70% of 225.

Do the math. 75% of 225 and 70% of 235 are only different by 4.25mm in the height of one sidewall. Surely this is not a significant difference.

You are somewhat correct as far as it goes. 75% of 225 mm is 168.75 mm and

There is a difference in the lower profile tires which hasn't been mentioned in sidewall stiffness (roll) which affects handling on-road but as I mentioned before the suspension of a Grand cannot really utilize the benefit due to its high center of gravity.

The poster's original question was, I believe, would there be any interference problem in his WJ. My answer then and now is No, there is no problem as both are listed in the manual and on the door placard as alternates.

FWIW I recently put 225-75X16s on my WJ but my original thought was to get the 245-70-16s

Not talking diameter ... talking total tire surface trying to get down to pavement through snow/slush. And my experience is that with my lightweight Cherokee, 225s are as wide as I want to go (with a tire that's also good on dry pavement) and still have decent control at speed in slush/wet snow. So let's just say the footprint of a 225 at 33 psi is 9" x 3" (27" square)... just a rough calculation. Then the total footprint would be 27 x 4 wheels or

J Strickland proclaimed:

The 225/235 is the sectional width, not the tread width. And now that you mention it, I can't remember where they measure it either... and the actual size is always rounded off anyway. Unless the tires involved have exactly the same tread compounds and patterns, comparing traction based on section [or tread] width is a waste of time, as it really depends more on the pattern and compounds.

tnx for all the info, the math, and the tire discussion - I was going to get the Bridgestone Dueler H/L - but it was a bit pricey ($700) if I only keep the 2001 Grand Cherokee Laredo another couple of years. I might go with the Firestone Destination LE ($450) and also go with the alt size of 245/70R16

I think that is what Jeff is missing, the 'section' width vs the 'tread' width.

It is very deceiving and I learned to only trust a tape measure for the 'actual' tread width way back in the 70's.

I just measured my new BFG AT's and the P235/75x15 tire has a tread that is only 7 7/8" or 200mm wide.

That is a far cry from the 'book' spec of 235mm wide. Well, over 15% difference anyway.

I also measured my son's vans tires and the P205/75x14's and the tread width is only 165mm, not the 205mm one would think!

I have a goodyear spare that is a P235/75 15 M+S and the tread is only

Now if I take a caliper and measure the sidewall width at the biggest part, (spare with no load on it) it is 235mm just like the book says. You can hold a 3' long straight edge on both sides of the tire across the center sticking out both ends and measure between them to get the section or sidewall width or use a big caliper.

Mike

One last nagging question on the slight sizing difference - What about the full size spare I have - 225/75R16 - Any major concerns on running 3 tires of the alt size - 245/70R16 along with the spare that is 225/75R16 -

The diameters calc out to a difference of .216"

For emergency use it will be fine.

If you intend to buy 3 new tires and use the Wrangler ST as the 4th you need to buy 3 more Wrangler STs.

You don't want to mix tires of vastly different qualities

Agreed, especially when the discussion is the difference in 225 and 235.

There can easily be other factors when the difference is, as Mike points out, a 11.50 and a 9.50, or whatever. But the OP asked about the differences in the performance of a 225 vs. a 235, and I don't think he will notice much, if anything, unless he also changes the tread pattern and/or rubber compound.

no - intent is keeping new full size spare intact as spare and putting all 4 new tires on the beast -

But there is no "lower profile" in these two tires. The OP was talking aobut the standard 225 and a possible change to a 235, by the way - not a 245. But the point remains the same whether a 235 or 245, the rim diameter is the same, and the sidewall is only changing a couple of milimeters. When the rim changes from a 15" to a 16 or 17, and the sidewalls get significantly narrower to maintain the same overall diameter, then we have the discussion you are talking about. But the OP is retaining the same rims, and the tire size is 225/75 or 235/70, and the difference in the diameter on these tires does not present the sidewall stiffness issues that you are bringing up.

My answer to him is also, no. There is no reason in the world he can't go from a 225/75 to a 235/70.

The 245 vs. the 225 could potentially cause issues with the fenders and/or the undercarriage, but I doubt it.

It would be a very bad thing to run 3 tires of one size, and a spare of another. It would be okay to run like this on the front axle to get off the freeway and to the next tire store, and the rear axle would behave as if it was going around a corner and won't like it but it would not explode getting you off of the freeway -- but could easily be damaged going more than a few miles, I'm thinking the range of about 50 miles would be the limit.

You are correct though, the difference is under a quarter inch, and this ought not be a huge problem. Having said that, my BMW had 205/60x15's on the ground, and a 225/55x15 in the trunk. I had a blow out on the rear and the spare caused the limited slip to make lots of noise. The difference in the tire size is only .06", and this is only about 25% of the difference you'll be having.

What BRay said.