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

Those tires (245-70-16) are listed as acceptable alternates on my 02 WJ to the stock tires (225-75-16)

There should be no problems

A consideration ... if you drive in snow, and will be using the 245s they may be too wide. I say this because the 235 is too wide for my XJ in snow ... but the XJ is a lighter vehicle I believe (unibody). I use 225 snow tires ... which unfortunately are due for replacement this coming winter :-(

I will second that. We lost a 'lot' of traction when we went from a 225 to a 235 while keeping the same profile even. Going to a wider 70 series adds to the loss.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Bowgus wrote:

There will be no difference (OK, the difference will be very small).

The 245 is 20mm wider, that's 10mm off of center, which is about 3/8". This should pose no particular problem for clearance inside the fenders.

The 70 or 75 represent a diameter change (larger in your case) of 0.4 inches. This will not present any problems for the speedometer or the gearing. Indeed, your speedometer is about 3 mph fast at 80mph right now, and will be about 2 mph fast with the larger tires. You will be fine.

The caluclation is * * 2 / 25.4 + = overall diameter. (you multiply by 2 to get the total sidewall height, and divide by

25.4 to convert from mm to inches.)

The width is the 225/235/245 number. The 70/75 number is the height of the sidewall.

A 225/65, 225/70, and 225/75 are all the same width. The difference in these tires will be the amount of sidewall, and the overall diameter -- the 65 will be the smallest and the 75 will be the tallest.

You might have noticed a loss of traction when you changed from 225s to

235s, because this would represent the width of the tire. A 225/75 and a 235/70 are within 10mm of being the same diameter.

I am a poor judge of what happens in snow, but it's difficult for me to see where one would notice a difference in 10mm in diameter. I have to suspect there are tread patterns and other things that enter into the discussion.

Could be, we went from Hercules 'terra Trac' to BFG AT's and it was a big disappointment. I believe there was over an inch of actual tread width difference.

Mike

J Strickland wrote:

Not if the change was from 225 to 235. These numbers represent the tread width in mm, and 10mm is the functional equivelent of 3/8".

Now, there is some misunderstanding (at least on my part) of just exactly where they measure 225 or 235, and the actual tread blocks being square on one tire and rounded off on the other might play a role in your perception of an inch difference, but the actual difference is 10mm. Your point being that you are looking at the tread patch on the ground is not lost on me ...

The 'book' width measurements are not at the tread, they are at the sidewall.

When I went from a 31x10.5" mud to a 33x9.5" mud, the tread on the ground went from 10.25" to 7.5". I got a 'radical' increase in traction.

Mike

J Strickland wrote:

Ddi you make this change on the same rims, or did you change rims too?

There is something here that one of us is missing because there is no point in giving a spec for tire size if that spec has no basis in reality. Why tell the consumer the tires are 9.5 when they are really 7.5, or 225mm vs.

235mm, if they are really something else entirely?

I'm not arguing that you find the 9.50s work better than the 10.50s, I have every reason to think you are correct in this. I don't have the same kind of driving environment where I am, and we find around here that wider is better, but we need to float above dry sand and dirt and the larger foot print works better here. I'm sure that if I brought my Jeep to your house, it would give me lots of trouble, especially in winter.

What I am suggesting is that if your tread on the ground is only 7.5 with a

9.50 tire, then there is another factor because the tread on the ground should be closer to 9 than 7. In any case, the OP isn't getting any benefit from this discussion.

His Grand should be fine for him with a 245/70 from the perspective of fitment. Whether or not it provides any better performance is beyond me, but my guess is that the actual tread pattern will play a larger role than the tire size.

The taller tire has a taller sidewall so the sidewall bulges out more. The measurements really are at the widest part of the sidewall, not at the tread.

That is why the OP 'could' end up with the same width tread with those changes in profile and height sizes, but a tape measure will tell for sure.

My 33" sidewalls look like this ( ), My 31's looked like | |

Mike

J Strickland wrote:

Well, yeah, I get that.

But, you said your 9.50s were 7.5 inches wide. The explanation you just gave would account for why the 9.50s measured 10.25 and the 10.50s were also

10.25. I can see where the 10.50s actually measure at 10.25, and I can see how 9.50s might also come in at 10.25. Where I'm getting tripped up is when the 9.50s get reported as 7.5.

If the 10.50s were mounted on 10" rims and the 9.50s are on 7" rims, then I can see where the 9.50s would be 7.5" wide, but if the 9.50s also went onto the 10" rims, then there's no way they would be 7.5". See?

I am only talking the road patch width with proper rims at proper pressure. The 'book' measurement is 9.5" which is the widest part of the sidewall. because the sidewall has the bulge, the tread is only made 7.5" to balance the shape.

Try it on some different off road or any for that matter tires. You will see radical varations from tread width to 'book' or sidewall width according to how tall they are.

A 50 series tire has straight up sidewalls so they are most likely to have the tread match the book size.

Mike

J Strickland wrote:

Well, with 4 tires, 225 to 235 adds up to almost 2 inches ... and in slush and so on at speed, it's enough to affect my puny little 3000 lb Cherokee. As to the heavier trucks, no problem I'm sure. Now if I went to 245s, yikes.

discussion.

so - you're saying that with the narrow footprint there is probably more bite to the footprint - ala the psi of the footprint - vs the wider tire that will distribute the weight more and not take a bite out of snow, ice, slush, etc -

hmmmm - any other thoughts on going from the 75 to the 70 (225 to the

245)

The 75 or 70 is the ratio of the height to the width ... or vice versa ... I think :-) So if the width is 235 (mm), and the ratio is 70, then the height is 70 % of 235 (mm) and if 75, then 75% of 235. I dunno ... some people like the look of a 70, or 75 ... apart from that, unless the diameter becomes so drastic so as to affect braking, I don't see that it's a consideration. I myself "like the look" of my 235/75 Michelins (summer tires) on my Cherokee.

It depends on where you plan to drive.

On the road, or in sand, you will have increased lateral traction from the wider tire. Off road, in snow or mud, you will have better forward traction.

Another consideration is your vehicle. A Grand Cherokee was not designed for maximum on-road traction. The 75 series tire will probably provide more traction than the Grand could handle given its high center of gravity.

A further consideration of tire type. You will have a better off-road selection with the 75 series and a better selection of on-road tires with the 70 series.

A final consideration in the discourse is appearance. There are different standards for 'city' and 'country' Jeeps.

Conclusion: What are your priorities?

In all fairness, Mike, the attitude of the sidewalls has to do withthe width of the tire, the width of the rim, and the air pressure. Assuming the airpressure and the rim width are correct, then yes, the sidewalls of a 50 series will very straight. But the 50 Series part isn't the reason. The reason is the 275, or whatever, part matches the width of the rim.

Jeff, they make the actual physical tread on a BFG 33x9.5" tire 7.5".

Putting the tire on a wider or narrower rim is not going to change this

7.5" tread width. It 'can' change the sidewall width which is why the 'book' says the tire is X big on the proper rim with the proper pressure.

The tread width has nothing to do with the 'book' size of the tire. This gets more pronounced as the tires get taller due to the bulge in the sidewall.

I would recommend you go play with a tape measure the next time you see a bunch of mounted large tires. When I figured this out back in the

70's it was a surprise to me too. There is a 'large' variation between brands.

Mike

Jeff Strickland wrote:

Adds up? What adds up? Not the diameter difference.

If one tire size is 10mm larger than another, you do not add 10 four times to come up with 40. (Adding 10 four times does equal 40, but the difference is not an additive difference, so adding the differences up is not something you would do -- well you might do it, but I wouldn't.)