ideal tire size

[gtagamet]

SHORT VERSION:

Which tire is best for all around wheeling?

255/85/16 (33x10)
285/75/16 (33x11)
305/75/16 (33x12)

LONG VERSION:

I've been reading all of your posts about tires and looking at your rigs for days; one can really get lost on this site!! Great photos. I will post my 'burb soon when the mods come in. But for now I'm stuck on tire size.

So now I'm lost in a tire size debate. I'm going to have to stay at or below 33's, but I can't figure out how wide I should go. We are building what is essentially a Baja chase truck. Slight lift, quality tires, quality suspension. But how wide should these tires be? i'm new to this discussion. I was told long ago that pizza cutters get to the hard stuff faster, but I think thats mud. On tidal flats, I avoid mud like the plague. I do drive on soft sand a lot - someone once told me the pizza cutters also work for sand, but thats confusing because after years of experience I know that float is better and there isn't really "hard pack" under 8 inches of soft sand. Its "feet deep" at least. If not tens of feet deep in the dunes. So I'm guessing wider tires are better there? Then their are trails, which is where these tires will be most of the time. Rutty, rocky, and @25mph on average. Not sure which tire would be most recommended for that? That's probably the terrain I hit the most.

I've been reading about the LT255/85R16 size that seems to get people excited. What is the ideal use for that size? And what's the drawback?

I know this is probably a can of worms. Sorry. Any input you have to share I appreciate.

 

[maktruk]

I'm not quite sure how you are doing your calculations, but the 305 is the widest tire and the 255 is the narrowest...

 

[maktruk]

METRIC TIRE SIZE CONVERSION CHART


15 inch tires sizes

Width Aspect Wheel Diameter Section Width
205/70 15 = 26.3 8.1
245/70 15 = 28.5 9.6
205/75 15 = 27.1 8.1
215/75 15 = 27.7 8.5
225/75 15 = 28.3 8.9
235/75 15 = 28.9 9.3
245/75 15 = 29.5 9.6
265/75 15 = 30.6 10.4
215/85 15 = 29.4 8.5
245/85 15 = 31.4 9.6
275/85 15 = 33.4 10.8
315/85 15 = 36.1 12.4
325/60 15 = 30.4 12.8




16 inch tires sizes

Width Aspect Wheel Diameter Section Width
305/65 16 = 31.6 12.0
345/65 16 = 33.7 13.6
245/70 16 = 29.5 9.6
305/70 16 = 32.8 12.0
225/75 16 = 29.3 8.9
245/75 16 = 30.5 9.6
265/75 16 = 31.6 10.4
285/75 16 = 32.8 11.2
295/75 16 = 33.3 11.6
215/85 16 = 30.4 8.5
235/85 16 = 31.7 9.3
255/85 16 = 33.1 10.0
265/85 16 = 33.7 10.4
305/85 16 = 36.4 12.0
255/70 16 = 30.1 10.0


The above numbers are computed values and some rounding has been done on the computed values. The actual tire sizes can and will vary depending on the tire manufacturer.

To compute the values in the table above, use the formula of:

HEIGHT = ((width * aspect-ratio * 2) / 2550) + rim size

WIDTH = (width / 25.50 )

For example, a 235/75R15 tire is ((235*75*2)/2550)+15) = 28.8 inches of tire height. The tire width is figured by 235/25.5 = 9.21

Metric tire sizes are stated as three numbers. The way it reads is:*

width / aspect ratio R tire rim size
235 / 75 R 15




P-Metric Size Actual Tire Width Actual Tire Height*

15-inch rims 205/70R15 8.7 inches 26.2 inches*
215/75R15 8.46 inches 27.6 inches*
235/75R15 9.25 inches 28.8 inches*
265/70R15 10.4 inches 29.6 inches*

16-inch rims 245/75R16 9.64 inches 30.4 inches
285/75R16 11.2 inches 32.8 inches*
315/75R16 12.4 inches 34.6 inches*
395/70R16 15.5 inches 37.7 inches*

17-inch rims 235/65R17 9.25 inches 29.5 inches
265/70R17 10.4 inches 31.6 inches*
255/75R17 10.4 inches 32.0 inches*
285/70R17 11.2 inches 32.7 inches*
315/70R17 12.4 inches 34.3 inches*

Hope that worked...copy/paste on a mobile phone yikes

 

[gtagamet]

oops that was a typo.

 

[comptiger5000]

Some of it depends on your vehicle. If it's full bodied rather than Wrangler-like, the narrower tires will usually be easier to fit and will involve less cutting. On loose surfaces with something grippy underneath, the narrow tire will dig in and grab better. Otherwise, the wider tire will often win for traction.

Do keep this in mind: A 255/85 is a pizza cutter in proportions, but relative to the weight of your vehicle, it isn't necessarily all that skinny.

 

[KevinsMap]

Otherwise, the wider tire will often win for traction.

I can tell you from personal experience that this is not true, unless your favorite terrain (and tire needs) are very specialized. Much has been written on this subject, but I will summarize:

As you correctly point out, there are variables. But the width of a tire is not even close to the most important variable in footprint size, or in floatation or traction in loose sand (or dust, gravel, snow, pumice... been there ;-). If you look at how a tire footprint expands as tire pressure decreases, most of the increase in footprint area is gained fore-and-aft, not out towards the sidewalls. This is independent of tire width. The tire footprint "lengthens" as the pressure lowers, or the terrain softens, and becomes like a short tracked-tread. A long blunt oval, fore-and-aft. This is the clue to a very complicated dynamic, what's going on with floatation and why one tire behaves better than another in the soft stuff. So (for two tires of identical diameter and construction, one wider than the other), while the wider footprint starts out just a little larger on hard surfaces, soft surfaces quickly erase much of the difference, and airing down erases it further.

None of what I write is true at the extremes, of course. My point breaks down with the need for a specialized tire. Look at the tires used in glacier or swamp country, and you will see where the "Cool Factor" comes from, and the natural habitat of such designs. But look at the profile of a swamp tire verses a glacier "Iceland" tire, and you will see two very different designs. They achieve flotation and traction very differently, with very different tread designs and sidewall profiles.

I have used tires of every geometry. Today, I use a traditional overlanding tire on my 2014 Jeep RubiconX JKU - about 10 inches wide. LT255/80R17/E KM2. They fit with no modifications at all. Only in the last 10 years have such a size been called narrow, and the popularity of wider tires is overwhelmingly the "cool" factor. And as with anything cool and popular, look to availability; the older skinny tires are just not commonly available (thank you BFG ... because most people want the cool stuff ;-) They want the cool lifts they never need (but others do, because big rock is their thing , and the "big meats" look they see so heavily promoted because it sells suspension components of all kinds... and so anything taller than my nominal 33's is not going to be skinny.

But not because they improve traction.

 

[Josie'slandcruiser]

Hi all,

My US$00.02 on this topic is that it depends on the terrain being traveled.

As an example, BFG MT tires. For more technical trails the 33X12.5 version is simply superior, with a wider traction surface. But for high speed rough road/trail driving the 33X10.5 should work just fine, while offering less weight and rolling resistance.

For the use you mention (Baja chase truck) I wonder whether a 31X10.5 tire would be sufficiently tall.

Regards,

Alan

 

[Kmrtnsn]

This is a pointless exercise without knowing the specifics of the application.

 

[Josie'slandcruiser]

From the OP:

>We are building what is essentially a Baja chase truck. Slight lift, quality tires, quality suspension.

This is a pointless exercise without knowing the specifics of the application.

 

[Kmrtnsn]

From the OP:

>We are building what is essentially a Baja chase truck. Slight lift, quality tires, quality suspension.

Wheel base? Track? Approach and departure angles? Suspension travel and capacity of the wheel wells as well as the current tire size all come into play when trying to figure these things out. Hell, .5" of backspacing on the wheels, depending on the tire selected could make all the difference. Knowing the stock tire size and the vehicle go a long way towards understanding oversize tire fitment issues. So I stand by my earlier post, this is a pointless exercise without knowing the specifics of the application.

 

[Robert Bills]

^ X2

What vehicle are your modifying to be a "Baja chase truck"?

Asking for the "ideal" size without specifics will not yield any meaningful responses.

 

[gtagamet]

Here's the info I have on the vehicle:

'99 k1500 suburban. Stock wheel size is 8Jx16 ET15 tire size 245/75R16. I have a set of 16x8 wheels with a 4" backspace that I plan to put on, tires TBD. I can tell that the new wheels have a -12mm offset. It appears that stock offset is 15mm - I assume positive. Is this a deal breaker? I knew about the different offset but my calculations indicated the negative offset would move the wheel outward if anything. Bad idea?

wheel base is 131.5 in (3,340 mm)

I appreciate your responses.

 

[Kevin108]

35 x 12.50

 

[rayra]

I've done a ****-ton of wheeling in the SW deserts on 31x10.50x15 tires and the troubles I had rarely had anything to do with my tire size.


late eta - I have 285/70-17s on my K1500 Z71 '02 Suburban. I had 31x10.50x15s on my C-10 pickup for ~25yrs.

 

[dumprat]

35x12.50 or there abouts. Not so big you can't get spares in weird places and not so small the diff drags in every mud hole or gets hung up on wet logs across the trail or road.