FYI Some 4 ply truck tires have a hgher load limit than the 6 ply LT tires ford provided.

There is no reason really and we don't even know that to be fact.

It used to be fact with the 18 over the 17 as they were taller as I recall.

20" is the same dia as the 18. I think trucks with 20's come with 18" spares.

The wheel will weight more, tire less pending which tires. Not a big net diff if any.

 

Larger wheels have more leverage on them and are inherently weaker unless there are large increases in the amount of metal used in the spokes. They are also much heavier--5" of extra wheel diameter weighs a heck of a lot more than 5" of tire sidewall..think of the circumference of the rim--a 20" wheel has 15"+ more rim length than a 15" rim (circumference=pi times diameter). And the tires will have longer metal beads, offsetting any loss of sidewall weight. Of course, you could go to a semi-truck type wheel that will hold more weight in very large diameters, but the weight gets way out of hand.

One of the performance car magazines (Grassroots Motorsports, I believe) did a performance test of a Honda Civic, moving from the stock 15" wheel to 16, 17, and 18" and keeping the same overall diameter. Each increase in wheel size made for a large weight increase in tire/rim combo and a measurable decrease in acceleration and braking performance. I believe the 16" wheel/tire combo gave the best handling (cornering G's) because the tire tread was much wider than the stock 15", with the larger combos slowing the car down there as well. Best acceleration was with the stock 15's.

As for tire weight ratings, LT tires are under-rated by 9% compared to P series tires, so an LT tire in a given size will generally have more capacity than its P rated counterpart when the P rated tire is adjusted for truck use. Weight carrying capacity of low-profile tires is often less because there is less air volume...so it is probable that a 20" tire with a given diameter will have less capacity than an 18" tire with that same diameter. (Going to extremes, think of the "rubber bands" that the ghetto guys use on 26" wheels...)

Again I maintain that the ONLY advantage of a 20" wheel on an F150 over an 18" wheel is looks. There is no performance gain to be had and in general there are losses in carrying and towing capacity, increased tire and wheel replacement costs, and less rim protection when encountering chuckholes and rocks. If you want to spend the extra money for looks, go right ahead, but don't fool yourself into thinking that any performance gain of any kind will be had.

George

 

I never EVER suggested there would be a perfomance gain. I just challenged the negatives some suggested by going to 20's.

 

Just for fun lets look at some of the shipping tires for a F150 (18 & 20)

The LT option:
Goodyear Wrangler A/T
Max load 2,535lbs, weight 49 lbs, Dia 32"

The 20" option:
Pirelli Scorpion ATR
Max load 2,403lbs, weight 42 lbs, Dia 31.9"

Other p rated 18's
Goodyear SR-A
Max load 2,604lbs, weight 39 lbs, Dia 32.1"

BF Goodrich Long Trail TA
Max load 2,601lbs, weight 43lbs, Dia 31.9"

So while the factory 20 has a lower max load than the fatory LT, the other 18's are in fact higher!

In any event even the 20's @ 2,403 = 4,806lbs which is over 300lbs over the F150s rear axle "breaking point" or 600 lbs over the max rec'd GAWR of what 4,200.

That said I'd still replace the factory 20s with LT 285/55R20 Toyos. Their max load is 3,305lbs and total dia 32.3" and at 52.5 lbs only a touch heavier than the factory LT 18's. Yes thats 10.5 lbs more than a factory 20, but the new engines will turn it...

 

Oh and I forgot to say... They'll look great!

Paying for those Toyos will be a diff matter.

 

I don't think you understand the basic concept that P rated tires on a light truck like an F150 need to have their load rating reduced by 9%, whereas the LT's are already reduced to reflect light truck use.

To put the P rated tires on the same basis as the LT, you need to take away 9% of the load capacity of the P's...2604 lbs turns into 2370. The 20's end up at 2186 lbs.

Go as big as you want. You will reduce acceleration, braking, and gas mileage, but I guess looks are so important to you that this doesn't matter

George

 

Hey everyones line in the sand is in a diff spot. The guy with 17's can say that to you with 18's too. A guy with p rated tires can say that to the guy with LTs. I can say that to the guys with a leveling kit too. The guy with a 2wd can say that to both of us. I use 4wd maybe 25% of the time. I need a supercrew 50% of the time, radio 75% of the time. Everything has its drawbacks.

That argumeent never ends.

 

Two things affect load capacity. Load Index, or Load Range, and air chamber. The larger the air chamber, the more weight a tire can carry at a given air pressure.
If you have two tires that are the same diameter, and similar width, but the rim size is an 18 and one is a 20, and they are the same load index, the one on the 18 inch rim will carry a larger load at a given air pressure, than the 20. The reason? Air chamber. The 18 has a larger chamber or air, and therefore can tolerate more load at a given a/p.

The other is load index. A Pmetric tire, will not have the same load Index as an LT tire. The reason is because they are not designed to do the same job. Pmetric means passenger tire, and LT means Light Truck Tire. So, they are superior in different tasks. The Pmetric rides nice and is quiet and comfy. The LT typically, is a little more truck like with less cornering manners and a bit stiffer ride, but will haul more load, and usually does much better on gravel roads. Tread design and brand also have an impact on this as well.

The 20 inch tires that come on these trucks are not well suited for heavier type use and gravel. You will not be happy with them in that vocation. I would recomend going to the 18s or 17's if you get an XLT. They will be much more forgiving and there are more LT sizes and options with those two than the 20's.
Many of the LT labeled tires will have a higher load index and will have some anti-chip compound in them. That will help on the gravel.

Ultimately it's your decision on what you go with. I have 17s on my XLT and love them. It came with Michelin's on it, and they are smooth riding and quiet. They are Pmetric, so I know what to expect. I live on gravel, and I tend to load it heavy at times, so my expectations are where they should be. I know Michelin makes this exact size and tread design in the LT version that is a Load range E, and I will put those on when these are done.

 

I just picked up my 2010 F150 Lariat Screw and I added the 20" Pirellis as upgrades.

I personally like the looks of the 20's rather than the 18's but know I will regret it when I have to buy new tires down the road.

 

My new truck.

Ford F150 - Thinking about getting a 2010 Ford F150 - Ford F150 .net

 

If you're going to be on dirt roads a fair amount of time, I'd go with the 18s. 20s are going to cost you more on tire replacement. I live on a very rough road (it's paved, but it's long overdue for another paving) and I have 17s on my 2010 XLT Super Crew. My friend over the hill has a 2009 F-150 FX4 with 20s and I notice little difference in ride quality on my road.

But if you're on dirt roads, chances are you're going to encounter rocks on that dirt road. A smaller rim will help with that because there is more rubber to get through. And since you mentioned how they do in mud and snow, if you're going to be in mud, I'd definetly go 18s. If every truck your dealer finds has 20s, just buy the truck and wear the 20s' tires out. Then sell your 20s and buy a set of 18s.

Let me know what happens. Whether you get 18s or 20s, at least you'll still be in a Ford! : )