Going from 20" to 17" -18" wheels - better tow capacity?

Just a point when figuring how much weight carrying capacity your truck has with P tires.
Fed regulations says this about P (passenger) tires on a truck; quote snipped for length

"When a passenger car tire is installed on a multipurpose passenger vehicle, truck, bus, or trailer, the tire's load rating shall be reduced by dividing by 1.10 before calculating the sum,"

Your current P285/50-20" tires may show a 2430 lb capacity. Now divide by 1.1 = 2209 lbs capacity. Keep them pumped to the max sidewall pressures when towing or carrying heavy loads. Then air down when the truck is empty.

Also diameters and capacities and tire weights can vary a bit between each tire maker for the same size tire.

500-1000 pounds of tow capacity or tire/wheel size difference between any OEM sizes will have an insignificant, immeasurable difference in towing power to the extent of wheel/tire combo weight is similar.

Towing any high walled RV is hard work. That's why we get 8-10mpg vs. 18-20mpg unloaded at 65mph. IT takes a lot of power to pull the parachute, err, TT through the air at 65mph. Find a hill and you find out how much reserve power you have.

2005 5.4L means 4spd trans. 2 gears to accelerate, one to cruise. Hit a hill and you'll be down to 2nd again and 4000+rpm. That's life with what you got. I had a 2003 F-150 5.4L/3.73. Same deal. Let it spin! Your 3v engine makes its peak torque around 3500rpm.

If your truck is 2WD, you may consider changing the differential gearing to 4.10. That will really wake it up for about $500 installed. 4WD trucks at least double and the value isn't usually there.

Looking at the revolutions per mile, the 20's are at 673 and the 18's at 681. Just a bit over 1% which is negligible to me. Speedo would be about 1 MPH slow @ 60 along with a correspondingly small change in effective gear ratio. (3.50 ish) I think you'd be hard pressed to tell any difference. Info from tirerack.com

Thanks guys – this is really helpful! Peg is right, we did finally figure out we need to check diameter and they are pretty close. The load capacity is higher on these current low profile aftermarket tires than the stock OEM would be. We bought the truck used last year and it came with aftermarket tires/wheels. Stock OEM for our Lariat should be 18” wheels but it appears there are F150s with 17” OEM as well.

@APT – when we towed a 4000 lb TT, it seemed to work hard uphills but maybe that is to be expected.

@ Skipnchar – we have no idea if the speedometer was adjusted or not, is that a standard adjustment that a tire distributor would do (or is that only done by dealerships)?

@ MFL – thanks for taking time to answer our questions offline – just to share with the group what we calculated on our aftermarket tires:

• The stock 17" tires on a comparable F150 are 255/65R17, which are 30.05 inches. The manual says this can haul 14,000lb.
• The stock 18" tires on my F150 Lariat are 265/60R18, which are 30.52 inches. The manual says this can haul 13,500lb.

From that, I figure that about half an inch of wheel diameter is worth about 500lb of tow capacity. If I use excel to do the math exactly, then I see that:
My current 20" tires are 285/50R20, which are 31.22 inches. This calculates out to 12,752lb tow capacity.

I should also add that our truck has 115K miles and two of the tires had holes patched since I discovered nails after buying it. I recall hearing horrible sound on freeway driving it home so maybe it happened them or they were already there – who knows. Now thinking aloud, am I compromising load capacity with two patched tires???

Looking at tire sizes/diameters/tire weights shows the OEM you mentioned;

P265/60-18" is 33" diameter at 39 lbs.

P285/50-20" is 31.2" diameter at 38 lbs.

P255/70-17" is 31.1" diameter at 38 lbs.

Because of the high geared 3.55 axle I would stick with the shorter 20" P tires you now have on your truck.

I would check the weight rating on the wheels. A lot of aftermarket wheels do not have a weight rating large enough to tow a trailer or put anything in the bed. I think you will be surprised how low a lot of the aftermarket wheel ratings are.

If they are aftermarket wheels/tires and it was done without changing the rear end gear then your truck is geared higher than your specs show it to be. If you came with a 3.55 you may well be into the 2s all of which will make it less powerful when towing. It won't make any difference in how WELL it tows but it WILL make a difference in the power you have. Change back to the OEM tire size listed on your DOT safety place in the door post and your trucks original specifications will be restored and you will have more towing power. It will NOT change your tow rating however. Tow ratings are a warranty issue and Ford is not likely to recognize any aftermarket changes you made in their warranty decisions.
Good luck / Skip

Relatively low profile 20" P-tires are pretty stiff which is one of the goals people want LT tires for on smaller wheels. I don't think I'd recommend changing the wheels/tires for a higher "tow rating". Are you unhappy with how the truck tows?

The diameter of the tires are what you need to check. A low profile 20" tire could have the same diameter as a 17" tire. A smaller tire diameter will, to some degree, help with towing.

The tire sidewall is where you want to check for load capacity. The max load in kgs/lbs will be listed for the max air pressure when the tire is cold.

Due to the lower profile, of the 20" after market tires, there wouldn't be a big advantage, going back to stock. There would be less than 1" in diameter difference. The 20s would be 31.2"s tall, compared to stock 18s at 30.5.

IMO, the gain would not be worth the cost. If there would actually be 2"s height difference (which there is not), it would make a noticeable gearing change.

Jerry