09dreamer

Hi Folks,

I have searched for a concrete answer here and haven't found one.

I have an 09 f150 s-crew fx4 with the short bed and 3.73 axle ratio. The truck sits on 35 inch tires. Stock tires were near 32 inches tall, but the wheel/tire combo was also lighter.

How do I adjust my towing capacity to compensate for my larger tires? I am guessing 15%? Is there a calculation to figure this out?

Thanks

11screw50

In theory, it should be less. Also have not seen a calculation for adjusting however, chances are you will run out of payload before you come even remotely close to the tow rating anyway...and the bigger, heavier tires certainly do not help that any.

Pugga

iTrader: (1)

I think you'll be hard pressed to find a calculation for capacity loss outside of deducting the added weight of the larger tires from the available GVWR. Otherwise, the difference is just like having identical trucks, one with a 3:73 rear end and one with a 3:31 rear end. Both are rated to tow the same amount but the 3:73 geared truck will just do it with less drama.

smokeywren

I don't know of the exact formula but here's what I would do.


The axle ratio is a big contributor to GCWR and thus tow rating. (Tow rating = GCWR minus the weight of the truck.) So determine the new effective axle ratio, then check the F-150 RV and Trailer Towing Guide to see the resulting revised tow rating.



Tow rating was 9,600 for a 2009 Supercrew FX4 shorty with 5.4L engine, 275/65R18 tires, and the factory trailer towing option.

Assuming your stock-size tires were 275/65R18, those had 32.1" diameter and 651 revs/mile. So-called 35" tires are size 315/65R18, with 34.8" diameter and 597 revs/mile. The percentage change in revs/mile = the percentage change in effective axle ratio.

651 - 597 = 54, divided by 651 = 9.17%.

3.73 minus 9.17% = 3.39 effective axle ratio with the taller tires

Closest stock F-150 ratio was 3.31, which had tow rating of 8,100 for a SuperCrew shorty.

So the tall tires cost you about 1,500 pounds tow rating and GCWR.

However, as 11screw50 noted, GCWR and tow rating are not your limiter as to how heavy a trailer you can tow without being overloaded. GVWR and payload capacity is your limiter. My F-150 has tow rating of 8,000, but I'm overloaded over the GVWR with my small TT that weighs less than 5,000 pounds when wet and loaded on the road.

So don't be concerned with the change in tow rating because of tall tires. Instead worry about the weight you haul in the pickup and the wet and loaded tongue weight of the RV trailer.


Here's the procedure I would use to determine your real-world tow rating:


1] Load the pickup with everybody and everything that will be in it when towing. Include all people, pets, tools, jacks, campfire wood, food and drink, options such as bed rug and tonneau cover, and the head of your weight-distributing hitch installed in the receiver hitch.


2] Drive to a truck stop that has a certified automated truck (CAT) scale. Fill up with gas, then weigh the wet and loaded tow vehicle (TV).


3] Subtract the weight of the wet and loaded TV from the GVWR of the TV. The answer is the payload capacity available for tongue weight of a TT.


4] Divide the payload capacity available for tongue weight by 0.13 and the answer is the maximum GVWR of any travel trailer you can tow without being overloaded. IOW, that's your real-world max tow rating. And it will be a lot less than 8,000 pounds.

Wannafbody

The tow rating is a marketing number based on various factors. The only issue that would reduce real world tow rating would be the larger tires reducing gearing.

Velosprout

Larger tires results in more wind resistance, more weight, lower payload, less mechanical efficiency, and lower towing capacity. For best payload/towing efficiency you would select sufficiently weight rated 17 or 18 inch wheels with E-load rated tires.

Dirttracker18

You are discounting the added weight of bigger wheels and tires. That depletes payload and causes greater rolling resistance plus less effective gear ratio.

Tow rating is not a marketing number it is an engineering factor. Marketers do not come up with it the engineers do based on specific factors which have been discussed at length on this forum.

smokeywren

Well, sorta. Tow rating (maximum trailer weight you can tow pull without exceeding the GCWR of the TV) is a calculated number, GCWR minus the weight of the tow vehicle (TV). GCWR is an engineered number. But manufacturer's tow number is usually overstated because the weight of the TV in the calculation is minimized to result in the maximum tow rating. If you subtract the tow rating from the GCWR, you'll see the max weight the TV can have and still achieve that tow rating. But if you weigh the wet and loaded TV, you'll find out it weighs a lot more than the GCWR minus tow rating.

Dirttracker18

In reality the tow rating is calculated without regard for payload.

The tow rating still stands assuming you do not load down the truck. It is based off of the actual weight of the vehicle. They cannot make assumptions about the amount of weight added to the truck after it leaves their hands so it is not overstated. They can only work with real world numbers and not assumptions.
Neither of these makes it a marketing issue and both are engineered numbers.