Trying to figure out what my actual payload limitations are with regards to TIRE load rating (before purchasing wheels & tires because I want to make sure they’re rated high enough).

I don’t think it’s as simple as: what is the weight that will be ON the bed and then making sure the rear tires are a higher value than that weight. Wouldn’t a good portion on the tires weight capacity be already taken up by the rear end of the truck/frame/bed etc?

Here’s my thought process, is this accurate? To get a true weight total that would be “to the tire” be to 1) get a scale weight of just the rear on the scale 2) subtract the weight of current wheels & tires to get a weight at the hub bolts 3) Add what I expect my payload to be (which would be a flatbed truck camper + tongue weight for a trailer I haul with it).

Is this tracking? I might be over thinking this but I want to make 100% sure. I’m looking to move from the 19.5s to 20s for LT tire but still dually and the flatbed camper is about 7k + some tongue weight for trailer. This is on a flatbed F550.

Thx for any feedback.

 

You should look at the GVWR of your truck. Manufacturers usually provide numbers for the entire vehicle as well as each axle. This usually takes into account the capacity of the springs, the axle, the bearings, as well as the wheels and tires. So get wheels and tires rated for twice the GVWR of each end.

 

The GVWR is 19500. My scale weight after flat bed leaves me with around 9900 payload. What do you mean twice the GVWR “of each end?”Cant be divided by 2 because weight at rear would be considerably more.

 

I usually see a GVWR for each of the front and rear axles, and they are not each half the total. If you get wheels and tires rated for the GVWR of the rear, you get two tires, each with that rating, to give you 100% load margin over the rating. If you can get tires each with twice the GVWR at the rear, then you have 300% margin.

 

All of these weights are measured at the ground. Weigh your truck, front and rear separately, at a CAT scale, and compare to your door sticker. Voila' !

And: Yes, I believe you are over-thinking it to some degree.

 

Yes, the GAWRs for front and rear axles allow the overall loading to have some flexibility on where it ends up. Got a plow? More weight will end up in front. Got a big cabover camper? Most will end up in the rear. Etc. Etc.
The weight ratings on the sticker aren't just about weight pushing down. They are also about braking, acceleration, cornering, etc.

This is why front GAWR plus rear GAWR often equals more than the GVWR.

 

Thanks for the replies. Yes, over thinking some but just don’t want 20” LT tires to be over loaded and risk a safety issue (family is always in the truck). 19.5s suck unloaded. Not worried about GVWR for anything other than tires (brakes, susp, axle etc are fine it’s a f550 with 19500 cap)

Tires are around 3750lb ea (3500ish for dual) so that’s 14k load rear. It’s a big camper, 7500lbs on the bed and about 1k lb tongue (with weight distribution hitch). I’m thinking I’ll be ok.

 

In the end, your math should be like this:

1. Are the 20" tires rated for less weight than the 19.5" tires were?
2. Are the 20" rims rated for more weight than the 20" tires?
3. If there are adapters installed, are they rated for more weight than the 20" rims?
4. Are the wheels and adapters hub-centric and fit the hub tightly when installed properly?
5. Is empty weight measured at the ground plus payload/passengers/etc. on front axle less than the 20" tire weight rating times two?
6. Is empty weight measured at the ground plus payload/passengers/etc. on rear axle less than the 20" tire dual-wheel weight rating times four?
7. Is empty weight measured at the ground plus payload/passengers/etc. on front axle less than the 20" rim weight rating times two?
8. Is empty weight measured at the ground plus payload/passengers/etc. on rear axle less than the 20" rim weight rating times four?
9. Is empty weight measured at the ground plus payload/passengers/etc. on front axle less than any wheel adapters' weight rating times two?
10. Is empty weight measured at the ground plus payload/passengers/etc. on rear axle less than any wheel adapters' weight rating times two?
11. Is empty weight measured at the ground plus payload/passengers/etc. on front axle less than the front axle weight rating?
12. Is empty weight measured at the ground plus payload/passengers/etc. on rear axle less than the rear axle weight rating?

If yes to all of these, then you should be good.

What rims are you using for this?