Tire pressure under load (camper)

[Brakish]

Hi folks,

This past weekend I had a Super Pacific Camper installed on my 24' 801. This adds about 375 lbs to the bed, and I'm trying to calculate my pressure when loading additional passengers and gear.

I'm trying to understand the load ratings and what pressure I should be running, I've been referencing this site (https://tirepressure.com/lt315-70r17-tire-pressure).

Currently, I'm running 38psi all around as is outlined on the door jamb. Though when starting to calculate the pressure per tire at say curb weight and GVWR, the numbers don't make much sense.

For the LT315/70R17C, the load per tire and PSI ratings are;
- 2535lbs: 35 PSI
- 2685lbs: 40 PSI
- 2915lbs: 45 PSI (The MAX rating for the C-load tire)

For the 801A Gen3 config, the estimated weight per tire is;
- Curb weight: 5,863lbs --> 1465.75 per tire
- GVWR: 7350lbs --> 1837.5 per tire

Trying to make some sense of this.. using the Load Inflation Table for LT315/70R17C, I calculated the "rate of change" or "slope" of the line from 35PSI to 45PSI (i.e., 0.0259326660600546). Then used this to calculate the PSI based on the Raptor's load (see table below). This leaves us with some alignment to the door jamb's recommended 38PSI.. Though at GVWR, the c-load tire does exceed its max inflation recommendation at 47.6PSI --> meaning if you're running at GVWR often, likely you should be upgrading to "D load" tires.

So what does this mean for my application?
Curb Weight + Camper + Humans + Gear -->
5,863 + 375 + 300 + 450 --> 6,888
That is about 1,722 lbs per tire. So I should be increasing all four tires to near Max PSI -- about 44?


Can anyone educate me as to what I may be overlooking here? Is there a simpler way to figure this out? Or any general guidance when adding say 250, 500, 750 lbs of gear to the truck how should we adjust tire pressure?

I know folks are going to point me to the chalk test but wondering if there was a quantitive approach to all of this..

I'll be headed on a trip this weekend, will look for open scales to get a quick measurement.

 

[The Car Stereo Company]

you are going to have some sag if on oem suspension. max payload doesnt necessarily mean you need to be at max pressure. its also dependent on where the load is positioned. i have heard people running new rear springs and at about 42psi on pressure, however it all depends on where you are driving. you might be better at lower pressure on trail somehwere compared to the pavement. i guess the best thing to do is try driving around with the load and see where and how the truck feels and sits and adjust accordingly

 

[Nex]

Welcome to FRF. Please post pictures of said prius that is towing the trailer.

I would look at your tires with 40 psi under load.

38 psi all around for normal driving is way too high. 34F/32R is a good starting point for you there as well.

 

[Brakish]

Thanks for the quick replies! I should've clarified this camper sits on the bed, it's not towed.

you are going to have some sag if on oem suspension. max payload doesnt necessarily mean you need to be at max pressure. its also dependent on where the load is positioned. i have heard people running new rear springs and at about 42psi on pressure, however it all depends on where you are driving. you might be better at lower pressure on trail somehwere compared to the pavement. i guess the best thing to do is try driving around with the load and see where and how the truck feels and sits and adjust accordingly

Makes sense, figured I would eventually upgrade to the Eibach HD Rear Springs (E30-35-060-03-02). But don't see enough sag at the moment to justify, we'll see how it looks loaded with gear.

For starters I'll experiment with 38 PSI front and 40 PSI rear on pavement. Off road I'll likely be down around 30 - 32 PSI.

Welcome to FRF. Please post pictures of said prius that is towing the trailer.

I would look at your tires with 40 psi under load.

38 psi all around for normal driving is way too high. 34F/32R is a good starting point for you there as well.

Lol thanks!

The 38 PSI today with the camper feels okay, but is a bit squirmy on large bumps at highway speed.

 

[Nex]

Feels that way due to high tire pressures. With that camper, give 34 psi all the way around and retest. Raptor uses the tire sidewall as part of the suspension.

I would look at rear springs before summer and a bunch of trips.

 

[Brakish]

Feels that way due to high tire pressures. With that camper, give 34 psi all the way around and retest. Raptor uses the tire sidewall as part of the suspension.

I would look at rear springs before summer and a bunch of trips.

Sounds great and makes sense, thanks a ton.