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If you’ve seen some of my other posts (like this one, and this one) you know that I’ve put a fair amount of time (and money) into improving the ride and handling of our Sprinter-based RV. But the one thing I haven’t addressed yet is probably one of the easiest and cheapest to deal with, and that’s tire pressure. And the good news is that even if you don’t have a motorhome based on the Sprinter, setting your tire pressure properly can help you too.
Now, a lot has been written – online, in magazines, in forums, and who knows where else – about tire pressure. And there’s probably some accuracy in all of it. So let me just say this up front – I’m not a “tire engineer”, or even a mechanic. What I am is an RV guy who’s come up with a way to set the tire pressure in my RV that – in my opinion anyway – leads to improved ride and handling, and still addresses the safety concerns about proper inflation. Now, other opinions may differ, but I’m going to share what I do.
But first, there are two common things that people do, and I want to explain why I don’t do them.
What I Don’t Do #1 – inflating tires to the maximum pressure on the sidewall
The first is simply inflating all the tires to the maximum pressure listed on the sidewall. All tires will have their maximum pressure stamped on the sidewall.
This is what mine look like, and on these tires (Firestone Transforce) it’s 80 psi. Now, inflating the tires to 80 psi will certainly provide all the load carrying capacity that the tires are capable of. But there’s a decent chance you don’t actually need all that, and riding with the tires at max will lead to a pretty harsh ride.
What I Don’t Do #2 – inflating tires to the pressure on the chassis plate
I’ll confess this is what I used to do, but the second thing I don’t do anymore is to inflate the tires to the numbers listed on the plate attached to the vehicle. Now, on my Sprinter, that info is on the driver’s seat pedestal.
This is certainly a better idea than just inflating things to the max on the tire, but this was put on by the manufacturer. They certainly knew their vehicle, but they didn’t know what particular tires you were going to use. They also didn’t know how loaded your rig was going to be – so they had to assume the maximum (in this case, 8550 pounds). So basically, these numbers aren’t optimized for you. Nevertheless, you can’t go wrong by using them, and that’s actually what I did for a while before I figured out what I’m about to show you now.
Step 1 – Find your RV vehicle and axle weight ratings
On my Sprinter, you’ll find them at another point on the driver’s seat pedestal, and they’re usually around the driver’s door somewhere.
My RV has two axles, so there are three numbers I’m interested in. The first is the Gross Vehicle Weight Rating, which in my case is 8550 lbs. The others are the Gross Axle Weight Ratings for both axles. For me, that’s 3860 lbs in the front and 5360 lbs in the rear.
We want these numbers because we’re going to head out and weigh the vehicle, and we want to be sure that we’re not over any of these weights. And speaking of weighing the vehicle, that’s the next step. But there are a few things we need to do before we head to the scales, and they all begin with “Fill the RV with…”
Step 2 – Load up your motorhome
Now, when I was thinking about weighing the rig, I wanted to be sure that I was weighing the vehicle as fully loaded as I could ever expect it to be. I wanted it full of stuff – so I had to wait until we were headed out on our next RV adventure to make sure that I had it full of food, bedding, fitness equipment, bikes, and anything else we might take on a road trip. You should do the same.
Fuel has weight, you’ll also want to fill the RV with fuel (gas, diesel, propane, whatever you use). The weigh station I use is a Flying J that also sells diesel and propane, so that’s easy enough.
You’ll also want to fill the RV with water. Head to the scales with a full fresh water tank and a full water heater.
And finally, you want to make sure that the grey and black tanks are full. Now, when I did this, I didn’t wait to fill these naturally… because, you know… eew… So I just filled them with water. I don’t ever anticipate traveling this way intentionally because that would be bad form – but we’re looking for a theoretical maximum, so that’s what I decided to do. Also, I want to point out that the RV is very heavy while doing this. So drive very conservatively, and if this is the first time you’ve done this, then make sure the tires are at least inflated to the pressures listed on the vehicle plate for now.
I also want to point out that it’s best if you can locate a scale that can weigh all of your wheels independently. The scale I use can’t do that, so I have to assume that the axles are more or less evenly weighted side to side. I happen to know my grey and black tanks are on the mid-line of the vehicle, and the batteries and fresh water more or less balance out, so for me, that’s not a bad assumption. But if your tanks are not located along the mid-line, weighing all the wheels becomes more important.
Step 3 – Weigh your rig
First, let me warn you – if you’re going to use a scale that’s normally used by truckers, when heading to the weigh station, you’re going to want to bring a long stick. Just trust me on this one. At the Flying J I use, here’s how it works:
I head inside first to tell them to expect me on the scale. Though this probably isn’t strictly necessary, I feel better about it. That way, if something unexpected happens, they know they’re dealing with a total amateur.
Then, it’s back into the motorhome and drive onto the scale, stopping where indicated. This is where you need the stick – the button that says “weigh me now” is set up for truckers, and it’s like 14 feet in the air. I can’t hit it without a stick. When you hit the button, they will ask you for a number – but you can just say “Private”.
Once you’re weighed, drive off the scale and head back inside to get your weight. They should give you something that looks more or less like this:
Now, if your waste tanks are totally full, dump them (again, easy at the Flying J) and head home to sort things out.
Step 4 – Validate your weights vs. the maximums
So the next thing we want to do is to check and make sure that we’re not over the max weights for either the vehicle as a whole, or any of the axles. Since I rebuilt our RV myself (details here), I can’t rely on the old manufacturer ratings, so I was sweating this one the first time I did it.
Good news! I’m under. Barely, but under. I came in at 8380 pounds of an 8550 max, so I’m good as long as Stef and I don’t gain much weight. But remember, this was as overloaded as I can make the vehicle, so we’ve got a safety margin here.
Also, I’m under on each of the axles. On the front, I’m at 3580 pounds of an allowable 3860, and on the rear, I’m at 4800 of an allowable 5360 pounds. Hopefully, you get good news like this too.
Step 5 – Obtain the proper load and inflation tables for your tires
For this you’ll need to hit the internet, or contact your tire dealer. What you are looking for is the load and inflation tables for your particular tire. I mentioned before that I run on Firestone Transforce HTs – but I also have a set of Blizzaks that I use in the winter. So I need two sets of tables. Here are some links to a few of the load and inflation tables I found online. You’ll need to find the right ones for your tires.
The interesting thing was, when I checked them, for the same size tire, they all seemed to indicate the exact same weight and inflation numbers… So either I’m in the Twilight Zone, or these things are more or less standard for E load rated tires. Either way, I recommend checking the ones made for your tires.
Step 6 – Determine your tire pressures from the tables
So I’ll just use the Firestones as an example here. (But the Blizzaks have the exact same numbers.) I need to mention that our Sprinter is a 2500, with the single rear wheels at each side. I’ll be using that when I read the tables. Here’s my page from the Firestone book.
So for the front axle, my weight was 3580 pounds. Since I wasn’t able to weigh each side individually, I’m just going to divide by two. 3580/2 gives me 1790 pounds per tire.
Now, with my size tire, in a single wheel configuration, it shows that I can get away with inflating my tires to just 45 psi (!) and they will still carry the 1790 pounds. But 45 psi seems awfully low to me. And remember that I wasn’t able to weigh both ends of the axle. So unless my load is exactly evenly distributed – 45 psi isn’t going to cut it.
Therefore, I’m going to select 50 psi for the front tires. That’s a cold inflation pressure. This will allow me a 300 pound difference – side to side – and the tires will still safely carry the load. And remember too, that the rig is never likely to get this heavy again.
Now, for the back tires, just dividing by half (4800/2) yields 2400 pounds per tire.
And from the tables, it shows that I should run 70 psi to handle this load. I could elect to run them as high as 75 psi and still be in the ballpark. But I am going to run with 70 psi.
This allows an 80 pound swing, side to side. That might not seem like a lot. But I know that my grey and black tanks are behind the rear axle. And since I don’t normally drive with them both insanely full – my typical running weight is about 60 gallons of fluid less than how I weighed. Water weighs about 8.3 pounds per (US) gallon. So normally, I’m about 8.3 x 60 = 498 pounds lighter behind the rear axle. That makes me pretty comfortable with 70 psi.
Step 7 – Inflate tires and go for a drive!
In my case, 50 psi front and 70 psi rear is a few pounds less than what the Mercedes Benz specified (by 5 and 9 pounds). But when I set the tires this way a drove a bit, I liked it. I found it to be less harsh – like when hitting seams in concrete highways – and it also provided good control when cornering. I am very confident that I am inflating the tires enough to carry the load. And since the Blizzaks have the same ratings, I have just one set of numbers to remember.
(OK. I don’t usually inflate my motorhome tires with a bicycle pump. But since I almost always travel with bikes and a pump, I thought I’d try it just to see. The good news is, it does work. The bad news is, it takes about half an hour per tire!)
Anyway, for my rig, my equipment, and my weights, 50 psi front and 70 psi rear were the answer.
How do these numbers shake out for your rig? There’s only one way to find out – so load up and head out to get your weight!
We’ll see you on the road.