Tire Inflation Dilemma
I recently had a stem replaced on one of my real duallies due to a slow leak. When the guy inflated my tires he set the rears for 80PSI and the front for 65PSI. He said all Class Cs should have 80PS...
Forum Discussion
Setting Pressure to Load is the thing to do. Take the Door Sticker for a "3" series Class C Chassis (Chevy 3500, Ford E350) and reference that Michelin chart for its tire size. You'll see that 65 PSI will support both the Front and Rear Axle Ratings, and "D" tires are rated at 65 PSI. The "4" series (Ford E450, Chevy 4500 and some Workhorse/Chevy) are upgraded mostly in the Rear Axle, and their Door Sticker will show Rear Axle rating that calls for 80 PSI which requires "E" tires. So, as Drew and others said, the pressures are based on the axle ratings.
So a couple factors kick in. First, Ford and Chevy aren't going to specify "D" on the front and "E" on the rear. The latest "4" series have enough front axle capacity that they fall into the need for "E" tires anyway. So Second, since "D" tires no longer "cover the market" the rubber companies have migrated to nearly all "E" tires in these sizes. Still, Load NOT Sidewall determines Pressure.
The Sidewall Factor... Put 80 in REAR tires when only 60 is needed... That's pretty much a So What. It'll ride a little harder but we can hardly tell from the cockpit. Do that on the FRONT and we WILL feel it in a harder ride. Maybe even in the steering wheel. But that's the least of it. The excess pressure reduces the contact area between tread and road. That compromises steering and where it shows up first is TRACKING. It may very well wander at speed. And Yes, the Fords seem to have more trouble with this. I told a tire shop I wanted 65 front and 80 rear in my set of new tires. I soon found out they didn't and put 80 in the fronts. This jumped out at us on the downhill side of a high bridge, at 65 MPH, on a gusty day. We'd done it dozens of times but that day we nearly lost it. Lowered pressure to 65 on the side of the road and handling was restored.
Humorous epilogue: Our tires fit some SUVs. Imagine how my neighbor's Hummer H3, about 5000 pounds, acted with "E" rated LT tires at 80 in all four corners. Shop said "Says 80 on the side..."
So a couple factors kick in. First, Ford and Chevy aren't going to specify "D" on the front and "E" on the rear. The latest "4" series have enough front axle capacity that they fall into the need for "E" tires anyway. So Second, since "D" tires no longer "cover the market" the rubber companies have migrated to nearly all "E" tires in these sizes. Still, Load NOT Sidewall determines Pressure.
The Sidewall Factor... Put 80 in REAR tires when only 60 is needed... That's pretty much a So What. It'll ride a little harder but we can hardly tell from the cockpit. Do that on the FRONT and we WILL feel it in a harder ride. Maybe even in the steering wheel. But that's the least of it. The excess pressure reduces the contact area between tread and road. That compromises steering and where it shows up first is TRACKING. It may very well wander at speed. And Yes, the Fords seem to have more trouble with this. I told a tire shop I wanted 65 front and 80 rear in my set of new tires. I soon found out they didn't and put 80 in the fronts. This jumped out at us on the downhill side of a high bridge, at 65 MPH, on a gusty day. We'd done it dozens of times but that day we nearly lost it. Lowered pressure to 65 on the side of the road and handling was restored.
Humorous epilogue: Our tires fit some SUVs. Imagine how my neighbor's Hummer H3, about 5000 pounds, acted with "E" rated LT tires at 80 in all four corners. Shop said "Says 80 on the side..."
