OT: Car Tire Pressure

[Crazy Chris]

After I had a tire checked for a slow leak, the woman at the tire shop told me she had them put in 40 psi on all four tires, rather than the 31/30 (front/rear) recommended by Subaru. She said that the latter provided a cushioned ride, while the former would get me a mile or two more gas mileage. Correct or not?

Thanks,
CJV

[rice rocket]

It's sort of true.

You'll also wear out the center of your tread though, instead of wearing it evenly, so I'm not sure if there's money saved in the end.

[FlashUNC]

Could also lead to a bit harsher ride.

[vsefiream]

I agree with both statements above. I raise the pressure a bit for long trips but run at the car manufacturers recommended rates the rest of the time.

[MattTuck]

I agree with both statements above. I raise the pressure a bit for long trips but run at the car manufacturers recommended rates the rest of the time.

I remember reading years back that this whole thing is a bit of hand waving. If I'm remembering right, this is what led to the whole Ford Firestone tire fiasco. That tires are designed for one pressure, car manufacturers are looking to give the car a specific 'feel', so their recommendations may not be ideal for fuel efficiency OR even ideal for the design specs of the tire.

If you went half way (ie. 35ish psi) I doubt you'd see a difference in wear, and you'd probably get some of the benefits of MPG.

As far as a "harsher ride", after this winter, the roads are giving a harsh ride regardless of tire pressure :rolleyes:

[benb]

It's exactly the same scenario as a bicycle. A 140lb guy and a 175lb guy riding 700x23 tires should not use the same tire pressure, but if they use the same tire it's going to have the same maximum safe pressure. (And the 250lb guy probably should go up to the larger size.)

If the car was right at the maximum weight the tire could handle I bet the car manufacturer's number would be right near the tire manufacturers maximum pressure. But no manufacturer is likely to want their car so close to the edge, so they spec a larger tire with a higher weight capacity.

I have a 2700lb car that has 215 series tires.. manufacturer says 31psi all 4 corners. The tires have something like a 42psi max. They basically specced a tire that could handle a car that weighed at least 1000lbs more. If I even run the tires at 34 or 35 I start wearing out the tire centers much faster then the edges and the tires will start performing poorly. I use a tire wear gauge to sanity check what the dealerships tell me.. they always always always overinflate my tires, I'm sure they'd like to see my tires wear out quickly so they can try to sell me new ones.

[RPS]

I remember reading years back that this whole thing is a bit of hand waving. If I'm remembering right, this is what led to the whole Ford Firestone tire fiasco. That tires are designed for one pressure, car manufacturers are looking to give the car a specific 'feel', so their recommendations may not be ideal for fuel efficiency OR even ideal for the design specs of the tire.

It’s pretty much like benb said above. All tires have a maximum load carrying capacity that is clearly marked on the tire’s sidewall, and it requires the tire to be run at the maximum pressure to achieve that load. When fully loaded the tire needs full pressure to minimize flex which causes heat buildup. When tires are underinflated for a given load they run hotter than normal and it can lead to failures. That’s what happened to Ford with Explorers in part because the Firestone tires were not capable of handling any unusual overheating.

Some manufacturers like Michelin publish load tables for their tires – particularly true for truck, RV, and trailer tires. The tires on the back of my Ford van require the maximum 80 PSI for those tires but at the front they only require 55 PSI. The difference is that the back axle is rated at 6,000 pounds and the front only 3,550 pounds. Same tires though.

[benb]

I wish more bicycle companies published load tables.. Michelin and Continental do, and if people followed them we'd have fewer silly tire pressure threads. :)

Of course IMO that is how Michelin is kind of "cheating". Their 700x23 is bigger then a Continental 700x23, so if I'm out of shape and 180lbs I have to run both of them at max pressure.. that's 120psi on the Conti but only 116psi on the Michelin so I get a better ride out of the Michelin. (Happy to currently weigh less then that)

[RPS]

I wish more bicycle companies published load tables.. Michelin and Continental do, and if people followed them we'd have fewer silly tire pressure threads. :)

Bicycle tire issues are a little different because they don’t overheat which can lead to separation in automotive applications. Running bike tires at lower pressure is more subjective IMHO.

[rice rocket]

What, you don't average 80 mph on your bike? ;)