I'm in no doubt that my current car's recommended pressures are higher than they need to be and that the reason is to improve head line figures for gas mileage. The puzzle is that they want to spoil the ride for a benefit that is probably less than 1%. There seems to me a lot of disinformation on the net about how much tyre pressures impact on mpg with figures as high as 10 % change in MPG. Frankly I think that's garbage for modest changes in tyre pressure and anything beyond modest changes are pointless because of the law of diminishing returns. Perhaps they are confusing changes in rolling resistance with fuel economy. The only convincing actual test I've seen suggested a 10% increase in pressure produced only 0.8% improvement in MPG at highway speeds. The improvement may be bigger at lower speeds when aerodynamic losses are not so dominant.
Have tire pressures trended up to improve gas mileage?
- Thread starter walterjay
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I've noticed this even within the same generation of a car. Eg the first model year of the BMW E90 has reasonable tire pressures, which trended upwards each subsequent year with no other relevant changes to the vehicle.
The ride on my late model E90 is incredibly harsh at the specified pressures, but far more comfortable when I use pressures specified for early model years.
The ride on my late model E90 is incredibly harsh at the specified pressures, but far more comfortable when I use pressures specified for early model years.
The rental jeep I have is the first vehicle I’ve come across that specifies different pressures front and back. 38/39 up front, 41 in the back. It definitely felt more “mushy” with all 4 at the 35psi they gave it to me with.
But did the mpg claims change year after year? cause that would be the reason to do it
I think all my cars specced different pressures fron and back, and different pressures for different engines and different wheel sizes.
On a longer timeline, my 1981 Accord called for 26psi, but I ran 30.
I compared the EPA fuel consumption for each model year (E90 ie sedan, 328i/330i RWD AT) and it didn't change significantly. They did squeeze out an additional 1 mpg on the combined cycle in 2007 which may be attributed to the increased tire pressures?
I went through the owner's manual for each model year and noted relevant specs below. BMW is very precise in specifying different pressures based on tire size/load rating and driving conditions. I chose a square set up out of simplicity, but the staggered tire pressures result in the same load capacity. Note that the first model year (2006) has two different recommended tire pressures and I'm not sure if the EPA fuel consumption for this year was for the lower or higher pressures. For what it's worth, the press cars were tested at the lower pressures. There was a facelift in 2009 with a wider rear track and stiffer springs and dampers. Perhaps you can see a pattern because I could never make sense of it:
Obligatory:
LOL, the full 107%
On a serious note, I use the tire pressures listed on the door jamb (or fuel filler door) as the minimum pressure I run. Most of my vehicles are or have been Euros, and they typically have different pressures for F/R as well as for travelling at high speeds.
On the GLK I typically keep pressures between 34-36 all around, and on the RAM they're 36-38 depending on whether I'm towing or not.
I've yet to see any weird treadwear patterns or premature wear in more than 30 yrs of driving.
On a serious note, I use the tire pressures listed on the door jamb (or fuel filler door) as the minimum pressure I run. Most of my vehicles are or have been Euros, and they typically have different pressures for F/R as well as for travelling at high speeds.
On the GLK I typically keep pressures between 34-36 all around, and on the RAM they're 36-38 depending on whether I'm towing or not.
I've yet to see any weird treadwear patterns or premature wear in more than 30 yrs of driving.
Low profile tires need more pressure to keep from damaging rims and sidewalls on square impacts, and also need more pressure not to overflex/overheat the sidewalls which would at best burn more fuel and can also damage the tire, and maybe a demand for sharper handling too?
In the past having low air pressure was an easy way to smooth out the ride on that all important first test drive, and the higher profile tires can take it better. My 2003 Tracker had 205/75R15's with a recommend pressure of 26psi. It was also 3000lbs so it didn't need much pressure with 6400lbs of tire load capacity.
TBH I was surprised that Kia would put tires as harsh and noisy as the 215/45R17's on the rental Forte I had. If it was a Miata ,BRZ or something that focused on handling, they would be fine, but for this car it wasn't adding anything.
Going back 50 years, Radials didn't have stiff side wall fillers in the early days, and you could run 26 to 28psi on C and C body cars for a soft ride but with jello-on-a-plate steering response.
I preferred Polyglas belted bias-ply performance tires during this era for sharper helm response and predictability - for the appropriate vehicle.
To answer the question, the door jam placard on most of our commuter class vehicles has been 32-34psi front for the past 2 decades.
I aired up the Ford to 35psi last week when it was unseasonably warm out. I expect the cold pressure to be a few pounds under that with the cool down coming to New Hampshire - following the Combined Gas Law.
I preferred Polyglas belted bias-ply performance tires during this era for sharper helm response and predictability - for the appropriate vehicle.
To answer the question, the door jam placard on most of our commuter class vehicles has been 32-34psi front for the past 2 decades.
I aired up the Ford to 35psi last week when it was unseasonably warm out. I expect the cold pressure to be a few pounds under that with the cool down coming to New Hampshire - following the Combined Gas Law.
I had a 2011 e91 with sports package, staggered 18” wheels. I later acquired an extra set of fronts and ran 245 on both axles. I would suspect BMW to have considered handling characteristics, but the year to year changes don’t make sense unless they were also changing OE suppliers of tire, etc.
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I never pay attention to the PSI in the door jam. I select it based on the conditions and goals.
If i recall correctly, Mythbusters tested tire pressures and there was a significant mpg percentage gain by running more pressure. So yes it`s a cheap way for manufacturers to gain a bit more mpg, also cars are heavier than they used to be.
It's many things...
Higher speed limits across the country, speed = heat and heat is bad
Heavier Vehicles = Heat and heat is bad
Lower profile tires probably have a harder time dissipating heat which leads to higher pressures and lower load ratings to compensate
The ford explorer debacle scared the industry into higher pressure even at the expense of ride quality. The exploders 26 psi recommendation made for a nice ride and hot tires, and tread separations.
and of course higher mileage expectations, better suspensions, and better sound insulation has allowed higher pressure recommendations in tires.
Higher speed limits across the country, speed = heat and heat is bad
Heavier Vehicles = Heat and heat is bad
Lower profile tires probably have a harder time dissipating heat which leads to higher pressures and lower load ratings to compensate
The ford explorer debacle scared the industry into higher pressure even at the expense of ride quality. The exploders 26 psi recommendation made for a nice ride and hot tires, and tread separations.
and of course higher mileage expectations, better suspensions, and better sound insulation has allowed higher pressure recommendations in tires.
One of my fav auto bloggers (Phil Greden) does a series on junkyard cars. He had picts of a door jamb placard in an old SUV that had different pressures for light load versus a full load. That got me thinking, along with the current (cold climate--pressures dropping naturally), to see the affect of dropping my tire pressures. The result--the ride comfort in my Honda Pilot (20 inch, 50 series) and Chevy Equinox (17 inch, 65 series) is GREATLY improved. Typically, we run a light load; rarely do we run fully loaded. So, I say screw the 35 psi placarded pressures both vehicles display. Both are are running 33 front and 31 rear, and I like that feel, a lot.
This fixes of the harsh ride in the Pilot; and I've lost the tire slap noise in the Equinox. BTW, I carry a portable inflator to enable factory pressure, easily at my discretion.
This fixes of the harsh ride in the Pilot; and I've lost the tire slap noise in the Equinox. BTW, I carry a portable inflator to enable factory pressure, easily at my discretion.
