Just made a trip from Florida to upstate NY in my 94 Roadmaster. I was hoping to get to the wagonfest in Geneva, but due several work related setbacks last week, I couldnt leave until Friday night so I just got here today. I'll be heading back to FL next weekend. During the first leg, I didn't seem to be getting great gas mileage (around 19-20) so I checked my tires and they were all low. They were all about 8 psi below the 44 psi max stamped on the side. They are fairly new tires and I just assumed they inflated them to where they should have been when installed. Anyway, after bringing them all up to 44 psi, my mileage improved to 23-24 for the rest of the trip. Does this seem reasonable to get this much of an increase? Car ran great all the way.
Yes, tire pressure does have a significant impact on fuel economy. Technically your tires are now over inflated because they are supposed to be inflated to the pressure recommended on the tire and axle loading placard in the driver's door area. The recommended pressure of 35 PSI for the stock tires is where the tires have the proper tread contact patch (the tread is uniformly pressed into the road surface) so that you get the best ride, handling, and traction, as well as better tread wear. By inflating the tires to the maximum on the sidewall (not the pressure on the placard) you are reducing sidewall bulge and rolling resistance, but you are also causing the center of the tread to have more contact with the road surface than the outer edges of the tread. This will cause accelerated tire wear on the center of your tread, and reduced traction. Because the sidewalls are stiffer, it may improve handling and cornering, however. It also will make the car ride harder. All that is for stock size tires. If you have aftermarket wheels and different size tires, then all bets are off, and running the sidewall pressure is your best bet initially (under-inflation is very dangerous so it's better to be over than under). The correct tire pressure can be approximated with lots of time, patience and baby powder. if you coat a patch of your driveway with baby powder and roll the car over it and measure the tread width of the track you leave, as well as examine the definition of the tread pattern you see in the powder, you can jockey the tire pressure up or down as needed until you get a nice, uniform tread patch across the powder. If you see sharp tread definition in the center of the tire but see dull definition on the tread edges and the total width of the tread pattern is less than the width of the tread on the tire, then you are over inflated. If you see sharp definition on the outer edges of the tread and dull definition on the center, and if the tread is somewhat wider on the powder than it is on the tire, then you are under-inflated. If you get the contact patch correct, in theory, you should have low rolling resistance, ideal traction and ideal handling. In actuality, you'll still sacrifice a slight amount of fuel economy and some handling for improved tread wear and traction.
Personally, I always run tires at the maximum on the sidewall. When I got the 89, the placard called for 28 front, 35 back. 35 was the maximum on the sidewall of the OEM tires. I didn't like the understeer that the differential inflation caused, so I immediately played with the pressure and found I was most happy when all 4 matched. When I upgraded to the Comp T/A HR-4, which was a 45 pound tire, I inflated them to their max, figuring that if the factory tire was inflated to max, the new tires should also e fine at max. They were rated for 80,000 km, and I actually got just over 90,000 out of them before they were worn to the wear bars. There was absolutely no unusual wear in evidence at all. Also, if you read the owner's manual when you have the vehicle loaded to its maximum, the tires should also be inflated to maximum as the max load stamped in the sidewall is predicated on the tires being a max pressure.
I also prefer to run max inflation on my vehicles as well, but I won't do it for a customer unless he/she specifically requests it, because you technically are deviating from manufacturer's specification. In order to keep my tread flat on my Suburban while running 51 PSI in my 235 75 R15 General Grabber HTS tires, I went with 15x8 Chevy Truck Rally Wheels to negate the effect of the center bulge that could occur. I used to pump up the front tires to the 35 max and drop the rears to 29 on my dad's Celebrity (it called for 29 all around) to negate the car's tendency to understeer. I liked it a whole lot better, and so did my dad.
I believe the manufacturer's specs are based on the OEM tires installed on the car when produced. Maybe it was appropriate at the time, but not anymore. Tire technology has advanced by leaps and bounds since our +/- 30 year old vehicles were produced. Also, recall the numerous lawsuits back then which were the result of under inflation using the manufacturer's specs. We always used the car's purpose as a guideline when servicing them; but, unlike today's shops, we knew all our customers. Mom's grocery getter got the lower pressures. Road cars got max inflation.
OEM inflation specs are often to maximize comfort. The Ford Explorer/Firestone debacle is a prime example. Minimal inflation pressures on a tall(cheap)tire to soften the ride of a crude truck chassis with predictable results. My own preference is to inflate all four to or near the maximum recommended by the tire mnfr. for better turn in and predictable handling. I don't mind feeling the road beneath me. I've never had uneven wear except on snow tires which get center wear from spinning the wheels, which is my own fault. I just did several thousand kilometers in Europe, some on unlimited Autobahns, with tires inflated to max. sidewall spec.. I can't imagine what turning at 200+k would have been like at the door placard specs. Fuel economy goes up, sometimes dramatically, with inflation pressures. Most, if not all, vintage wagons would have manufacturer inflation recommendations aimed at comfort. OP's results are not surprising. It's also worth noting that eliminating sidewall flex will decrease heat and lengthen tire carcass life.
On a 30+ year old car I agree with you. I meant on newer cars. Something that isn't old enough to vote or buy a pack of smokes. I agree with you also. Ford no doubt lowered the tire pressure too low to appease the people who thought they wanted a "rugged, rootin' tootin' SUV 4x4 off road thunder chief" when in actuality they really wanted a wagon, but were swayed by marketing hype and they of course bought the cheapest tire they could get away with, resulting in catastrophe. Also, as far as Autobahn speeds, then you are supposed to fully inflate the tires to get the full speed rating, so I agree with you there also.
I dropped the air pressure down to around 36 psi all the way around to drive around locally near my mothers house in MA where the roads are full of potholes and frost-heaves. The 44 psi was just too rough. When I left yesterday for home, I put 'em up to 44 psi again, and so far on the trip (1000+ miles down), she's doing 23.6 mpg. running around 72-76 mph on cruise control. ran north Georgia at 81 mph on cruise and I could see she was using more gas, so I dropped back to 75. That seems to be the sweet spot. I can live with 23-24 mpg and all this comfort. The tire pressure definitely makes a big difference.
It isn't just tire inflation pressure that affects mileage. Wind resistance increases as the square of the velocity of the vehicle through the air. So your speed doubles between 40 and 80 mph, but the wind resistance increases by a factor of four. In going from 70 to 80 mph, a 10 mph increase, wind resistance increases by about 30%. I never understood these guys in their big Silverados or F-150s, which don't get the greatest mileage to begin with, going down the interstate at 85 mph. Their gas gauges have to be going to the left as fast as their speedometer is going to the right. Slow down, be safer, and save a little fuel.
It sounds to me like you did see a boost with the tire pressure. If you go 55 as opposed to 65 on two-lane highways you'll see another fuel economy boost and for me, It's not as stressful to drive at that speed. I still recommend anticipating other drivers though, Some might not understand.
This is very true. My diesel suburban will do 28 MPG at 55 MPH. At 60 it is down to about 26.5, and by 65 it is down to 25. At 70 it falls to about 23. Faster than that is drops like a rock. That is why I love my cruise control, because I have a lead foot and tend to gain speed on my own.
