Discussing Tire Pressure or PSI

In my 2004 Nissan Frontier I run 40 PSI and I can't remember what they recommend, but it's definitely less. I ran 44 PSI for a few months and I noticed that I was stiff enough that I was wearing my tires faster and dropped back down to 40 and I was happy with what happened after another few months where I barely noticed wear.

When I have a big load I go up to 50 PSI. My tires are rated at 55 PSI, but this is based on the logic that a heavier load means the contact pad on the bottom of the tire was bigger. I didn't want a blow out. I followed the advice of a local grandpa who's tried everything when it comes to tire pressure changes and looking at my tires and feeling how they were cooler with more PSI when the truck had a heavy load all matched up with what he said.

On my 1999 Chevrolet Prizm they recommend, if memory serves 32 psi. I ran 40 to 44 for a long time following the same grandpa's advice (not my grandpa, just saying he's played with this for a long time). I put up with this for a few sets of tires wearing out at 1/2 what the tire manufacturer recommended. I decided to try 36 psi and my tires still wore just as fast. I went to 32 psi and finally my tires were lasting a lot longer. They only look half worn and I've driven as far wearing out the other tires. (same brand)

You see the problem on my 1999 Chevrolet Prizm is that it has no cam adjustment. So running my tires softer like the manufacturer recommends under my circumstance is preventing the inside of the tire wearing.

Recently I posted this opinion about how tire pressure effects a car's miles per gallon (edited):

Air pressure? Just do what the manufacturer recommends and you'll have better mileage and long tire life. That's putting it simple. Let's go complicated. Putting extra air in stiffens up your tires, reduces rubber movement in the tires and you'd think it's raise your mpg. What actually happens: your tire real height (after the weight flattens the bottom of your tire) goes up and your car will go "fewer miles". You see the taller your tires the farther you actually go, but your miles goes up slower from the car's perspective or on the odometer. This will appear to reduce your gas mileage. I ran the math on my mpg going up and down with this experiment. It was equal to how many apparent miles I was driving from taller and shorter tires.

Long story short apparent mpg changes from these up to an inch changes in tire radius didn't change my actual real mpg at all. It just appeared to be changing.

I used to run my tires with higher psi - and chose tires that could handle it. All I got was tires that wore faster because they were more sensitive to alignment never being perfect no matter how much you get alignments and a lower apparent mpg...the worse consequence is that over-filled tires increases stopping distance and if you accelerate quickly you have less traction. Just keep it simple and do what the manufacturer recommends unless you have custom wheels - you'll have to rethink it at that point.

Back to something new:

When I tried the putting more or less air in a car it was on a 7,000+ mile car trip. I was in a 1996 Saab 900 SE. I ran the math on how far the Saab was going according to it's odometer. I ran a ratio off of that and how tall my tires were at a "recommended" tire pressure versus stiffer pressures I was into at the time. So as I changed the tire height I knew that the actual circumference of the tire was changing. I ran the circumferential changes as a ratio against how far the car was going more or less distance against the car miles per gallon. Equal to how much better my car appeared to be doing on it's miles per gallon was the ratio of how less much less distance the car went due to the tires being shorter. Exactly equal. So I believe that, at least based on this one experiment, it's a myth on how much your tire pressure really effects mileage. I even messed the car up for one of the drive tires having more or less PSI by 6 PSI than the other and it still wouldn't effect the car's miles per gallon (of course when you include the ratio of how much farther or shorter the car is ACTUALLY travelling).

Note: When I say exactly equal I should say that the amount of change was 4-6 decimal places in or using 6-8 significant digits since the mpg was in double digits like 36 miles per gallon. That car has active readouts so I didn't have to fill up at every PSI change to get the numerical changes. I ran these experiments for about 1/2 the trip. The ratio of change kept being consistent up hill, down hill, across relatively flat terrain and over mountains.

by AutoBravado

Car Miles Per Gallon

I can't make this claim for every car miles per gallon (MPG), but my Saab 900 SE, for sale, has gone from 23 mpg to 33.5 mpg in the city just by upgrading to Iridium Spark Plugs. My other car's miles per gallon barely increased for adding in Iridium plugs, but it more consistently gets 44 on the freeway instead of 42, but the real difference for that car is that it has a lot more power.
Email me with interest in the Saab: autobravado.com@gmail.com


I have a few theories for why the above differences exist for the miles per gallon:

You see, the Chevy Prizm, referred to as other above, has already been modified so extensively for getting better mpg, that it is more difficult to get more out of it. I'm sad to say, that as these cars have been competing for which vehicle is faster, since the iridium spark plugs the Saab 900 SE with V6 turbo, is now the front runner. For a long time the Saab could out corner and out drive they Chevy Prizm when it was loaded with stuff because of better torque, but I'm quite excited for this car's miles per gallon boost, and hopefully I can get even more out of the Saab. Ah, one more factor, which boosted this cars miles per gallon: I got it a badly needed new battery! It runs so much better!


Common trouble for your car's miles per gallon:

I know, I know, you probably hear this all the time: don't drive drowsy, but in my recent cross state trip, the only time they Chevy Prizm did poorly - 32 mpg - is when we switched off drivers, yet the new driver who was just sleeping, was no longer awake enough. On the same road, going in the other direction in a hilly area that stayed about the same elevation, and after a night's sleep, I got 46 mpg! You see! Something else I discovered is that going 55 and popping it out of over drive increased the car's miles per gallon from the 44 to the 46 mpg. I'd still rather drive 75 when it's legal, but it was a long construction zone, and curiosity paid off. There's the all the evidence I need. It's not just about being responsible and safe, driving drowsy is expensive!

Related article:
Truck Miles Per Gallon
The article above is a come back to my original and wider ranged advice on the Truck or Car's Miles per Gallon. (Still totally applicable to any gas powered engine, especially if you're serious enough about your car miles per gallon to make some changes and do some upgrading.)

by AutoBravado