V6 fuel mileage

[mitmaks]

You're supposed to check your tire pressure COLD, that's what the general rule is for checking pressure.

[Nd4SpdSe]

Really? I've been told to to check it when it's warm since that's the pressure you'll actually be driving on. Tire pressure doesn't mean squat when you're not actually driving, IMO, lol

[MrMazda92]

Cold test is the way to go, it makes sense when you think about it as the initial mounting of tires(they go on cold) It's a silly way to remind yourself, but it works for me.

[wytbishop]

You check them cold because of Boyle's Law. A gas' pressure rises proportionally with its temperature.

[Ryan]

I think that the people who make tires understand this, and they understand that most people would never think of checking them warm, or what constitutes warm, so they specify cold, because thats easy.

So I imagine they'd determine what pressure is best at the tires' average operating temp, cool the tire to room temp, and then see what the "cold" pressure is. Something like that, probably more complicated with more math.

[wytbishop]

Keep it simple. It's much easier to define "cold" than it is to define "warm".

[MrMazda92]

Let's not forget that temperatures vary greatly...

In a test city, say it's 65 degrees outside for the majority of the day.
65 degrees inside the tire is "cold" for the sake of this discussion.

Okay, so let's say you drive for 10 minutes at a normal pace; Your tire air temp. has gone up to exactly 78 degrees, wow we're living in a perfect world of exact round numbers!

Now to be fair, you test the pressure after 45 minutes of tire-shredding turns, and high speed straights out in the country. Your air temp in the tires hits a whopping 93 degrees!

It's reasonable to assume that the pressure will change in varying heat... The only "constant" is the cold temperature. That being said, you should adjust your pressure periodically to avoid the obvious effects of cold/warm weather change.

If 51 PSI is great at 70 degrees in the summer, 51 PSI at 45 degrees in the winter is pretty good too. Cold test from now on people, there are plenty of reasons...

[_-Night-Shade-_]

Wow Nd4SpdSe, no wonder you got amazing fuel economy! I'm sure I'd see a significant gain pumping up my tires to 40+ PSI but that's a little hardcore for me, that's some hypermiler tactics. And as much as I like good fuel economy, I like even tire wear more. Especially once you get into expensive grippy tires - you want those to last. I keep mine at around 32psi and I don't usually pump them up again till they get to about 25psi.

One of these days I'll throw on my stock rims with 205s before going on a lengthy trip and let you know what kind of gains I get.

[MrMazda92]

Remember Igor, it depends on the tire too...

My current Humhos are rated 51 PSI max. Running those in the 30s would be disastrous.

I'm assuming you're running a larger wheel, and a smaller sidewall than stock size as well; That plays an obvious role as well.

[mikeinaus]

incase anyone was curious ive added some injector cleaner to my gas and my mileage seems to be slowly improving...

[_-Night-Shade-_]

Remember Igor, it depends on the tire too...

My current Humhos are rated 51 PSI max. Running those in the 30s would be disastrous.

I'm assuming you're running a larger wheel, and a smaller sidewall than stock size as well; That plays an obvious role as well.

Tires are not made specifically for a particular car. That 51psi is for safety - that's the max pressure the tire can safely handle. It doesn't mean pump them up to 51psi on your MX-3. As per the manual for MX-3s the tires should be at 28psi for standard tires (I'm assuming this means summer) and 32psi for all-season/winter tires (M+S) in the factory tire size. This wouldn't necessarily be the same for another car - it would have a different weight and distribution. On a heavier car it would probably have higher psi as more weight is being placed on the tire. The only other time a different tire actually makes a difference is with the actual compound (as in racing compounds that have a much higher coefficient of friction and will actually lower fuel economy in exchange for grip) and, to a much lesser extend but still counts if you want to get technical - the shape of the tread pattern. But these are all minor things. For regular purposes just the tire width makes the biggest difference as that directly translated to the size of your contact patch.

[MrMazda92]

For fun I'll take pictures of my Kumhos at 28 PSI for you when it stops raining here. When I had them mounted, Schwabs left them at 32 PSI a corner and it looked like I was ready to drive on sand. Not all tires are created equally, at least not in Oregon. For simplicity sake, think about the sidewall. All other factors being equal, a tire with a shorter sidewall will take less pressure to maintain it's shape.

[Ryan]

I pumped my winters up to 40 PSI today, and I run my summer tires at 40 daily, and 45 racing.

If you ever go circuit racing, I dare you to run sub 30 PSI. Have fun re-beading your tire, and you won't be able to read the PSI you should have used either, it'll be rubbed off on the road.


Unless of course you race a quad, then 6 PSI should do you.

Definitely subjective to the tire/application.



Interesting Tidbit, Ford released one of their large SUV's, and people were complaining the vehicles rolled over too easily.

Ford suggesting reducing the tire pressure to reduce traction (Thanks, Ford)... and it worked, except now people speeding down Arizona freeways were having blowouts.

[MrMazda92]

Honestly, I keep debating whether or not to get another SUV for that exact reason. I think it looks terrible, but I might lower mine an inch and a half to lower the center of gravity... IDk yet.

[mikeinaus]

from what my teacher has been saying recently, tire psi is calculated for each car by its weight, handling characteristics, desired comfort, tire size and a few other things. when a car says to use 32psi on each tire it is referring to the stock tire size. all the calculations engineers make on a car are based around there set stock parameters, when you change these parameters the end results are of course going to be different. you should only fill more then this when you are carrying a heavy load. ie 4 fat chicks in your mx3 youd probably want some extra pressure. however when you change to larger or smaller rims with larger or smaller side walls all of their calculations go out out the window. he suggested filling low profile tires to 90% of their maximum rating as they need to be stiffer.

think about it 35psi on 3" of side wall gives you a large room for error if you hit a rock or bump at high speed, the chances of bottoming out and causing damage to your rim are slim. however 35 psi on 1" of sidewall is more likely to bottom out and have chances of causing damage. therefore they need to be stiffer to reduce damage.

psi is also calculated on the shape of the tire. the larger the sidewall the more round the tire is, if you over inflate these tires most of the surface pressure of the road is going to be on the centre of the tire which will cause uneven wear. the smaller the side wall the more square the tire is, under inflating these will cause the center of the tire to buckle in slightly and cause the outsides of the tire to wear faster.

also the maximum psi on the sidewall is based on the maximum weight rating the tire can support safely. over inflating and under weighting is not a good thing. im guessing if you wanted to know the actual psi you should run your aftermarket tires on, you should divide the weight of your car by the maximum supported weight of the tire and get a percentage, then take this percentage and apply it to the maximum psi of the tire then +/- 10% and id think youd be in a safe range.