I'm confused
 

As the titles says, I'm completally confused. After driving 147,7 km (85% highway 15% urban) and applying ALL hypermileage tips (no AC, windows closed, using cruise control, approx 10 km/h BELOW speed limit, no braking, 5th gear nearly everywhere... And I spent a total of 12.86 L. That is 8.7 l/100km of 95 octane gasoline (27.04 MPG).

I just don't know what am I doing wrong, I drive an american-made Chevrolet Evanda RTX (133 hp). Last week I've got 33 MPG in another trip, and for some reason this one went better than that one I made today.

Any advice?:confused::confused:

Weather? Maybe this time you had headwinds - not strong enough to notice at the time, but enough to increase areo drag. Maybe the temperature? Rain? Mechanical problem since the last time? More red lights or traffic?

Each trip is different, even if you always take the same route. It's the long-term average which will show you whether a new mod is working.

Now that I think about it, it was colder than usual. Plus the fact that I decreased tires' pressure to those specified by manufacturer.

By the way... Maybe this doesn't have anything to do but... My father filled up the oil tank 2 cm above the maximum level. May that affect fuel economy? If there's too much oil, that oil may get sticked to a plug and more fuel would be needed to maintain the engine working. Maybe THAT is the issue! Am I wrong?

An overage on oil should spill out at some point, but won't hurt FE on most engines.
Unless the oil was of a thicker grade than need be.
Low tire pressure does hurt FE! Anything south of 40 PSI hurts, the lower the worse.

And do heat up the planet! Cold makes the air more dense and prolongs the cold start. Really cold starts are dramatically bad.

But I decreased tires pressure to those specified by manufacturer, not any lower. Neither higher

The relation between tire pressure and rolling resistance has been thoroughly investigated in this thread: http://ecomodder.com/forum/showthrea...tire-2721.html

Audi advices the tire pressure it feels gets its cars sold best. Which usually means compromising handling and grip for 'comfort'. If economy were their sole concern the figures would be double what they are now.

Honda does that too. My Insight needs 8 PSI on top of the OEM pressure to get rid of sidewind sensibility and other nasties. Road noise increased a bit, but so did cornering speed and general car control. I feel so much safer now.

One consideration to make is the max tire pressure as stated on the tire sidewall. The tire was designed to handle that pressure, but not above.

When you raise the pressure on used tires do it gradually, especially if you are not certain that the tire has never been damaged before.
Check the tires for deformation immediately after airing up and again after some use.
Hitting a kerb or a pothole with a soft tire can damage the carcass; riding a flat tire on the rim does that too. Those tires may blow when you raise the pressure; ironically, it was low pressure, not high pressure, that did the initial damage.
They would fail in the long run even at low pressure.

As said, cold hurts FE a lot.
Overfilling with oil might also do that, depends a little on engine design.
It is always a bad thing to overfill! Try to get rid of the excess as soon as you can. The crankshaft is running through a lot of oil, which can start frothing, oil pressure may also rise.
A bit excessive perhaps, but this guy overfilled too. He put in 5 liters with the dipstick low, so probably 4 liters too much.
http://youtu.be/TyVjlP2Qpg8

But if I rise pressure to 40 PSI (3 Bar) (starting from 2.3 bar) wouldn't it be so high that I would lose a lot of grip?

You would not know unless you test it, right?

There's a corner at a junction coming off the highway where two lanes go right.
I could do that corner on the OEM pressure at about 55 km/h max. Any faster and the car would not hold the inner lane.
When I raised it to 40 PSI i could do that same corner at 59 km/h!

Last winter I had a slow leak in my right rear tire. I almost lost it in a corner, going nowhere near what I thought would be the limit. At work I saw the sides bulge, it was at about 15 PSI.
The tire was plugged.
A few weeks later the same happened. This time the tire looked OK, but it had just 28 PSI or something.
The valve had started to leak; I saw it bubble when I put a wet finger on the stem. Some dirt had got in between the valve seal and its rim. One short tap on the pin and the bubbling ceased.
Aired it up again, low grip problem gone.

Now my tires aren't yours and my car isn't yours, so it may act differently.
You need to test it.
Grip is grip; it has the same benefit on braking distance as on max cornering speed, but the latter is much easier to test without precise instrumentation than the former.
So find a safe corner with enough runoff space and try it with varying tire pressures. Then you'll know - and learn something useful about the behaviour of your car too.

I do recall reading a post from someone who did performance tests on road cars on a track, including a slalom test. He wrote he got best times in the slalom test when the PSI was around 55 or 60, way over max sidewall for most tires.

Now lowering tire pressure is a good idea when you are stuck in sand or snow. But you'd have to go really low for best effect - 10 PSI like - and air right back up again once you get unstuck.

If you are worried about grip in cold conditions do buy winter tires! The difference between winter tires and all season tires in the snow is gigantic, blowing anything you can achieve by meddling with tire pressure completely.
I was cynical about winter tires but gingerly bought a set because my Insight had full summer tires which were acting up in the cold wet.
What an eye opener. Another thing you can only appreciate fully if you try it yourself.

If your drive is anything but level, then the cruise control could be contributing to poor fuel economy. It tries to hold a steady speed, up hill and down, which means it could be using extra gas to climb up, then braking on the downhill side. See various posts on "Driving With Load" or DWL.

Thermostat OK?

But are all of you 100% sure that tire pressure doesn't affect grip? Anyways, I've read that tires with high pressure tend to wear down faster from the centre than from the sides. Do you strongly recommend me inflating my tires way further than those recommended by the manufacturer? (I think maximum PSI for my tires is 50. Must check).

It's quite strange that I don't lose any grip with a higher tire pressure.

So, what pressure would you recommend me, if my car weights a total of 1370 kg + 55 kg (me)?
Keep in mind that sometimes I'll carry up to 5 people + charge in the trunk (+350 kg)

Yes, thermostat is ok. Maybe Cruise control is what affected my mileage, as I noticed that it was braking rather than letting the car go downhill with an increasing speed

There is ZERO truth to that assertion with modern tire construction. The main drawback with higher PSI is reduced ride quality which can vary tremendously depending on the car.

Those that think tires wear in the center faster most likely are the same that think a car needs to be warmed up before driving. They are still living in the past.:eek:

I'm sure it does affect grip, in a positive way.

Wear should be even with modern radial tires, where the wires in the carcass are evenly distributed over the track width.
It would just wear slower as the thread bends and scrapes less when it hits the road.
My two sets of tires (summer and winter) have worn evenly at 40+ PSI, with lots of thread depth left after 45.000 miles combined.

When you distribute the same weight over a smaller contact patch, that patch gets pressed into the road deeper, making it harder to slide.

Best pressure is up to you. The further you go the better it gets BUT the improvement recedes quickly.
Going from say 30 to 40 PSI can give you a 5% gain in overall efficiency.
Going to 50 PSI would only add 1% to that, and another 10 PSI would only add 0.1 % or so. Comfort and road noise may increase at those pressures, so that would be a clear sign to take it no further.
Max sidewall is another logical boundary. While most tires can handle way more than that you'd take it outside its intended use.

Whatever you do take small steps and check the tires after airing up, and again some time later. If a tire deforms it has been damaged and needs to be replaced asap regardless of pressure.

There is a grip loss, but the drop off in grip isn't a straight line.

You will see lots of disagreement on this point, but there is a tendency towards center wear with elevated inflation pressure, but there are other factors that contribute more, so it is not always easy to see.

I'm kind of the odd man out here, but I don't think the increased inflation pressure is such a good idea. It also causes groove wander, harsh ride, and increased impact and puncture damage.

You will find many people for whom the increased fuel economy out weighs any other factor - and some will even deny that there are any negative issues.
Personally, I recommend no more than 5 psi (0.3 bar) above the pressure listed on the vehicle tire placard.

As you see, opinions differ.
You need to test and find out what works best for you.

My opinion is that if you live at the edge of grip, you shouldn't be asking for advice on teh interwebs.

If your tires never squeal while cornering, pressure won't change your life. If they do, you need to explore things a bit more on your own.

:confused: yeah.... noooooo....

Having aired up my Winterforce to the max of 44psi and going for a drive in the snow, I can tell you that there is a LOT of grip lost running that high of pressure compared with the recommended 32psi.

http://i1002.photobucket.com/albums/...F63E9BA544.jpg

During the winter I had them at 36psi for the most part but the second the snow came back it was back to 32.

You may want to read up a little more on how sipes, studs, compound and pressure work to grip the snow.

Mind you, how often would you see this:

http://i1002.photobucket.com/albums/...203_115044.jpg

I'd suspect that's because the loss of traction is in the snow itself, not in the snow/tire contact. Same for e.g. driving off-road in sand.

On winter tires the thread needs to flex a bit to loosen up the snow.
40 PSI in mine works fine with me, but 44 PSI in yours may be too much.
It depends on the tire type, vehicle type, tire wear and of course weather and road conditions.

The rule of thumb is that there is no rule of thumb.
You have to adapt to the situation and be aware of changing conditions and the effect that may have.
Test the grip when you can safely do so, then you know where the limit is and stay well away from that limit when you need to stay on track.

Thank all of you for your answers. I'll slightly rise tire pressure, to 2,4-2,5 bars. I just bought an injector cleaner, as I suspect there is some carbon build up on the cylinders, whigh also reduces FE. To prevent this from happening, I'll put my car in a high rpm for little periods of tine.

Just to nitpick: If the Google results I'm getting for Chevrolet Evanda are correct, you drive a car made in South Korea by GM's Daewoo division (although it's branded as a Chevy).

From what I've read about Daewoo products, they tend to do worse than other vehicles in their class for fuel economy. While there are things you can likely still do to improve your fuel economy, your expectations may be a bit high.

They're certainly nice looking cars though, so there's that.:)