I'm confused

[Xusi]

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?

[Piwoslaw]

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.

[Xusi]

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?

[RedDevil]

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.

[Xusi]

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

[RedDevil]

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.

[wickydude]

Originally Posted by Xusi

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?

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

[Xusi]

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?

[RedDevil]

Originally Posted by Xusi

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.

[Piwoslaw]

Originally Posted by Xusi

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?

"Inflation pressure does not affect grip": Autospeed article.