Gas mileage

KCRam · June 1999

probably true, Shift.

Diesels enjoy running steady rpms all day. And at 1600 rpm, I was at my torque peak (420 lb-ft), so I was getting maximum "twist" the whole time - no need for shifting, even on mild hills.

Generally speaking, any engine will get its best economy just above its torque peak, hut it's up to the manufacturer to select transmission and axle ratios that translate that rpm to a usable speed.

Just as a comparison, I got 24.3 as i explained at a ste speed of around 57. If my average speed is 67, my mpg drops to 19.5-20.

akjbmw · June 1999

I agree with the cruise control having a possible negative effect on mpg. When I drove my ol 2002 in the days of lines at the gas pumps, I made it a habit of NOT pushing harder on the pedal to maintain speed on a hill. I let the speed decline and then downshifted if necessary. On the downhill side, I have to admit that I let the speed creep above the normal speed I had been traveling. This had a significant effect on the little four banger and single barrel carburetors efficiency.

jgrundman · June 1999

I don't recall where I heard this but regarding getting better gas mileage when city and highway are quoted as 17 / 27, the best way to improve your mileage is to stay out of the city...

jshu · June 1999

Just got a new Caddy with the 300hp northstar V8 and I'm kinda shocked at the mileage I'm getting.

With the old 4.9l V8 which was rated at 16/25 I usually get 19/28. The new northstar is rated at 17/26 but I'm hardly getting 19mpg on the highway! and 14 to 15mpg in the city if I'm lucky.

I don't have a lead foot, infact the engine hardly revs past 2500rpm and usually cruises around 1500.

Is there something wrong with my engine or is this normal for the northstars?

thanks!

civilized · June 1999

It is true that keeping 55 mph will be a good idea to increase the gas mileage. I have 99 Civic EX sedan and have got upto 30mph mileage. Considering that I have been driving in the city and car is pretty new, it is not bad.
But I have been noticing that around 50 mph the tachometer (engine speed) is pretty low (below 2000 rpm) and as I go near 65 the rpm also increases in almost the same proportion as speed. SO I find tachometer touching 3000 at 67mph. I have been trying to drive at 55 mph (and it is very difficult unless everybody in your lane does so) just to see how much the mileage difference it is going to make (I hope a lot).
Has anybody experimented with tachometer ?

guitarzan · June 1999

Chadz, unfortunately, I know some theory, but would be very poor at detailing exactly how the vacuum is caused in the engine.

Civilized, 55 very well might be the best cruising speed, I'm not sure. However, a bigger point is to remain at the same speed, no matter what speed you are going, for the best efficiency. Acceleration is what uses most of you gas. If you take freeway trips at a steady 55 vs. 65, I'm not sure you'll notice any difference. Just try to keep it steady.

ggosselin · June 1999

Guitarzan-
If you don't know how an engine produces vacum, you don't know too much. Put simply, an engine is nothing more that an air pump. Vacum is created because you have pistons trying to pull in the air or air/fuel mixture into the cylinders but there is a restriction -the throttle plate(s). When the engine is at idle, the plate(s) are fully closed, restricting the amount of air that is admitted into the intake manifold. TaDa- thus producing manifold vacum that is used to power many accessories. Even when the plate(s) are partly or fully open,(gas pedal is to the floor) there is some restriction due to the fact that the engine is trying to pull many hundreds of cfms of air through an opening that only allows a few hundred cfm at at time. When you downshift- the term "engine braking" comes into play because you suddenly shifted to a lower gear and increased engine speed- the engine makes so much vacum in the manifold that this itself slows the car down. Assuming your foot has let off the gas pedal, this will be the case. In a diesel, however, this is not the case. Diesels have no throttle plates to control the air volume entering the engine, instead they change engine speed by the amount of fuel that is or is not injected into the cylinders. Because of this, they have very little vacum naturally created by the engine. In most larger trucks, they have seperate vacum pumps mounted to the engines to control things such as a/c, emis. control devices, etc. This is why they produce such a large amount of torque vs. horsepower.
Sorry, my mechanical engineering degree took over- sometimes I feel the need to explain things like this to people with questions like the ones here.

guitarzan · June 1999

The engine slows down due to vacuum? I thought the engine slowed due to the force of compression.

I said I'd have trouble detailing it, not that I don't know! And my trouble stems from when I posted that, if Edmunds is right, at 3am. I just couldn't figure out how to say it at that time. Thanks

ggosselin · June 1999

Guitman- sorry I came off a litle harsh, sometimes, I get carried away. Technically the engine does slow down due to compression- in this case, the lack of it. Like this- the pistons are trying to suck in their normal amount of air at that engine speed, which happens to be in the upper (off idle) rpm pange. But one thing slows down the air from making its way to the cylinders- that pesky throttle plate. Say for example, your car weighs 2k Lbs. and you suddenly shift into 2nd gear @ 55mph, the momentum of the car is what speeds the engine up, but the engine can't take in all of the air needed to match that rpm, due to the throttle plate(s) being colsed. Massive amount of vacum is produced due to this. The engine acts like a brake and slows the car down because it is having a hard time taking in air from a blocked off hole.
-Later

wrw2 · June 1999

We have a Mercedes C230 Kompressor which has a 5 speed automatic (185 hp, supercharger). Should we manually shift to maximize fuel economy or leave the transmission on "D"? We have to climb a lengthy inclining hill everyday on our way to work. When we leave the transmission in "D", I end up pushing the accelerator substantially to get adequate acceleration. Would it be wiser from a fuel savings standpoint to manually shift?

Any help would be appreciated.

guitarzan · June 1999

Probably. If you are pressing the gas far down, you very well might save fuel by downshifting. If the car moves out much easier once shifted, you know you did the right thing. I've never owned one, but have driven several Mercedes, and they seem to only kick-down after putting the accelerator past the floorboard. "Let the gear do the work for you." This can save the engine from strain also if it is not shifting at a point it really should.

radracer · May 2001

A good way to get good fuel economy is to not fill up your gas tank all the way. I always drive with about 10 gallons instead of 18.5. I also have my tire pressure set 1-3psi above normal. Staying around 45mph gives you the best gas mileage. If you have a car that the peak torque at a low rpm like 2200 or 3000, driving at that rpm will also give you great gas mileage, but I wouldn't reccommend it if the peak is over 4000. It's also a good idea to drive 55 on the highway, as going 65 consumes 13% more fuel. Let's face it, we're back in the 80s now!

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