DDMan

I know the best way to save gas is to make sure the car is tuned up and not do harsh stops and starts, but I'm curious about the best speeds to cruise at?

I prefer to drive 80+ on the freeway, but at 85 I am right up to opening up the secondary (much larger) butterfly valve on the TB and that has to kill gas mileage (though will definately get to my destination quicker). Also, while driving around town should I stick it in 4th (better torque) or 5th (lower revs)? If I put it in 5th and have to accelerate I really have to push on the pedal to get the car to move.


Thanks,
Dan

P.S. This is for a 1988 Audi 90Q, but in reality applies to almost all cars here.

90/90Qs

I *believe* most cars are tuned for nominal economy at 55mph

DDMan

Thats so crazy. Here in Sac there is only 1 freeway with a 55mph speed limit, and everyone still does 70!

At 55mph would take FOREVER to get somewhere, espeically long distances (not to mention can be dangerous on the freeway). My friend and I went from Sacramento - Oceanside (south of LA) and back in 13 hours...did close to 90 the whole time in his I-6 4L Jeep and still got 16mpg.

ThetaTau87

On a long highway drive the slower you drive the better milage you will get. This is because the relationship between wind resistance and the power required to overcome it is a square. This means to go 2 times as fast it requires 4 times the power (2 squared) and to go 3x faster it takes 9x the power (3 squared) etc...

For everyday driving the best technique is to accelerate quickly up to your cruising speed then shift directly into your highest gear to cruise. If you are done accelerating in 3rd gear shift direclty to 5th and skip 4th. Cars burn the most fuel when accelerating so if you accelerate in the shortest amount of time (w/o going WOT) you will save the most fuel. Once you are cruising avoid ever having to use your brakes. This wastes your momentum that you gained by burning fuel to accelerate and tosses it away as heat and brake dust. Keep a nice distance to other traffic so you can stay at a constant speed and coast when ever possible. As soon as you see a light ahead turn yellow put the car in neutral and coast. Coast on the exit ramp from the freeway or up to any stop sign. Driving like this improved the mileage in my CQ from 19 up to 24 mpg! The best part is that you still get to accelerate briskly to get up to cruising speed. The trick is to avoid unecessary accleration while cruising due to following other drivers too close or trying to drive faster than everyone else.

520

The reason is you think you are doing 90mph "all the time" but your actual avg. speed is much much less as you have to slow down and wait until others move out of way or what have you.

If everybody drove the speed limit, followed the rules, paid attention to road ahead and not accident on the other side of the highway, switched lanes well in advance, signaled properly everybody would be getting much better gas mileage and would be getting places faster.

Ever noticed that if you go the speed limit between the lights you actually "catch" the green again, instead of speeding up to it, having to stop and then take off again - this ties to what thetatu87 was talking about.

When in rush hour traffic, leave about 3 car lenghts in front of you, put it in 1st gear and go real slow - if traffic in front of you speeds up - do not follow but keep your pace, you can't beat anybody anyways and you'll catch up to them again - I know many may think this kind of driving is annoying but I don't care...I still get to the next light/intersection at same time as they do but without having to speed up, brake, shift gears, etc....and I'm sure I save some gas, though that's not my goal, I don't mind it either.

Good way to save up some gas is slightly overinflate your tires. it'll be harsher ride but more firm and your car may even handle better.

DDMan

And today I started doing it. Wonder if it will make a diff.

And I guess I wonder about the freeway speed because at say 55, you are only at 2000rpm (hypothetical) and only have 90lb/ft torque (which is barely enough to overcome wind resistance at that speed), but at 70, you are at 3000rpm, and have 150lb/ft available (which is barely enough to overcome wind resistance at that speed too because the engine is more efficient because its closer to the power band).

So at both those speeds, even though the air resistance is more, there is more power available to overcome it without pressing harder on the gas.

I guess it all comes down to how the engine's torque curve is set up. If its flat, the faster = more gas is true, but if it has a peaky tq. curve, you might be able to go faster, but once you reach that peak the gas mileage will plumitt because your losing torque as you speed up.

If this logic is wrong tell me....maybe I'm under estimating the power of air resistance. So I guess a better question is how are our engines set up?

DDMan

I also have oversized tires (205/60/14 instead of 195/60/14).

And actually we AVERAGED 85mph the whole way. I didnt want to get the ticket so when I drove I did 80-85. Mike did 95-100 when he drove.

Oh, and I totally agree about the "if everybody...." stuff, but you can't change millions of people's driving habits. I normally do the slow go in rush hour and it is nice, but everyone is also a jerk and cuts into the open space in front of you. Pisses me off, but oh well. I normally dont drive in rush hour/stop-n-go traffic so I avoid that.

ThetaTau87

What you are saying about the engine being most effiecient at the torque peak is true. That is the point where the engine is most effiecient at converting fuel into energy. However, the power required to cruise at a constant speed is very low in comparison to peak power of the engine. At this low power consumption level the engine speed is more of a factor on fuel economy than at what RPM the engine is most efficient. Spinning the engine at 3000 RPM vs. 2000 RPM will burn far more fuel due to the higher engine speed than you will save by having the closer to its peak torque.

When accelerating it is very important to be close to the torque peak, because then you are using a significant amount of the power output of the engine. When you are cruising you want the engine at the lowest RPM possible.

CvTech

you are so wise.

520

right.....got my tools in da trunk? I need 'em