Sorry guys but you will never convince me that speed is a factor in gas mileage. I used to work on a Nascar Busch team and we would get about the same gas mileage at 170mph as 120mph. The reason being the engines, trannies and rear's were built completely different for each of those speeds.

You can do 100mph with 60hp just like you can with 800hp. The difference is how long it takes to get up to that speed. With a short enough final gear you would only need a small amount of HP to keep a car rolling at 100mph while crusing at 1000rpm.

So you guys can keep arguing about it but speed only matters when you talk about one individual vehicle. A slower speed may get better mileage for one application but may actually increase it in another, and that's the bottom line.

 

I don't really give a rats ass who you worked for. Test it for yourself, whatever way you can. I'm lucky I have a computer that spits out instant MPG. I notice a huge difference (1-1.5 mpg) between 60 mph and 75 mph.

You're pushing more air, more air requires more horsepower, and horsepower requires more fuel. It doesn't get any more basic then that.

If fuel mileage was better at higher speeds, then logic states it requires less horsepower to run at higher speeds. So you're telling me when you "back off", your vehicle speeds up? Please, get yourself out of the hole you've dug while you can!

It doesn't matter if you have 60 hp or 800 hp at 100 mph, what matters is what the engine is making at that time. If it takes all the juice of the 60 hp engine, then obviously at that time it's making 60 hp. The 800 hp will also be making 60 hp cruising at 100 mph.

You know... pressing on the throttle more creates more horsepower, less throttle = less horsepower.

 

also, slow that 170 mph car (I assume it's a restrictor plate) down to 120 mph, and suddenly it will get better fuel mileage.

The 120 mph car is losing a lot of mpg because first of all it has more horsepower, second it needs to bleed of speed going into the corner. This wastes momentum that you need to use fuel to make up.

You're comparing apples to oranges.

Take any one car, one car, and it will get better fuel mileage going slower.

 

I agree overall, but I have to wonder how gears and rears fall into this. They have to matter somehow. I can run 60 in top gear on flat ground, but if I hit any kind of hill, I have to drop one, and raise my RPMS by 500. I'm moving the same air either way, so it's more efficient to stay in top gear, but if I hover right at this breaking point, I wind up changing gears constantly, which will put more wear on my transmission than if I pick the gear it will run in at this speed (9th, direct) or increase my speed slightly to get away from this margin, and reduce shifting.

I also think there must be a coast factor. If I run 65, I can come up the hills in top (10th, overdrive) gear 70% of the time, then I let off at the top and with no fuel on the pedal, I think the engine computer considers this a zero fuel state, so I'm not burning anything on the descent. I kick in the Jake to hold my speed, and I don't think this burns any fuel either on a modern ECM-controlled engine, and then I get back into it hard in the bottom third, to try to come back up the next one at 65.

I think this must save fuel over running 60 in direct, and save tranny wear compared with running 60 in the highest possible gear in any given situation, but I don't have a computer in this truck, so I don't get realtime feedback.

I don't pay for the fuel anyway, and I don't really care that much. The O/O I drive for seems to be a lot more of a fuel guzzler than I am, with his big W900L that he lets idle all the time, so I'm not worried.

 

I'd need more proof than someone merely telling me on here that 30-35mph is the most efficient speed ina car. All of my trucks (suv's) never kicked into 4th until 40, turning a lower rpm.

"Well, I think its true, so it must be" mentality :wink:

 

Those on board computers aren't accurate, so go ahead and keep swearing by it. It's such common sense that I can't even dumb it down for you to explain that final drive ratio means more than speed and horsepower.

 

Buddy I'm the one "dumbing it down".

Anyone with half a brain knows the horsepower required to push air increases exponentially with speed.

changing diffs won't magically get you back a ton of fuel mileage unless the engine is wayyy out of it's range.

Also, you're comparing two completely different cars. And no, not just a "diff change" either.

And yes, the onboard computers are fairly accurate. They know how much fuel your injecting, and they know how fast you're going.

They might be off(mines about 2-3% off) but they're off by the same amount all the time. They don't magically change.

You're going completely against the grain of what is common knowledge in our business. There's a reason big companies run a bit slower then the average joe. Do you think they're doing this for the hell of it?

Until you offer some proof, you're just arguing for the sake of arguing. There's tons of white papers, even aerdynamic wind tunnel testing showing the gains of running slower.

 

Get over yourself allan, running slower does not always mean better mpg and that is all there is to it. You are not always right, and it is obvious you just like to argue. Have a nice day and be sure to stay out of the middle lane while you "saving fuel".

 

If I'm not right about this, counter my points, and refrain from ad hom attacks.

 

I have countered your weak points already there is nothing more that I need to counter. And if you think I'm attacking you, then you are definitely in the wrong profession.

You have offered nothing but your opinion and a reference to an article that you read somewhere and that is all. If going slower saves you fuel in your vehicles that is fine and dandy. But that is not always the case, just like in my 97' ford dually power stroke, i got better fuel mileage at 72 than I did from 60 up to 72. So hmmmm looks like you were right.

As I said get over yourself, you do not know everything. I know it's hard to believe being a trucker and everything but the sooner you realize that the better it will be for everyone.