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Better Gas Milage

Mileage efficiency is based in part on engine speed. When driving normally on the highway, say 60-65 MPH, most cars run at about 2,600 RPM. Why not add another gear (Highway OverDrive) so most highway driving could be accomplished at say 1,500 RPM plus or minus? What would it save us in gas, and how much higher would CAFE be?

No, it would take more gas, not less, as your engine would have very low torque (power) at that speed. Not to mention it would not be at all safe. Try driving around town at 30 MPH in only your top gear and see how the engine responds. It would be even worse at twice that speed running the same RPMs due to air drag.

Gee, I’ll bet none of the engineers working for any of the world’s carmakers has ever thought of this! I suggest you patent this idea quickly before someone steals it.

The engine needed to provide that much torque wold be so large it would defeat the purpose. Small, efficient engines can’t provide that much torque at low engine speeds.

The 7.0L V8 in the Z06 Corvette has been know to exceed 25+ on the highway. An engine need not be small to be effcient.

Yeah, but I’ll bet city mileage SUCKS!

PS. Yup, 15 mpg city!

My Thunderbird with 3.08s and an AOD 4-speed. It’s turning about 2000 rpm at 70 in 4th. It also has a V8 with enough torque to keep the car moving down the road at a steady speed and have some power in reserve. Consequently, being a V8, it can’t do much better than about 26 or 7 mpg. (I realize there are more efficient V8s…)

When it was a V6, I had different final-drive gears (3.27:1) and that made it turn a slightly higher RPM, but it routinely pulled 30+mpg.

500+ HP and a combined 20 MPG overall. That is way more efficent than 130 HP and 35 MPG that the typical econbox achieves.

I am not an engineer so excuse my ignorance of the details you raised. My observations started when I noted that my car will creep forward while idling on level ground unless I put on the brakes. Why can’t it move itself forward on level highway in a higher gear at a lower RPM than is currently offered? When torque is needed, don’t all automatic transmissions shift down to a lower gear?

Isn’t this concept already in use and has been for mamy years?

My 4cy 88 Ranger was a example of when it didn’t work, that little truck couldn’t pull 5th gear up any kind of hill,and would not accelerate on the level in 5th gear.

Every car has a sweet spot for any given speed and road conditions, including hills, total loaded weight etc. Any final gear ratio is a compromise. Manufacturers today are generally trading off mileage and pep (acieration) when they choose that gearing. Modern engines generally have higher sweet spots than the old big V8’s of the 50’s.

If you choose to put in a taller gear, there is a good chance that you will end up with lower mileage and less pep.   Do it just right you may be able to increase mileage (this will vary depending on city vs highway) a little and loose a little a little pep.  

If you really want to increase mileage and do it without investing a thin dime. Search for hypermilling. A few slight changes in your driving habits (basically slower and avoiding using the brake when it does not impact safety) can make a real difference in your mileage. 

CAFE would not change.  If the auto maker could add significant mileage to CAFE by changing the gears, they would have done it years ago.

First off not ALL cars running at 60-65 are running at 2600 rpms. In fact my 4-runner at 60mph is about 1600 rpms.

Yes by adding another gear or a different rear-end and lowering the RPM’s there’s the potential to get better gas mileage. But it’s a fine line at having enough torque to maintain speed on highways and high winds and even a slight up grade at such low RPMs. The engineers HOPEFULLY have figured this out. There is the potential for the better gas mileage…but the car may NOT be drivable. And if the torque is too low you’ll actually end up using MORE gas.

You don’t need an extra gear to travel a 1,500 RPMs in top gear. You can do it right now with the car you have if you don’t mind going slower. After all, in normal ranges, velocity has a bigger effect on fuel economy than engine speed does.

What you seem to want is more speed with the same efficiency as slower speeds. You would have to give up on power, which you would need to climb a hill or pass another car. There is no free lunch. If the car was capable of that, you would not be happy about not being to accelerate when necessary. It would have very little power at highway speeds. You would have to downshift to go up a hill or pass someone. Downshifting uses a lot of gas. After a while, you would wish you had more torque so you didn’t have to downshift so much.

Most small efficient engines are low displacement engines. When they are shifted into higher gears, they need extra RPMs to maintain speed. So gear ratios end up being developed to balance fuel economy with other needs, like acceleration.

A great deal of the gas used on the highway is used simply moving the car through the air. The wind resistance. Pursuant to your earlier comment about the engine moving the car forward at idle, you have no meaningful wind resistance at 2 mph. You have an enormous amount at 65 mph. Your engine needs very little power to move the car forward from idle, but it needs to be at a good spot in its power curve to keep it moving at 65. It needs power at that speed.

Heck, you could move the car forward on level ground all by yourself!

Heck, you could move the car forward on level ground all by yourself!

…just like Fred Flintstone!

The fuel consumption is dependent on both engine load and RPM. An example of a load point that is too high for a given RPM would be ?lugging? the engine when letting up the clutch to the point where you here knocking in the engine. This means the engine combustion is far from optimal and therefore less efficient. The same principle applies at highway speeds.