Optimal speed for fuel efficiency

[2011accent]

I am trying to figure out what the optimal speed for fuel efficiency is in my car without going through a lot of fill-ups.

This calculation seems complex because of:

1. Gear selected/Gear ratio
2. Wind resistence
3. Finding out the most fuel efficient engine rpm while the engine is under load.

I've seen some people state that the optimal speed is for fuel efficiency is the speed when the car is in the highest gear at the lowest rpm without lagging. I know what lagging is in a manual tranny, but I'm currently driving an automatic and I'm wondering if a car that is equipped with an automatic tranny could lag, or be in a state of too low rpm. My car runs in high gear down to about 35 mph at about 1,500rpms. What I'm thinking is that maybe the most fuel efficient rpm in high gear is higher. I think that the make and model of the car isn't necessarily relevant, but the engine type(a torqyengine vs. a horse power heavy engine/ a sports car engine vs a truck engine) could be.

On a side note, I wonder if the engine in my 2011 Accent is optimized for torq, horsepower or mixed.

[southerncannuck]

In my Fit my best economy is at the lowest speed that will maintain 5th gear (it's an auto), or about 34-35 mph.

[RedylC94]

With most automatics, you'll get the best steady-state fuel mileage at the lowest speed at which the torque converter remains locked up in the highest gear. If you are doing pulses instead of steady-state, things get a little more complicated.

[ALS]

If your looking for a sweet spot in almost all cases, that point is between 40-45 mph depending on gearing.

Motor Trend put together a constant speed / fuel economy graph for the cars they were testing this month.

[RedylC94]

Note that the Cruze in the above graph did not have an automatic tranmission, which probably explains why it didn't level off around 40 mph as the others did.

[abcdpeterson]

at steady speed 45mph turns in my best numbers.

[JohnM]

2011accent, you are correct about the optimum speed for manual transmission (low speed and lugging) as shown for the Cruze in the chart above.

Outside temperature is also a factor.

The manual Cruze (assuming in warm weather) shows about a 5 mpg improvement over the 7 year old Cobalt. See below:

[ItsNotAboutTheMoney]

Originally Posted by 2011accent

I am trying to figure out what the optimal speed for fuel efficiency is in my car without going through a lot of fill-ups.

This calculation seems complex because of:

1. Gear selected/Gear ratio
2. Wind resistence
3. Finding out the most fuel efficient engine rpm while the engine is under load.

I've seen some people state that the optimal speed is for fuel efficiency is the speed when the car is in the highest gear at the lowest rpm without lagging. I know what lagging is in a manual tranny, but I'm currently driving an automatic and I'm wondering if a car that is equipped with an automatic tranny could lag, or be in a state of too low rpm. My car runs in high gear down to about 35 mph at about 1,500rpms. What I'm thinking is that maybe the most fuel efficient rpm in high gear is higher. I think that the make and model of the car isn't necessarily relevant, but the engine type(a torqyengine vs. a horse power heavy engine/ a sports car engine vs a truck engine) could be.

On a side note, I wonder if the engine in my 2011 Accent is optimized for torq, horsepower or mixed.

An AT picks the gear so it will downshift instead of lag or stall.

As RedylC94 notes you want to be in top gear with the torque converter locked.

With the lock on it's goes into "stick mode" so you'll have rpm varying with speed instead of with the gas. So, if you vary your gas pedal pressure a bit and the rpm doesn't change you know you're locked. (I noticed this phenomenon before I knew what was happening).

Once you can identify when it's locked then you want to find at what speed it'll lock. Bring it up to a good speed on level ground, ease off the gas and you should see the rpm drop or jump to match the speed and ratio. Our Honda Civic locks readily at 45mph, for example but can lock a bit below that.

Once you know the lock speed you want to accelerate briskly up to that speed, get it into lock and hold it. In an automatic, Driving With Load is your friend. Maine being the Pine Trees On Hills State will limit you but you'll learn where you can DWL and where you can't. Lowest speed in lock is ideal, but in hilly areas you need a higher speed in the dips to give you room to lose some speed uphill.

[JonNC]

INATM,

I've found your post to be very true over the past year.
My Elantra (auto) can shift into 6th with lock up around 43-44 mph and hold that down to the high 30's (~1100 rpm) or so. I can stay in 6th down to about 35, but I'll lose lock up on anything but perfectly flat, or a decline (where I'm either "pulsing" or in "N" anyway depending on circumstances).

The OP needs to memorize your last paragraph, it's gold.
As long as you can get into a rhythm of "pulsing" (increasing) your speed down hill and then let off (loose speed) as you go uphill you should be able to pull some pretty good numbers (and use less fuel, which is the more important factor).

[shadescape]

my 09 Escape has the information center- it has MPG and when clicked on has the bar graph- pretty easy to understand where it gets the best fuel economy- would think most newer vehicles have something to this effect- course a Scan Gauge or Ultra Gauge would be better

besides actually testing, retesting, testing, & verifying- all things considered- it takes a person who is concerned with getting the best mpg over the long run

different driver, different concerns, could throw everything out the window