Best cruising speed for highest MPG.
 

I was curious to see if anyone has figured out the best HIGHWAY speed to set cruise control on for the highest MPG?

I think for cruising (in 6th) driving at the lowest speed possible without stalling. IIRC there was a study some years ago showing that, on average, cars were most efficient at around 45mph. It would be nice to know what the specific range is for the twins.

Would you believe in 1100 miles my car has yet to hit 65? It's been about 62 or 63 indicated. I never need to use a highway.

Anyway, I believe I've heard using Cruise Control can actually negatively impact MPG. Depends on a few variables. As far as the best speed overall on a highway, I'm going to guess 65 mph, but be willing to lose a little speed on inclines and pick up a little naturally on declines.

This is not a matter of opinion - it's basic physics.
 

It's physics, not opinion. You need to determine the engine speed that is most efficient for gas consumption. Then, find the speed at which the highest gear the engine is turning at that number of revolutions. Set the cruise control at that interval.

Of course this assumes a level road. Under changing elevation, you may have to downshift to maintain the correct engine speed without lugging.

My off-the-cuff guess is that optimum fuel consumption, without lugging, is probably around 1800 rpm, or roughly 45mph in 6th or thereabouts. This is obviously sub-optimal for normal driving.

If you drive above 45mph wind resistance increases at a much faster rate, so driving faster will increase fuel consumption regardless of rpms.

Maybe we should limit the scope of this to highway driving then?

Highway, the slowest speed you can maintain safely and smoothly. You don't want to be going too slow that hills drastically slow you down nor faster than necessary. Best MPG is going to be like Havoc and Frisson said, typically 35mph-45mph in OD but varies depending on conditions.

BTW, topics like this should go in Tech area.

Pick the lowest highway speed that you are comfortable and safe driving at, and adjust upwards if conditions warrant it to maintain the general highway flow. Use cruise control, but keep in mind that it will cause the car to consume extra gas to maintain speed on hills. If you have the opportunity, slow down on hill climbs and allow the car to speed up on descents - that can save fuel as well (note: do not coast or put the car in neutral). Using cruise control on relatively level and uncongested highways will usually boost efficiency.

You might want to dig up a test that Popular Mechanics performed measuring the most efficient way to accelerate a car to highway speed. They found that accelerating slowly is not the most efficient method. Instead, they recommend getting to your terminal speed at twice the maximum speed of the car. In other words, if your car goes from 0-60 in 7 seconds under maximum acceleration, you will get the most efficient performance by going 0-60 in 14 seconds. They found that the gains in getting to the highest gear quickly is more efficient than getting to it as slowly as possible.

Should be fairly easy to figure out. Just get on a flat stretch of freeway and set the cruise control at different speeds for a mile or so each, in increments of 5 mph, and see what the instantaneous mpg readout is. My guess out of the above choices is 60 mph.

Cruising has a time factor involved... does no good getting better MPG at a much slower speed if you are going to be on the road for possibly DOUBLE the time compared to a faster speed that isn't as efficient - but not 1/2 as inefficient. Stick to as fast as you can on interstate/long distance travel without excessively consuming fuel: it's a compromise/balance of time and fuel consumption.

On my Tacoma, cruise speed is ~3k rpm which works out to about 80mph...that gives 27mpg on 1,000+ mile interstate trips, and the Tacoma is an aerodynamic brick. I'm sure slower would be better for the actual fuel consumption rate. But the fuel consumption rate is only 1/2 the equation for cruising....you also have a time factor for the time spent on the road while consuming fuel; by going slower you are increasing the amount of time fuel is being consumed.

45mph average for 1000 miles is ~22.2 hours
75mph average for 1000 miles is ~13.3 hours

Is burning fuel for ~8.9 more hours really saving much more fuel? Only if the fuel consumption rate is at LEAST 1/2 of what it is at 75mph... and I don't see how the Gallon/hour rate would be less than half if you got say 28mpg cruising at 75-80mph and 35mpg at ~45mph. I'd much rather save 9 hours than a few gallons.

45mph on the interstate is not cruising, it's more akin to hyper-miling. Cruising is being somewhat efficient with the fuel by keeping the motor in it's most efficient torque rpm range while also being efficient with the trip time - in other words you are trying for as short a trip time as possible without burning through fuel at too high a rate.

Generally cruise speeds should be ~75-85mph but the speed is really determined by the torque rpm the motor likes the best for sustained higher speeds. All my vehicles have gotten EPA or better rated highway MPG at these speeds. I also don't use cruise control...cruise control doesn't work correctly for the ups and downs of overpasses and hilly sections. You should carry excessive speed into the base of the incline with gentle acceleration before hand and use the incline to slow the vehicle back down to crusing speed via engine braking and either maintain speed going down (if going to be flat for a while) or use the declining slope to build up the potential energy (if another incline is coming up). Only if on flat terrain I will use Cruise Control...but there's still traffic that needs to be negotiated which requires kicking off the Cruise control to keep from being shoehorned behind 18 wheelers and other "turtle racers" and such [now those are really annoying things that will really eat at your MPG's! Keep right people and don't take MINUTES to pass somebody while both of yall are going 10-20mph below the flow of traffic!]

Airplane "cruise" speeds are not at the stall end of their operational airspeed envelope but rather a whole lot closer to their Vmax speeds.

How does time figure into a miles per gallon/fuel efficiency discussion? You have miles driven and you have gallons of fuel burned. There is no time factor in that equation.

The issue of what your time is worth, especially on long trips, is another discussion entirely.

It depends on the drag coefficient of the car but it is probably somewhere around the 50 mph range. Once the car starts pushing air rather than sliding through it the amount of energy required to achieve each additional MPH grows exponentially.

An interesting automotive factoid that demonstrates this is that the Bugatti Veyron only requires 270 hp to reach 155 MPH but needs the other 730 HP to reach it's 253 MPH top speed.

1mph in 6th gear manual car :)

I dont know how accurate but when i was going 75 on the interstate my read out was showing 37 mpg as the average.

The minimum posted speed limit will get the best mpg, if you can do 45 in 6th it will be fabulous.