Speed vs Fuel Efficiency

[Looking4Ward]

The Texas 130 toll road near me has a speed limit of 85. I'm pretty sure my MPG sucks whenever I use that road.

 

[Chuckanut]

My old Honda Accord had a 5th gear that was essentially an overdrive ratio. As a result I got better mileage at 65 than at 55mph.

I did a similar test with my current vehicle, a hybrid. 42mpg at 55mph. 46 mpg at 65mph. Wind resistance is real.

 

[Senator]

I think 2 hours time saved is worth $25

+1. Especially when driving to a vacation spot.

 

[calmloki]

I track indicated miles and actual fuel filled for each fillup. I can't drive 55. I notice about a 10% drop in fuel economy in states that pump 10% ethanol. Really love supporting the corn farmers that way, though I wish the corn was used as food instead. The other huge and frustrating/baffling difference is when driving at altitude. The difference between 4000' and sea level is 10-15%. Temperature doesn't seem to be the factor, neither speed, and I just can't imagine that air resistance makes that big a difference. I would expect the BMW engine management computer to compensate for altitude, which takes me back to air resistance.... 2000 BMW 528it

 

[2soon2tell]

I'm a part time dealer trade driver for a local Toyota dealer. On long drives I sometimes check the speed vs mileage data. On most of the cars I have driven the sweet spot is 55-65 mph. Any speed over 65 and the mileage falls off drastically.

 

[yakers]

I pay a lot of attention to MPG figures but often am willing to burn MPG to save time. One car is a Ford Focus Electric, I get very precise actual watts per mile figures for each trip. I am averaging 243 watts per mile for 13K miles, very efficient, works to 110 MPGE (miles per gallon equivalent for actual price of gas Vs what I pay in electric) (except when I plug into free charging sites then MPGE is, well you do the numbers ) This car loves going slower, best mileage is probably @ 45MPH. I tend not to go over 60 MPH when I can, sometimes I have to drive faster like in the car pool lanes or sometimes even in the slow lanes in Los Angeles, of course at times they are not moving at all. Above 60 MPH the efficiency drops way down despite this being a streamlined body design. I drive slower more because of the limited range of the car, 80 miles, than trying to max efficiency. The car can accelerate well when I need to, but is software limited to 85 MPH.
Then I have a Jeep diesel Liberty which has its own efficiency profile. I would probably get 29 or 30 MPG if I could drive it at a steady 55 MPH but it falls down to 22 in mostly city driving, 24MPH at 65 and down from there, maybe 22, when I go 70+ on the I-10 from LA to Phoenix. But it is efficient for towing, I have a small travel trailer which I get 20 MPG towing @ 55 to 60MPH.

 

[Walt34]

It doesn't seem safe to drive at too different of a speed from everyone else, plus it would drive me nuts.

You're right. It isn't the speeds so much, as the differences in speeds that create the volatile conditions that make for collisions. If everyone is driving 80 mph with only differences of 1 or 2 mph, that is actually safe.

It's the dorks that insist on going 55 or 95 that mess everything up.

 

[CoolChange]

I'm a part time dealer trade driver for a local Toyota dealer. On long drives I sometimes check the speed vs mileage data. On most of the cars I have driven the sweet spot is 55-65 mph. Any speed over 65 and the mileage falls off drastically.

This is way off topic and probably best suited for its own thread in Life after FIRE:

• Can you tell us a bit more about how you landed this gig and approximately how much they pay or otherwise compensate you?
• I have thought about doing something similar occasionally, maybe delivering/moving RV's. Others have mentioned something similar in the past.
• Any specific advice, warnings, etc. would definitely be appreciated.

I have a friend who does this occasionally for new jets. So much cooler than what I am considering.

 

[Koolau]

On the mainland, I drive a Buick Lesabre with the 3.8L V6. I always do the fueling and fill to the brim to keep the tank-full comparisons as close as possible. The car has an instantaneous fuel mileage gauge which can also monitor cumulative mileage per tank. I have found that the tank mileage is virtually always 8% high compared to my calculations. In steady state driving (relatively flat and constant speed) the mileage seems to get BETTER up to 70 or 75 MPH! This is confirmed by the mileage per tank gauge AND actual miles per gallons added at fill up. Back when my tachometer was sitll working, 75 turned out to be about 2300 rpm (seems quite low for that speed, but that's what it always read at that speed.) My guess is that the car was "tuned" to give great mileage at highways speeds. In town, I get closer to 20 or even less.

I usually get 31 MPG or better on a long trip at 65 to 75. AC does affect this somewhat. In fact, watching the instantaneous gauge for a half mile at, say, 38 mpg and turning on the AC, the instantaneous reading drops 4 mpg, virtually every time (all else being equal). To achieve the best mileage (with AC comfort) I turn the AC unit on and off as needed, using the recirculate function. If practical, I AC going down hill (even slight) and turn AC off going uphill (even slight). This practice seems to improve mileage by as much as 3 mpg overall compared to just leaving the AC on. Full disclosure, DW has full veto power over the AC game I play.

Not certain any of this is helpful to anyone. Still, it's kind of interesting that an old "boat" like the Buick can get 30+ mpg consistently with amazing comfort and reliability. As always (and especially on this thread, heh, heh) YMMV.

 

[harley]

I don't really care, as long as you slow people stay in the right hand lane.

 

[travelover]

Here is some actual data from CR. More fuel efficient vehicles suffered more loss in MPG at higher speeds than less efficient vehicles.

Tested: Speed vs fuel economy

 

[Car-Guy]

I don't really care, as long as you slow people stay in the right hand lane.

This probably belongs in the pet peeve thread but I agree.

 

[ERD50]

Here is some actual data from CR. More fuel efficient vehicles suffered more loss in MPG at higher speeds than less efficient vehicles.

Tested: Speed vs fuel economy

Might seem that way at a glance, but I plugged that into a spreadsheet, and...

While the highest mpg @ 55 mph (Honda Insight 1.3-liter 4-cyl. - 51.9 mpg) saw the largest % drop at 75 mph (70.33% compared to 55 mph), the second highest mpg @ 55 mph (Toyota Yaris 1.5-liter 4-cyl. - 42.5 mpg) saw the lowest % drop at 75 mph (80%[corrected] compared to 55 mph).

The others were in a tight range of 74% to 77% compared to 55 mph. (edit/add: - even though their 55 mph mpg ranged from 23.8 to 40.3)


I would kind of expect the best to see the most drop though. Those smaller engines are probably closer to maxed out at 55 mph, and the extra load at 75 mph would be lugging them more, and increasing fuel consumption more %-wise? Then again, maybe they are really hitting their peak efficiency curve (combo of torque & RPM)?

-ERD50

 

[HFWR]

On my 2014 scion , I have found the onboard mpg computer to be off about 15% actual mileage is worse than the computer says, when I tracked the real miles , and gallons used over 5 fill ups.

I've cross-checked "actual" mpg v. the "computer". They're reasonably close, plus I allow the pump to stop when it stops - i..e. no topping off - so that is probably a slight variable as well.

Like horseshoes and hand grenades...

 

[Midpack]

Here is some actual data from CR. More fuel efficient vehicles suffered more loss in MPG at higher speeds than less efficient vehicles.

Tested: Speed vs fuel economy

Of course they do. Had they included a Suburban it would probably have been "the best" falling from an (estimated) 20 mpg @ 55 mph to 15 mpg @ 75 mph - a drop of "only" 5 mpg! Percent is the logical way to compare. Note that the highest mpg cars are still the highest at higher mph, and the lowest remain the lowest - isn't that the relevant comparison?

 

[ERD50]

Of course they do. Had they included a Suburban it would probably have been "the best" falling from an (estimated) 20 mpg @ 55 mph to 15 mpg @ 75 mph - a drop of "only" 5 mpg! Percent is the logical way to compare. Note that the highest mpg cars are still the highest at higher mph, and the lowest remain the lowest - isn't that the relevant comparison?

I don't think either measurement is the most relevant.

Seems that most people are focusing on 'how much extra will it cost me to drive at 75 mph versus 55 mph (all else being equal)' - they can then weigh this extra cost versus the time they save.

So if I did my math right (converting mpg to gallons per 100 miles), the largest $ savings is the Mercury Mountaineer, which is in the middle of the pack as far as % delta, but since it is the lowest mpg in the bunch, this translates to the largest dollar savings per 100 miles.

(edit/add: ) Numbers - assuming $3.50/gallon, the Mountaineer will cost you $4.96 extra to travel 100 miles at 75 mph versus 55 mph. In the Yaris, it will cost you $2.06 extra to travel 100 miles at 75 mph versus 55 mph.

And a 100 mile trip will take 1.8181818182 hours at 55 mph, 1.3333333333 hours at 75 mph - a savings of 29.0909090909 minutes (approximately ).

-ERD50

 

[Midpack]

I don't think either measurement is the most relevant.

Seems that most people are focusing on 'how much extra will it cost me to drive at 75 mph versus 55 mph (all else being equal)' - they can then weigh this extra cost versus the time they save.

So if I did my math right (converting mpg to gallons per 100 miles), the largest $ savings is the Mercury Mountaineer, which is in the middle of the pack as far as % delta, but since it is the lowest mpg in the bunch, this translates to the largest dollar savings per 100 miles.

-ERD50

And a Suburban or a Humvee would be even "better" even though the total cost would be higher per mile.

"Most people?" I'd guess the person who buys the Honda Insight couldn't care less that he/she would spend incrementally more to drive 75 mph than someone with a Mercury Mountaineer. They're going to spend less on fuel than less efficient vehicles at the same speeds.

And someone with a Suburban doesn't consider MPG a priority. Their prerogative, some people truly need larger vehicles for a variety of reasons.

 

[ERD50]

And a Suburban or a Humvee would be even "better" even though the total cost would be higher per mile.

"Most people?" I'd guess the person who buys the Honda Insight couldn't care less that he/she would spend incrementally more to drive 75 mph than someone with a Mercury Mountaineer. They're going to spend less on fuel than less efficient vehicles at the same speeds.

And someone with a Suburban doesn't consider MPG a priority. Their prerogative, some people truly need larger vehicles for a variety of reasons.

As I said 'all else being equal' - I'm thinking along the lines of, I already own the car, what will my trade-off be on this particular trip, with this particular car that I own. Is it 'worth it' to me to spend $x.xx to get there xx minutes early? I believe that was the focus of the OP - can you optimize the trip for the car you own?

To use this information for a new car purchase ( a very different calculation ), I'd be comparing a total cost of ownership based on the weighting of the speeds I typically drive, and maybe even make the decision at that point to keep my highway speeds lower.

Fortunately for me, I don't drive enough to have to do a deep dive into those numbers. A vehicle with 'decent' mpg is good enough for me, I just found this to be an interesting puzzle. As you are fond of saying, YMMV.

-ERD50

 

[ERD50]

...
"Most people?" I'd guess the person who buys the Honda Insight couldn't care less that he/she would spend incrementally more to drive 75 mph than someone with a Mercury Mountaineer. ...

For a fuller explanation - I would say that the Honda Insight owner certainly could (should?) care what it cost them to drive 75 mph versus 55 mph.

I agree with you that they may not care what the Mercury Mountaineer driver's costs are. But why would they?

But even if they spend less on fuel overall, they probably are still concerned about saving what they reasonably can. I know I am - fuel cost isn't a big issue for me, yet I try to conserve. A $1 saved is a $1 saved. At the end of the year, it really makes no difference if that $1 saved was on a $50 purchase or a $100 purchase. Your balance sheet is $1 further in the black.

-ERD50

 

[aaronc879]

When considering the cost vs time saved with regard to speed, should we also consider wear and tear? Does going 75 vs 55 wear your tires faster or put more strain on the engine thus reducing it's lifespan? If so, that would need to be factored in when deciding if it's worth it to go faster or slower.

 

[InTheSticks]

When I drive the RV, I try to keep it under 60 so I get 8+ MPG. If I push it to 70, it drops into the 6's. The motorcycle gets 60 MPG and it's smiles per mile are off the chart.

 

[mpeirce]

Anybody here trying to optimize fuel efficiency on long distance driving trips?

Not really. I bought a diesel that gets excellent mpg (low to mid 40's on the interstates) even when I'm driving 75 or 80 mph.

I usually care more about making good time when it's a matter of getting from A to B (Ohio to Florida or Ohio to California or wherever).

I got the higher mileage variant of my car (E250 rather than the E350) so that I could spend lots of time driving and not feel guilting about spending too much money on fuel.

I love to drive.

 

[photoguy]

When considering the cost vs time saved with regard to speed, should we also consider wear and tear?

I think it should be considered but I'm not sure what the effect (if any) there is on vehicle health and tire longevity at the speeds we are considering.

The other issue I think about is fatality rate given vehicle speed. Kinetic energy is proportional to v^2 so a crash at 75mph has 33% more energy to dissipate than 65 mph. Of course there is Walt's point driving at a speed significantly different than the flow can increase your accident likelihood.

 

[Midpack]

Anybody here trying to optimize fuel efficiency on long distance driving trips?

Not really. I bought a diesel that gets excellent mpg (low to mid 40's on the interstates) even when I'm driving 75 or 80 mph.

I usually care more about making good time when it's a matter of getting from A to B (Ohio to Florida or Ohio to California or wherever).

I got the higher mileage variant of my car (E250 rather than the E350) so that I could spend lots of time driving and not feel guilting about spending too much money on fuel.

I love to drive.

Aside from what vehicles we choose to purchase, I doubt anyone tries to solely optimize fuel efficiency while driving. I'd guess we all consider the trade offs of driving time, MPG, speed limits (and other) to arrive at a speed that fits our own priorities. Some people probably drive as fast as they dare or are comfortable at (lowest time spent), some drive at posted speed limits or some fixed differential (ie, 5 mph over posted speed limits) and there are some hypermilers out there. To each his/her own.

 

[HFWR]

I readily admit that I paid a premium to drive a Prius. I'm a low-mileage driver, at about 30k right now, 34 months in. Of course, the Prius does save money on fuel, especially compared to the Silverado I traded in.

So, I haven't calculated it recently, but I could have bought a car for $5-10k less, with reduced mpg, and would likely not notice the difference that much on a daily/weekly basis.

The reasons I did buy a Prius: I assume fuel will generally be more expensive in the future, I plan to travel more during retirement, and add in the fact that DFW spends much of every summer at pollution level orange, so doing my small part to reduce localized pollution.