Mileage vs Speed

[jwrezz]

We had to have our FD ladder truck serviced and I was the ride back for the driver of the rig. Wind was negligible and elevation change wasn't much. Ride from Parsippany NJ to South Plainfield NJ. Following the rig that never got over 56, but was frequently under 55, I got 4.1 mi/kwh. Ride home doing 65 I got 2.7. I gotta look at maps for elevation cuz that seems an amazing difference just from a 10mph wind resistance increase!

Oh and I saw some cool rides at their shop!

Sponsored

 

[jwrezz]

Ok elevation change was about 350ft. So I went down while traveling at <55 and up headed back. I still think I'm gonna try to keep it at or below 55 as much as possible. Those mornings when late for work can't be helped tho!

 

[Mach Turtle]

Thanks for the interesting data! Now the really nerdy thing to do would be to ask, does this match the theory?

The energy to climb 350 feet in a 7500-ish pound Lariat ER is close to 1 kWh. Since you seem to have used about 9 kWh driving back and more going there, that could make about a 10% difference... each way.

Theoretically, aerodynamic drag goes with the square of the speed, and (65/55)^2 = 1.4, so that's a big difference. If there was a headwind or tailwind, that makes a lot of difference too, and if you were following the other truck very closely (how well does the cabin air filter take care of diesel fumes?), you'd get a further drag reduction.

So these numbers are impressive but should not be surprising. What is surprising to me is that you got 2.7 mi/kWh while doing 65! That seems really good for such a speed.

 

[davehu]

this works for ICE vehicles too, of course. A decade ago traveling from Dallas to Denver I got about 18mph in my Chevy Avalanche. of course it's flat with a slight headwind. After wondering around the mountains for 3-4 days we cleared the mountains and from Denver to Dallas got 25mph! I was elated until getting home and realizing what made the difference for me was the 4280 elevation change!

 

[jwrezz]

Thanks for the interesting data! Now the really nerdy thing to do would be to ask, does this match the theory?

The energy to climb 350 feet in a 7500-ish pound Lariat ER is close to 1 kWh. Since you seem to have used about 9 kWh driving back and more going there, that could make about a 10% difference... each way.

Theoretically, aerodynamic drag goes with the square of the speed, and (65/55)^2 = 1.4, so that's a big difference. If there was a headwind or tailwind, that makes a lot of difference too, and if you were following the other truck very closely (how well does the cabin air filter take care of diesel fumes?), you'd get a further drag reduction.

So these numbers are impressive but should not be surprising. What is surprising to me is that you got 2.7 mi/kWh while doing 65! That seems really good for such a speed.

The most I did was 65. There was some off-interstate driving and traffic.

 

[RickLightning]

Yeah, people don't get it. That's a 35% savings. Even if your electric bill for the truck is only $1000 a year, that's $350 before taxes...

And, some get it and don't care.

During the previous fuel crisis in 2008, I drove to work at 65 and got there like 3 minutes later. I forget the savings, but it was like 15% or something. Like $10 a day. $50 a week...

My son now has a commute of 43 miles each way. His car gets like 23mpg. I explained the math, and the money, and he gets it. And doesn't do it, because he just can't...

Lets' assume his 23 becomes 26.

86 / 23 x $4.10 = $15.33 per day
86 / 26 * $4.10 = $13.56 per day

He commutes 4 days a week, 50 weeks a year, that's 200.

200 x $15.33 - $13.56 = $354 a year, after tax. Assuming a combined 30% tax rate, that's $505.71 a year. Put that into your retirement account for the next 20 or 30 years...

That said, I drive trips at 73mph. Yes, I could save by going slower, but I have the money and I enjoy driving, passing, accelerating...

 

[Adventureboy]

Wind power is exponential, not linear. The difference between 55mph and 65mph is 65% additional power required to overcome the resistance. By the math, your 4.1 mi/kwh translates to 2.5 mi/kwh so your 2.7 mi/kwh isn't bad.

 

[Mal106]

Somewhere I saw a speedometer marked in the time it takes to go 10 miles. Obviously 10 mph it takes an hour. 20 it takes 30 min, a big change. At 60 it takes 10 minutes, at 70 it takes a bit over 8 and a half min a change of less than 1 and a half minutes; not much of a change at all. That makes the dollar value of slowing down seem a bit less of a sacrifice in arrival time.

 

[RickLightning]

Somewhere I saw a speedometer marked in the time it takes to go 10 miles. Obviously 10 mph it takes an hour. 20 it takes 30 min, a big change. At 60 it takes 10 minutes, at 70 it takes a bit over 8 and a half min a change of less than 1 and a half minutes; not much of a change at all. That makes the dollar value of slowing down seem a bit less of a sacrifice in arrival time.

Except...

Let's say I'm driving 1,000 miles. Going 60 takes 16.7 hours. Going 70 takes 14.3 hours. Saving 2.4 hours is real time.

 

[kuzzin]

In my experience (48k miles in 31 months) it is usually about wind resistance. The photo was from this morning when I had been running 60-80 mph but hit San Antonio traffic 20-50 mph with lots of big rigs moving wind out of the way. Average speed almost 47 mph including time parked with AC blowing cold.

Sponsored