How to improve gas mileage in this situation?

I know you quoted ALS, but I can't help thinking this is in response to my comment about being a road hazard. If it is, you must have missed the part where I referenced MY route from Phoenix to Savannah and in particular, the 80 mph speed limit through Texas where no one, including semi's, goes anywhere near 65. I have noticed that many truckers hover closer to 70 than 75 across Arizona and New Mexico on both I-10 and I-40, but when it comes to Texas, they seem to throw out the mileage rule book.

 

Actually crossing into Georgia, everyone speeds up. <GRINS>

My first Prius fill-up was from Fort Worth to Shreveport, my first extended highway drive, and I got 39 MPG! I briefly thought about turning around and taking the car back. Instead, I conducted an experiment of driving ~2 hours at fixed speeds on cruise control and using pump calculated MPG. I soon learned:

• 60 mph -> 53 MPG
• 65 mph -> 52 MPG
• 70 mph -> 49 MPG
• 75 mph -> 39 MPG

The last one so surprised me so I repeated it on the Birmingham to Huntsville leg and confirmed it.

Since then, I've done multiple long distance trips via Alabama, Arkansas, Oklahoma, Kansas, Missouri, Tennessee, North Carolina, South Carolina, Virginia, and Georgia. What I've found is there are some moving van companies that appear to have a policy of 65 mph. The others vary depending upon local enforcement. I figure moving companies don't like to pay for breakage.

Of course the second best is a pickup truck with loaded hay trailer. I prefer following alfalfa. Livestock, not so good.

Bob Wilson

 

Followed a few of both in my time, too much debris with the hay trailers for sure.

I get upper-40's at 70 and mid-40's at 75. The only time it really drops off is at 80 where it dives as low as mid-30s if I'm pushing against a headwind. The section that still befuddles me is Amarillo TX to Moriarty NM, though it is a constant 3,000' climb. It does go into the 40's if I bypass Moriarty and wait to refuel in Albuquerque NM. Sadly, I haven't gone in the opposite direction with the Prius yet to see the difference. In late January I'm going to have an opportunity to repeat part of a trip from July and I'm going to try to stop at the same 5 stations to refuel.

 

When you get a chance and it is safe, take a photo of the MFD display after:

• reach a target high speed on level ground, 60-80 F, little wind
  • prefer cruise control but not a stickler
• reset a trip meter
• after 10 miles, photo (or record) MPG and speed
  • in ideal world, some speed, same section, reverse direction

Here is an example:


I've got this chart but the high speed end only has my test results:

I would like to cover more points at higher speeds that due to traffic and road conditions are difficult to cover. Posted at 75 mph, we have few flats, actually only one I-65 section, near by.

Bob Wilson

 

There is a section of I-10 about 15 miles from here where the speed limit increases to 75 with an exit about 12 miles further west. I guess I could take some time one day to run out there and do 3 r/t passes at 65, 70 and 75 just to see the actual difference in a somewhat controlled environment. I only use Trip B when we're on a trip anyway, so resetting it multiple times won't mess up my data recording.

However, since I'm not the one striving for max mpg here, it still won't get me to go slower than the speed limit, even through Texas. We're getting almost double the mileage we got in our van, so even when we get relatively poor mileage, it's better than it was.

I must admit though that around town, I'm finding it quite a bit of fun to see just how good I can do by altering some driving habits (while still not going under the speed limit). I get up to speed more slowly, don't speed and don't use cruise control (so I can pulse/glide some, I guess that's what it is). Anyway, I've always been more concerned with mileage around town because there are so many variables and other factors to consider on our trips, mostly time, and it doesn't take a test for me to believe slower speeds results in higher mileage.

 

I usually do three passes:

1. Look for road hazards (like the ones with lights on top) and traffic.
2. First pass which includes wind and elevation changes
3. Second pass which reverses wind and elevation changes

Then I average the two runs and that is my data point. In my case, we have tires right-sized calibrated against mile-markers and GPS.

Bob Wilson

 

I know our speedometer doesn't always agree with our Garmin GPS as far as speed goes, has varied up to 2 mph at times, but I never though to match the odometer to the Garmin or highway mile-markers, mostly because I don't now what I'd do if they don't match. I've always treated mileage data as just a point of reference and not an accurate absolute. Since Toyota uses so many different OEM tires, I don't know if they all match in circumference, etc., or how much any variance might affect mileage calculations.

 

Get the revolutions per mile for your current tires. Then go to Tire Rack and use the tire specifications for revolutions per mile. Then find one whose revs per mile will bring the mph and odometer back to zero error. Let me share an example:

• 2 mph over at 65 mph
• 65 mph / 67 mph ~= 97% error
• Need a tire that turns slower by this amount

So going to Tire Rack, looking at 15", Yokohama tires because they support 51 psi:

	size	max load	max psi	revs per mile	% revs
1	P195/65R15	1279 lbs	51 psi	831	100%
2	P205/65R15	1400 lbs	51 psi	818	98.4%
3	P205/70R15	1499 lbs	51 psi	787	94.7%
4	P215/60R15	1411 lbs	51 psi	827	99.5%
5	P215/65R15	1510 lbs	51 psi	801	96.4%

The candidate tires closest to 97% error are:

• P205/65R15 - 21 lbs (my first choice, 17.991 kg/tire)
  
• P215/65R15 - 22 lbs (second choice, 18.830 kg/tire)

Bob Wilson

 

Though it is not apples to apples we did that section of I-40 on our cross country trip last year. Overall MPG for the trip was 51, and on that section we managed mid-upper 40s. I used cruise but would switch it off on the hills if it pushed out of ECO zone, 2nd avoided unnecessary accelerations, 3rd made sure to only use regen brake. Drafted if it made sense. Also Michelin Energy Savers helped. ECO was always on, and a/c set at 82 with air recirculation, a/c makes big difference.