We recently finished a 4000 mi trip in our '03 HCH CVT, from Reno to Minneapolis and back. Having read some of the tips offered in these forums, I tried to improve over my usual FE by
1) inflating the tires to 45psi,
2) changing to Mobil 1 oil (5W20 - I couldn't immediately find 0W20), and
3) keeping my speed to 65mph maximum (except downhill).

The results were good - overall trip was 54.1 mpg vs 47.3 lmpg at the start of the trip.

Some observations: The higher tire pressure really works! I can coast forever on a windless level road. However my wife and I both suffered from the harsher ride - we were traveling up to 500+ miles per day and it felt like 1000. She is a real trooper - not much complaint even about minimal A/C, although it was clear she would have preferred to be in her E-class Benz.

They sell "unleaded plus" back in the corn country which contains 10% ethanol, but I think it is a bad deal. I bought two tanks of it, thinking 'higher octane, lower price - good deal', but looking at the record we got 1143 miles on 23.33 gal of the stuff (49.0 mpg) whereas we got 3000 mi on 53.24 gal of normal 87 octane unleaded (56.3 mpg).

We ran into fierce headwinds twice and it was devastating to watch my trip B average just drop and drop. I really didn't want to slow to 30-40mph on the interstate and I'm not sure it would have helped much.

I got the impression that elevations above 6000 ft are tough on FE, but central Wyoming (which is high) is also where we saw headwinds. Does anyone have data on FE vs elevation?

I set a personal tank record of 757 mi and maybe could have stretched it to 800 but we were in the middle of N. Dakota on the interstate with gas stations pretty sparse.

The best day's run was the last day, from Salt Lake to Reno. 520 mi with only a couple hundred feet rise (but several 2000 ft hills in the middle) and no wind, 62.8 mpg indicated on trip B which equates to about 59 mpg by hand calc.

 

Hi Solecondad:

___Excellent post in regards to your trips particulars and your FE observations. I cannot comment on the other questions in good detail just yet but here are a few possible answers? Our E10 blend here in the Midwest does cause a FE hit unfortunately The mountain drivers receive a nice boost in FE and I am not sure it has to do with thinner air or if it has to do with the combustion process possibly being a bit rich or overcompensated to the lean side by the ECU afterwards? Those that drive the mountains are usually hurt with the continuous ascents and do not completely recover on the descents so overall it is probably only a boon for a few stretches or segments only? I know Billy6 and Troy run the mountain states and receive excellent FE in the process so who knows? A Prius II owner I am now friends with out of W.VA. runs the foothills and receives some stupendous FE in his as well although I doubt the slightly lower overall air pressure in W.VA. has anything to do with his vs. Billy’s and Troy’s.

___Anyway, nice stats and I hope you enjoyed the trip. Or at least recovered from it Tell your wife she deserves huge kudo’s. Mine doesn’t mind 36 - 40#’s in the X-Terrains in the MDX (max sidewall just 35 #’s) but she will not forgo A/C at anything above 74 degrees in the day when it’s sunny and 78 at night. It is a small price to pay for marital bliss as I am sure you have discovered

___Good Luck and thanks for the post.

___Wayne R. Gerdes
___Waynegerdes@earthlink.net

 

What a great post, full of useful information for all of us inquisitive and analytical folks to digest. A couple questions:

- were the speed limits on the interstates mostly 70 or 75? and what was your experience in terms of being passed, and passing, other vehicles?

- what PSI do you normally run your tires at? I'm wondering if you noticed a difference from the recommended 32 to 45psi, or whether you noticed a big difference from 38 (where I have mine) to 45psi.

Thanks.

 

were the speed limits on the interstates mostly 70 or 75? and what was your experience in terms of being passed, and passing, other vehicles?

Yes, generally 75. I was passed by most other vehicles on the road, including a couple of Insights and a couple of Prius - all going about 75. Got past an occasional truck.

what PSI do you normally run your tires at? I'm wondering if you noticed a difference from the recommended 32 to 45psi, or whether you noticed a big difference from 38 (where I have mine) to 45psi.

Ah, sad to say my tires were at 25-30 psi. I just hadn't been paying attention for the last year or so.

 

Fuel efficiency increases with elevation. The two reasons are intake manifold inefficiency and aerodynamic drag. Both improve the thinner the air is. If you've seen the Honda R18 video you've seen the demonstration of improving fuel economy by holding the throttlebody wide open and switching to an atkinson cycle cam profile from the standard otto cycle cam profile. Anyway, the bottom line is that creating vacuum is very inefficient and the thinner the air is, the less vacuum the engine has to create for any given airflow rate.

The second of course is the fact that aerodynamic drag is a function of the mass of the air you're going through. At higher elevation, the air is thinner, and thus there's less mass, and it takes less energy to maintain a given airspeed. Er, groundspeed in this case. If you scroll down a bit on this linked page you'll see a table of air density from sea level to a million feet. We'll pick out 6000' since that was your example. At 6000', the ambient air density is only 11.78psi. That's just 80% of sea level's density. As such, you could cruise at 65mph and only experience the aero drag of doing 58mph. The rolling resistance and mechanical friction is still all the same as going 65mph since those are a function of tire and engine speed, which are the same regardless of air density, but at 65mph, aero drag is the majority of drag anyway. Figure that total loss is about equal to 60mph at sea level.

Now if we go through the calculations, if you normally would get 55 miles per gallon at 60mph at sea level, then that same fuel consumption rate is all that's needed to create the power to go 65mph at 6000' elevation. 55mpg @ 60mph = 1.091 gallons/hour, so 65 / 1.091 = 59.6 miles per gallon. That's an increase of 4.6mpg, of 8.3% better FE. We can see from this that obviously even though there's 20% less air, we did not get 20% better FE, but it's still a nice boost. The only thing working against you is that most high country has a lot of hills, and hills generally decrease efficiency because you can't take them in top gear. Any time you drop out of top gear your mechanical drag goes up. At low speed, mechanical drag in a low gear is much higher than aero drag even. Ideal would be hills gentle enough that you can stay in 5th gear, or that the CVT can stay around it's top ratio.