Which effected my MPG more: elevation or weight of passengers?

[prr]

I took a quick freeway MPG for my Highlander Hybrid 2006 yesterday. I gassed up, then we went out of town, and came back. This morning, I gassed up and was disturbed to see my mileage on the 111 mile two-way trip was only 22.5 MPG. When I did an MPG test earlier this year, I got 26 for pure freeway. I don't know how I got so low--I was driving 60-65 on the freeway, which was most of the trip.

Looking back, I saw two factors that might have contributed, although I'm not sure at all that either one did--which is why I'm posting here.

(1) There were more people in the car. I am estimating about 500 pounds more of people than there has been in the past (our whole family went). EDIT: Actually I went back and checked, and I got a long out of town MPG at 26, with almost as much weight as we had in the car last night, so I no longer think weight was a factor at all.

(2) it was not a flat grade. We drove from our town some 15 miles from an elevation of 3000 to 3800 feet, then went down a steep grade (6-8%) for 20 miles or so, and our final destination was at 1200 feet. Then, we drove back home the same path. Now I was thinking that the uphill drive shouldn't have hurt our MPG, as we coasted down that same stretch going there, but could we be hurt more by an uphill trip on the same path, than helped by a downhill trip down?

Its hard for me to see how either of these factors could have brought the MPG down so much. Even our pure city MPG is 24, and our combined is 25. Granted, most of those other mileage checks are with only myself in the vehicle. I've been gassing up at the same station for most if not all of the fillups the last few months, including the previous 3 fillups, so I don't think there was a discrepancy about which station said I was full at this level vs. another station saying I'm full at that level of gas in the tank. Of course I double checked my math, and my numbers are right.

Normally, my freeway MPG checks are longer, most if not all of a tank. I just limited it to this 111 mile trip because I've only owned this car for a few months, and this is only the second pure freeway MPG that I"ve taken, so I had to make it shorter.

Of course, I'll want to get another MPG estimate for freeway sometime soon, but I don't know if the shorter distance might have had anything to do with this?

I know the maintenance history of teh car. It is in great shape, has been gassed up at Costco most of its life, I've been going there since I bought it, and I did throw in some techron to clean up any deposits a few tankfuls ago, so it really shouldn't be anything mechanical--and even if it was, there was no sign of this up until yesterday, so I don't see that as an issue.

Any thoughts?

[prr]

OK should have googled first, but I just didn't see that weight as bringing me down all that much.

Weight of vehicle with only me is about 4300 lbs, with the extra passengers last night it would have been at least 4800 pounds. So that would be a about a 10% increase/decrease (depending on how many people are in the car) from the other weight. Based on the figures in this link (https://www.h3xed.com/blogmedia/Rica..._MPG_Study.pdf, I could expect 1-2 MPG hit by increasing the weight by 10%.

But still, that is a good 4 MPG.... Perhaps the incline had something to do with it as well?

[prr]

Using numbers from https://www.huffpost.com/entry/gas-m...ight_n_1951174, adding 100 lbs will take the MPG of a car getting 31 MPG, down to 30.3 MPG--some 0.7 MPG per 100 lbs. So that ratio would have taken 3.5 MPG off my mileage.

[prr]

Sorry for the slip up, but as I edited my first post, I don't think weight was a contributing factor to the drop from 26 freeway to 22.5, because that earlier 26 freeway MPG was also with lots of people in the car--in fact, the same exact people as our trip last night.

I realize that extra weight can bring down a car's MPG, but it shouldn't be responsible for the drop from 26 to 22, as there wasn't any more weight in the car last night.

[Vman455]

You've got a short fill on one freeway trip--don't sweat it. There's so much margin of error here that it's impossible to say what's responsible. It could be any combination of:

-pump error
-tank error from the short fill
-weather: temperature, wind speed and direction (huge source of error just right here)
-traffic
-rate of acceleration/deceleration

People like to complain about the way EPA ratings are determined--but there's a reason it's best to do it on a dyno in a temperature-controlled lab: you remove all these other variables.

[oldtamiyaphile]

I used to tow a 600kg enclosed trailer, usually at least once a week.

My best tank without trailer is 6.0l/100km. My worst trip with the trailer is 10.0l/100km.

That equates to 0.00666l/100km per kg. So best case for a NA engine you might loose 0.2L/100km per hundred pounds.

This is worst case scenario stuff. 100% city use (10-15mph average speeds), there's an extra axle and extra aero drag.

If you use the ecomodder calculator, add in trailer aerodrag and rolling res and it seems weight makes zero difference to a hypermiler's economy.

[prr]

So you're suggesting that this isn't a valid sample.... OK I'll (hopefully) find sufficient reason to go on a more extended freeway trip that will give me a more true figure.

Originally Posted by Vman455

You've got a short fill on one freeway trip--don't sweat it. There's so much margin of error here that it's impossible to say what's responsible. It could be any combination of:

-pump error
-tank error from the short fill
-weather: temperature, wind speed and direction (huge source of error just right here)
-traffic
-rate of acceleration/deceleration

People like to complain about the way EPA ratings are determined--but there's a reason it's best to do it on a dyno in a temperature-controlled lab: you remove all these other variables.

[vskid3]

Does your Highlander have any sort of MPG gauge? Even if it's not exactly the accurate to the mileage at fillup, it should be able to help you see how trips compare to each other.

[prr]

Yeah, MPG for previous owner was 24, both city & freeway. So overall, I'm doing slightly better, but this last run was a bit worse, than their average.

At this point, of course I'll read all responses, but I'm just going to ignore this as a short trip outlier, and I'll look to get another mileage MPG soon.

Originally Posted by vskid3

Does your Highlander have any sort of MPG gauge? Even if it's not exactly the accurate to the mileage at fillup, it should be able to help you see how trips compare to each other.

[JRMichler]

Some other factors:

Temperature. My truck gas mileage improves by almost exactly 1 MPG for every 10 degrees F temperature increase. That's for temperatures from -20 deg F to about 75-80 deg F. At higher temperatures, the increased air conditioning load balances the decrease in drag.

Elevation. On a particular 72 mile trip, I used to get 3-4 MPG better driving north. There was a net elevation difference of 800 feet, with north downhill.

Wind. A 20 MPH headwind will decrease my gas mileage by about 3 MPG, with a similar increase if it is a tailwind. Crosswinds hurt mileage similar to headwinds.