engineermike

FWIW, the biggest factor that causes diesels and small engines to get better fuel mileage is the reduction or elimination of pumping losses - that is. . . the pistons working against a vacuum in the intake manifold. Diesels don't run vacuum, and smaller engines are more heavily loaded at cruise so they run higher manifold pressures. That said, since IC engines make power based on mass flow, it takes less manifold pressure to achieve a certain power level [cruising, for instance] at colder temperatures. p=PV/RT, so if Temp goes down, Pressure must go down too in order to maintain the same density. So, at the lower manifold pressures, you are deeper into vacuum and the pumping losses are greater.

A-Dub

I think Houston started the winter fuel in November. Noticing a 1-1.5mpg drop on my EB and my wifes 4runner.

Woolaroc

Dang was hoping Houston wouldn't get the winter blends haha! I may be moving there in a year or so from Oklahoma (Oil and Gas of course).

A-Dub

I bought my truck in Dec of 11, so I didn't really notice the mileage drop last winter during break in.

Im an OK transplant as well

Buck50HD

If your mileage drops significantly more than 0.5 mpg, it's not the winter fuel blend.

bcb97

Any way to back up your claim? I looked into this the other day and it all depends on what the ambient temperature is. If it's colder, your MPG will drop. The colder it gets, the worse your MPG is. So people in Florida might not see much of a decrease in MPG with winter gas versus someone in Wisconsin who might see a drastic change.

I saw this test on a Honda Civic (it's calculated)
Speed---- Ambient Temp------- MPG
1. 60------------95----------52.98
2. 60------------85----------52.62
3. 60------------75----------51.16
4. 60------------65----------49.12
5. 60------------55----------47.22
6. 60------------45----------44.67
7. 60------------35----------43.05
8. 60------------25----------41.54
9. 60------------15----------39.41
10.60------------05----------38.09

engineermike

This is excellent information. It's a fundamental part of the way an IC engine operates to lose part-throttle efficiency in colder ambient temps. See post 21 above. This is, by and large, the biggest factor is a loss of winter fuel efficiency. The loss varies as a function of temperature. It should be obvious that there weren't 10 different "winter" fuels used in the test, therefore something else is going on.

The energy content change in the fuel is less than 2%, or less than .5 mpg, which is where I think Buck50HD was coming from.

Buck50HD

If you drove the same day, same temp, and switched from summer to winter blend, you would only see about 0.5 mpg difference. It's only a couple percent from the fuel alone. The rest of the loss in mileage is due to the temp drop. Aerodynamic losses account for 1% every 10 degrees. So, dropped 40 degrees? You just lost twice as much from drag as you did from just the fuel alone. Also, the fluids are higher viscosity, much more drag. Plus what engineermike mentioned has nothing to do with the fuel.

What I'm getting at is that the change in fuel is a very small portion of the loss in mileage. All the other factors are what you notice.

klumb15

I've always thought that my decrease in winter fuel economy was more from the cold than it was from winter blend fuel but I got some very contradicting evidence recently when I took a trip up north over Thanksgiving, then another over Christmas. I live in the Detroit area and I go to visit my family over the holidays who live in Michigan's Upper Peninsula where I was born and raised. It's about 500 miles 1 way. Prior to the trip, ambient daytime temps were consistently in the 40's and I was seeing my normal 17 city mpg and 22-23 highway mpg, which I saw all summer. Going up to the UP, my truck pulled an average of 22.5 for all 500 miles with ambient temps in the mid 40's the entire way. A cold blast came through while I was up there, and my trip back included temps in the mid 20's and snowy conditions. I was still able to get 21 mpg for 500 mile journey back, but my entire 6.5 ft bed was filled with firewood up to my BAKflip cover. The additional weight could have easily costed me a 1-1.5 mpg loss and I don't think that loss was due to the outdoor temp being 20 degrees cooler. After that weekend, I filled up in the metro detroit area and my mileage immediately plummeted 3 mpg less than what I was seeing prior to the trip to about 14 city mpg and 18 highway mpg, where it has remained since then. I took the same trip over Christmas with temps in the mid 20's and I got 16.5 mpg on the way there bucking a pretty strong headwind. The way back was same temperatures with a 5 mph tail wind which yielded me 17.9 mpg.

The point is the temperatures were pretty consistent on both trips, yet I saw a substantial decrease in mileage that I suspect is due to winter blend fuel. This experience really made me question the impact that winter blend fuel has on my truck.