New state, change in gas mileage

Good morning!

I recently moved from Denver, CO to North Liberty, IA. I’ve been here about 2 months, and I’ve noticed a marked decrease in my gas mileage. There are several variables at play here. Which is the most important?

  1. Altitude - I am at a much lower altitude than I was before, which I understand will increase resistance.
  2. Octane rating - The manual for the car recommended using midgrade fuel. In Colorado, this was 87 Octane. Now that I am in another state, 87 is low grade, and 89 is midgrade. Since I was using 87 octane before, I used it again.
  3. Speed - in Denver I commuted ~40 miles on the highway every day. Traffic was bad in the morning but I usually worked through rush hour at night so I could drive 65 MPH on the way home, easy. Now I live in a small town, I only drive 8 miles a day, and my average speed according to my trip computer is just 25 MPH.

My average MPG in Denver was between 24 and 26. Here in Iowa I am only getting 19.7. Since I drive so little now, I don’t mind the lower rating - 279 miles between fillups is still enough for a whole month of driving. Mainly I’m curious as to which of these factors plays the biggest role in the change in gas mileage. However, I also want to be sure that using 87 octane fuel in the lower altitude environment is not going to damage the vehicle. If it will, then I can switch to 89, the local midgrade rating.

Thanks for your advice!

For the octane question, does your manual give an octane number? What exact wording does the manual use? What kind of car is this?

Did you use E10 gas in both places?

To answer your main question, I’m pretty sure the short in-town trips are the biggest factor here.

All of the things you mention could have an effect, but air resistance is only a big factor at high speed.
Better look at your manual. Cars require higher octane numbers at lower altitude. Use what it says.
Assuming you don’t have a hybrid, the low speed short trips are probably at fault.
Be careful you aren’t buying that high ethanol Iowa crap.

Yeah I think the short trips are doing it but that’s not a big difference in mileage. On weeks that I just drive around town, it seems like I can use as much gas going to a store as on the highway. One thing in Iowa though is that its all 10% ethanol and it might be your car is a little older now, but I really don’t see the difference as significant.

Only driving 8 miles a day at 25 mph average is much different from driving 40 miles per day at 65 mph. The little driving you’re doing at low speeds means the engine barely gets warmed up, so it operates at lower efficiency, and you’re doing more stop and go.

If you were doing exactly the same type of driving as in Denver you’d see similar MPG, but you’re driving much differently now, with predictably worse fuel economy.

I only drive 8 miles a day, and my average speed according to my trip computer is just 25 MPH.

You already answered your question.

If the manual says to use mid grade keep using it. You need higher octane at lower altitude.

Using 87 octane in Colorado is like using 89 octane in Iowa. Your manual recommends midgrade so you should be using that anyway.

I think your driving habits are the major cause. Fuel economy is terrible for the first 2-3 miles of driving until the engine warms up.

What was the percent of ethanol in the CO fuel, and in the IA fuel? More ethanol means less energy in the fuel, thus lower MPG if all other factors are the same or close to the same.

Thank you all for your input! As I said, I knew all of those factors would play a role, I just wasn’t sure which one was the biggest issue; my interest in the solution was purely scientific curiosity. I didn’t expect to really be able to get any different performance. It sounds like the rural driving is to blame more than any other factor.

The car is a 2012 Chevy Impala LTZ. I did go back to the manual to verify, and it doesn’t actually specify “midgrade”. I had the association in my brain that Midgrade=87, but in fact, the manual specifies the octane rating, rather than the grade:

Use regular unleaded gasoline with a posted octane rating of 87 or higher. If the octane rating is less than 87, an audible knocking noise, commonly referred to as spark knock, might be heard when driving. If this occurs, use a gasoline rated at 87 octane or higher as soon as possible. If heavy knocking is heard when using gasoline rated at 87 octane or higher, the engine needs service.

So, I have been doing as the manual says, just remembering wrong. I have no knocking or symptoms, just the reduction is mileage rating, which is expected.

Melott, you said not to get the high ethanol fuel. My engine is capable of accepting E85 - was your warning meant to warn against using E85 in a standard engine, or a statement about the quality of E85 whether your car can take it or not?

You’ll suffer about a 30% reduction in mpgs with E85.

You can experiment with E85. I have, and found that the lower (tax-subsidized) price at the pump does not compensate for the lower MPGs.

@AlexG E85 has a lower energy content than gasoline, and your car will get lesser fuel economy when running on it. Generally 25%-35% less fuel economy, it’s one of the trade-offs of E85.

"You'll suffer about a 30% reduction in mpgs with E85."

…and because we all have to suffer with ethanol as a result of the “Iowa lobby”, it wouldn’t surprise me at all if high ethanol content gasoline was standard fare in that state.

It’s necessary to use a lower octane at higher altitude, due to less oxygen in the air. That’s why the octane numbers on the pump for low/mid/high-test fuels are different in Denver than in Iowa. I think the reason for your reduced mpg is probably what’s already been described above; however, there’s a chance your engine isn’t reaching the proper operating temperature due to a bad thermostat, or something is amiss in the engine function that modifies the air/fuel mixture due to altitude differences. That would usually be a problem with the air flow meter or the MAP.

If your manual says that 87-octane gas is required, using a higher octane won’t improve your fuel economy.

So lots of things could be causing this loss of mileage. I’d bet on the in-town driving or the E85, if he has started using it.

It's necessary to use a lower octane at higher altitude...

Not really. Higher altitude allows use of lower octane gas, but doesn’t necessitate its use.

All of the factors you mention have an effect on your mileage. Your Chevy’s computer automatically leans the fuel to air ratio at high altitudes and enriches it at lower ones. Denver is the mile high city with one of the capitol’s steps at 5280’ above sea level. North Liberty IA is only at 772’. That’s a big difference in the fuel to air ratio needed to make your car run right. I know I always get better mileage touring around CO than I do at home, and I’m about 700’ higher then you are.

This vehicle has an EPA rating of 18 MPG city, 30 MPG highway (13 MPG city with E85), your results are normal.

there’s a chance your engine isn’t reaching the proper operating temperature due to a bad thermostat

On a 2012 vehicle? Come on, how many thermostats have you had to replace on late model vehicles?