Sudden MPG Drop (3i w/SkyActiv)

[skylinedrifter]

For the first couple of days, my trip computer was telling me that I was at 50+ mpg on my commute to work each morning. While using cruise control at 55 mph I was hovering around 58mpg. This week, my trip computer tells me I am averaging about 42.5 mpg on my morning commute, and cruising at 55 mph gives me about 48.5 mpg.

What happened?

I have made no changes to the car, this was using the same tank of gas, I haven't changed my driving style, the tires look good, there wasn't a sudden addition of extra weight... There's just no rhyme or reason that I can figure. Granted, 42.5 mpg is nothing to cry about, but I can't help wondering if I'm suddenly going to experience another significant mpg drop sooner or later.

Does anyone have any thoughts?

 

[Jamis]

Manual or automatic transmission? Are you shifting at the same rpm all the time? My Civic's mileage will drop off markedly if you shift above 3K rpms and really drop if you dip into the VTEC cam's range. I don't know what the variable valve timing rpms are on the Skyactiv motor.

 

[skylinedrifter]

I've got a standard transmission, no I haven't changed my shift times, but this happens when I am cruising in one gear at highway speed. Shifting isn't much involved.

 

[njaremka]

its best to calculate gas mileage at the pump, not with the MPG display. I would only trust the display to guide me in my driving style for better fuel economy.

 

[Jamis]

Does the computer do instant mileage and/or average mileage? I'm still learning about the Skyactiv system, so I need to go read the OM. I agree on the tank by tank suggestion. It will even out the overall mileage, so you aren't seeing transient issues on a day by day basis. Also, look into the Fuelly website to help you keep track of mileage and gas expense. It will also let you see what other drivers are getting from a particular vehicle.

http://www.fuelly.com/

 

[skylinedrifter]

I agree that the tank to tank measure is going to be the most accurate, but why would the computer have such a dramatic change in readings literally overnight? It makes me nervous, AND part of the reason I bought the car was due to the MPG I was getting during an extended test drive. To see it drop 2 days after purchase is disappointing, at best.

 

[msp5]

Well you have to under stand the computer had no real data to average mpg with only 5 or 6 miles on it.Now that your driving it and the car is getting acustomed to the way you drive,Its giving you a more concrete avg. mpg on your display.Im getting around 44 mpg avg highway driving with my skyactive 3.

 

[skylinedrifter]

The 50+ mpg average was after approximately 250 miles, which should have been plenty of time for the computer to gauge effective gas mileage, especially since I am an unusually conservative driver. Adding a bit of weight to my worries that there might be a problem is the fact that I've been driving a loaner car for the past couple of days while the dealer installs some options for me. I've been getting 50+ mpg average again.

So, while it's in the shop, I think I'm going to have them look it over.

 

[skylinedrifter]

So, either I'm crazy or whatever the "problem" was, it self resolved.

Long story short, I didn't have the dealer look at the unit while they had it. When I got the car back, I was lucky to cruise at 47mpg. Then, I reset my trip computer again and now I am back to cruising at 57mpg, averaging 50.5mpg combined.

Anyway, I think these fluctuations may just be part of the SkyActiv experience and I just might need to learn to live with the occasional "low" fuel economy. *chuckle*

 

[andersun16]

Good to know. I am picking mine up tonight and will let you know if I have any similar fluctuations.

 

[ryukensfj]

Tell us what you get when you calculate miles driven/gallons pumped. You may have changes in gas quality, or temp changes that may affect how the computer calculates the mpg.

 

[skylinedrifter]

I've been within about 1 mpg when I manually calculate tank to tank, which can probably be attributed to variances at the pump. The MPG drops seem to happen while on a single tank, so changes in gas quality are unlikely. Air temp is a possibility, though.

 

[VoleakySport]

Filled-up the tank last night, and ended up with the worst mpg's for my Skyactiv automatic, to date. I got 24.3 mpg doing 100% city driving, mostly rush hour traffic, using the A/C all the time. My daily driving routine and habits are the same, just a little more A/C usage because of the heat wave we are having here. The thing that really changed, and could be the culprit, is I put on a larger diameter tire, it is now a 215/55/16 instead of the OEM size 205/55/16. The new tires are allegedly LRR. Theye are Yokohama Avid Ascends. But, I do not know how to adjust the calculation for the half inch taller tires, so I can get a completely accurate mpg reading. Can anyone tell me how to calculate accurate mpg for a plus size tire? Thanks in advance.

 

[cymx5]

From new, the car simply does not have enough data to get an average...DUH! (braindead

My Skyactiv3 Automatic started showing an consistent average after about 1000 miles. Thus far, I have tried a few tanks of 10% ethanol 87 octane and one tank of 10% ethanol 93 octane. My averages over about 1,500 miles are: 30MPH and 35.8-35.9mpg. I get anywhere between 34-36mpg average on the 87 octane and got 38mpg on the one tank of 93 octane (calculated w/ trip computer & actual fuel pumped at the gas station). If only the government would kill this ethanol bulls*** and let me find out the FULL potential of the Skyactiv engines. I'd expect to see just over 40mpg if we weren't forced to burn corn.

 

[Justin2129]

were you resetting the trip computer each day before you went to work, or were you just letting it average for the whole tank?

 

[cymx5]

I reset the trip comp every tmr i fill up. The car computer is averaged since the day i bought the car.

 

[skylinedrifter]

I reset my averages with every fillup. I also reset my trip computer and I use the same station and the same pump every time. If I hypermile, Honestly, I am finding that air temperature is the biggest determination of my mileage and that I am getting the best mileage at about 80f. How... odd. 0_o.

Gotta say, 2 months on, I am still in love with this car.

 

[ngd298]

Filled-up the tank last night, and ended up with the worst mpg's for my Skyactiv automatic, to date. I got 24.3 mpg doing 100% city driving, mostly rush hour traffic, using the A/C all the time. My daily driving routine and habits are the same, just a little more A/C usage because of the heat wave we are having here. The thing that really changed, and could be the culprit, is I put on a larger diameter tire, it is now a 215/55/16 instead of the OEM size 205/55/16. The new tires are allegedly LRR. Theye are Yokohama Avid Ascends. But, I do not know how to adjust the calculation for the half inch taller tires, so I can get a completely accurate mpg reading. Can anyone tell me how to calculate accurate mpg for a plus size tire? Thanks in advance.

http://www.tacomaworld.com/forum/tirecalc.php?width1=185

 

[VoleakySport]

Thank you ngd298, but I have since switched back to the oem size 205/55/16, this time I am using the Yokohama YK580. All is good now. The 215/55 Ascend was an XL load rating, they road to stiff and heavy.