So I'm trying to figure out why I have been getting considerably less mpg's. On a specific 300 mile drive, I have consistently averaged anywhere from 52-58 mpg's. These are highway miles at a speed of about 78 mph. The last 4 times I've made the drive, I have averaged 43 mpg. Those 4 drives have come in the winter, but the cold temperatures shouldn't account for that much of a drop in my opinion. I'm leaning more towards my 12V battery being old and needing to be replaced. Last week, it was completely dead, and the car wouldn't start. There was an interior light on though, so I don't know if the battery is bad, or it it just got drained because of that. I have read many threads saying that a dead 12V could potentially cause a drop in MPG's because of the engine having to divert more energy to charge it. Other issues that have been happening. In this past summer, it would refuse to switch over from the ICE to EV mode until after about 10 minutes. I though this could potentially be from the battery being overheated, or the 12V needing to be charged. Also this past winter, I frequently have drives where the ICE stays on for 10-15 minutes before it finally switches over to EV mode. Any help would be much appreciated! I've been driving myself crazy trying to figure out why my 2015 prius with 41,000 miles all of a sudden has long highway drives where it only gets 39 mpg's!
I see you have doubts about your 12 V battery, why don't you charge it up and have it tested and eliminate one piece from the puzzle. There are so many things that will cause low gas mileage and winter time doesn't help one bit especially with wind and cold temperatures.
I don’t have a charger, I’m a college student with limited resources. I wish I had things like a Scangauge and all of that stuff! I’m thinking amour just buying a new 12v at Pepboys for $160. Do you think the problems I’ve described most likely point to the 12V being old?
cold weather/heater use, winter gas formulation and tire pressures are huge factors. the 12v could be a culprit. have you owned it since new?
I have owned it since new. I do pay quite a bit of attention to the common things that affect MPG. I always keep my tires at 38 psi, I hardly ever use the heater in the winter unless the car has been running for 15 or more minutes. Last winter I experienced the expected 3-5 mpg drop because of all the normal factors (cold air, increased ICE running, gas formulation), but something is different this winter. I have experienced an additional 5 mpg decrease or more from last winter.
12v isn't very old, worth testing though. have you ever needed a jump start? 40k isn't much mileage. are the air and cabin filters clean? oil level good? are you still running the original tires?
I just had to jump start the car, but there was an interior light that was on when it started back up, so it might have drained the 12 volt. I’m not sure about the air or engine filters, oil level is slightly above where it should be, and I did change tires from the OEM’s to Michelin Defenders at about the time I saw the 10-15 mpg decrease. I wouldn’t think the tires would make that much of a difference.
If you question the battery take it to Pepbpys and have it tested so you don't waste money on a battery you may or may not need.
All great comments and I also have Defenders on mine and I saw a slight decrease in mileage but not very much. I'm leaning towards 1. Tire air pressure, 2. 12V getting weak, 3. Check to make sure you don't have a stuck brake caliper. 10 mpg with mostly highway, I'd lean towards the caliper. I lose at least five mpgs when the dealer resets my tire pressure back to factory 33-35 psi. I run my tire pressures at 42F,40R. BTW it is recommended that you change out the 12V every 48 months.
Make sure your tires are inflated at least to spec if not higher. AutoZone will test your 12V battery health for free.
Even though it's an old thread, it's still an interesting topic. On a 300 miles drive on the highway at 78 mph, the OP is getting exactly the mpg that he should, based on BWilson's chart here, which is 43 mpg. My question is how he could have averaged 52-58 mpg at 78 mph previously (assuming round trip), unless he had a massive tail wind in each direction over multiple trips? He would have needed to average 63-70 mph round trip to have averaged 52-58 mpg.
