2008 with electrical gremlins and low mpg

Things that can reduce mpg- wrong tires (not LRR) or low pressure, incorrect wheel alignment, weak hybrid battery, missing or damaged body panels or air dams, dragging brakes, engine /ICE problems (low compression, misfire, restricted exhaust, improper fuel trim/ control, weak thermostat), or possibly a wrong input of some kind that causes the ICE to run when it shouldn't (like a request for heat or A/C).

Do you have halogen or HID headlights? Does the dimmer control for the dash illumination work? Do the tail lights work?

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You sure they're 17" wheels? Gen2 Prius comes standard with 15" wheels and the Touring version of Prius comes with 16" wheels which ends up causing slightly less MPG.

So if you really have 17" wheels I'd say the car is running great and unless they're low profile 17" that's where you're losing all your MPG.

 

Resetting the 12v is indeed a good way to go... But in general 37-38mpg is within range of normal and boosting it higher is probably as easy new spark plugs, mass air flow sensor & throttle body cleaning, new air filter adding low rolling resistance tires at inflation PSI in the 40's... Also many with bad MPG have clogged cats due to getting their oil changed at oil change places that don't know that Prius hold less than 4 quarts and don't care if they add too much oil... And having your oil level above the full line not only kills your MPG in the short term it makes the inside of the engine real nasty, especially if the owner goes too long between oil changes and all that nasty travels out the exhaust pipe and can clog up the catalytic converter and because you're in California replacing that will cost near as much as you paid for the car.

 

If you have the Touring model, you also have HID headlights. The headlight issues you describe are most commonly associated with the HID bulbs. Replacing them with the quality OEM bulb would probably solve that.

Edit: On second thought, if your car is a 2005, it couldn't be the Touring model, which was not available until 2007. It could still have HID headlights, though. Does it have factory fog lamps? That would mean the headlights are HID.

2nd Edit: Scratch that, I see you are talking about an '08.

 

Agreed... Those HID have unreliable ballasts that cost a fortune... It was a failed design. I converted a 2009 HID system to Halogen bulbs. Easy re-wiring job and you just buy the bulbs, the sockets and you need the most common cheap online lenses that bulb sockets mount on, which are less than a $100 for a pair. Here's links to bulbs and sockets I bought for less than $30:

Genuine HELIOLITE Halogen 12V 9003 H4 Headlight Bulbs Hi-Low Beam | eBay

https://www.ebay.com/itm/New-Headlight-Socket-for-Jeep-Wrangler-1955-2006-Dorman-84790/184437381933

 

Just gotta google priuschat headlight conversions... There's lots of misinformaion on that subject though. For some reason people think you need to buy a high beam low beam switch. But it's way simpler than that. There's a wire that goes to the little motor in the HID and there's another wire that goes in behind where the bulb is and both these wires have a black ground wire. So you twist those two ground wires together and the other two wires are postive lead for high beam and low beam. The part I got stuck on is I was trying to wire from the car's wiring harness and the color coding on those is confusing so it's easier to tap into the wires after where the HID wires plug into the harness. The color coding isn't perfect that way, but easier.

 

You can always just convert to LED and save yourself alot of hassle of buying new cheap non oem lenses and use what you already have. You can follow the guide here

 

Does an LID Lens work with LED bulbs? I can tell you I know for certain that an LID lens will not work with a Halogen bulb because there's no clip to hold in place, nor receptacle to hold in place the Halogen bulb in an LID lens.

 

My '08 would get between 42 and 48 mpg regularly on my daily 140 mile commute. Lower on the way down due to 2000 foot elevation change. It had HID headlights and fog lights, and I only had to replace one of the HID bulbs. Your electrical gremlins are a strong indicator of poor 12 V battery health. Do you know how old your 12 V battery is? It's that little motorcycle battery in the right-rear corner of the vehicle. Have that load tested for sure. A 12 V battery in need of recharging or replacement will cause a drop in mpg due to the charging circuit constantly trying to bring the battery up to spec. Can't think of a good reason for headlights turning off after a bump in the road. Loose connectors or corrosion/poor connections at the 12 V battery, which could cause poor recharging and also lead to electrical gremlins.

 

In the video they show how to use the stock HID housing with the LED bulb so yes it does.


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