Prius Prime 2017 Low/Unusual MPG

Never mind miles per tank. This one has a smaller tank than your previous models, and different people have different ideas of how far down to drain their tanks before refueling.

What MPGs are your getting, now vs the old ones? Have your driving patterns / routes / speeds change?

The replacement tires could well be an issue. I don't remember "air suspension" being an OEM thing. Other mods may also not be beneficial.

 

I believe the Prime has an 11.4 gallon tank. Not that much smaller than your older Prii, but it may be noticeable.

Wider tires/wheels do seem to reduce MPG a bit, so going back to the original size would help some, but it won't be enough to make up all the difference you are seeing. It would also give an opportunity to make sure you have a low rolling resistance tire model, though numerous drivers do prefer to sacrifice a bit of MPG to get better tire grip. But these shouldn't be enough to account for your full MPG shortfall, so I suspect something else is also happening.

BTW, what is the tire size mounted now? If the diameter is different than OEM, it will alter all your speed, odometer, and MPG readings.

I'd also wonder what other mods the prior owner may have applied. E.g. did he "upgrade" brakes too, and end up with something creating noticeable brake drag, reducing MPG?

For a second wheel set for seasonal tire swaps without twice-annual dismount/remounting, I acquired a set of Plug-In-Prius wheels from craigslist, also dressing up my regular 2012 a bit. You might be able to find original or suitable wheels from the same, or from another on-line marketplace, or from a junkyard. Fresh new OEM wheels from Toyota are likely too expensive.

 

Those wheels only come in 18" & 19" sizes and the weight starts at 32 pounds and goes up from there. OUCH!!
Rotiform CCV Wheels & CCV Rims On Sale

That'll make a big difference. And if the tires are the wrong size, that'll throw off your speedometer and odometer.

Also, if it is really air suspension, that means a compressor and that means gas to run the compressor.

 

safety amount may not be the same as gen3 either, idk

 

I'm seeing the original tire size as 195/65R15. Those tires you now have calculate out as barely 0.1" taller, which should shift the speedometer and odometer something like just 0.4%, not enough to be meaningful contributors to your mpg problem.

But the 30 mm wider tires do add to air drag, plus whatever else wider and heavier tires and wheels do (I've never used plus-size wheels, so look to others for more detailed commentary).

As for that air suspension, I'd think the compressor is filling up some sort of air shocks/lifts, not the tires. The tires aren't flat or low before startup, are they?

BTW, what are your tire pressures? Those wide tires probably want something lower than the standard 195s to avoid jarring your dental fillings loose, but make sure they aren't too low, i.e. something below 30 psi.

 

I hope you are asking only for a target pressure to set them, not for how to actually check the pressure that is in them now.

For a starting guestimate, try something close to what a regular Liftback with upsized wheels recommends. Here is from the 2016 Liftback Owner's Manual, look down at the bottom for the 17" tires, Toyota recommended 33 Front, 32 Rear.


This may need some refinement, ask around for others who actually use other-sized tires. I don't.

As for how to mod or un-mod the suspension, that isn't my field. Hopefully others will chime in with advice.

 

I'm no suspension expert, but I would think the air suspension would be a total replacement. I can't imagine a good way to add air shocks to what's already there due to space limitations and handling concerns.

As for the wheels and tires, weight is a huge factor because it takes more energy to get those large masses of metal and rubber spinning. And they will want to keep spinning so will take more braking energy when you want to stop. They are essentially flywheels like on big industrial machines like presses.

Heavier wheels and tires also reduce the sprung to unsprung weight ratio. It's a big concern in motorcycles since the bike is very light and it's hard to make a wheel that both light enough for comfort and strong enough for safety. For comfort, you want as high a ratio as is practical. That's either a heavy car (sprung weight) or light wheels & tires (unsprung weight). The closer the weight of the wheels & tires to the weight of the vehicle, the harder it is for the springs and dampers to absorb the bumps and control the rebound. Here's a quick video on the topic:

 

Other than OEM struts & other parts, I wouldn't know what to suggest. I like the way my Prime handles and feels and I wouldn't change a thing. There may be equivalent 3rd party parts, but I don't know what they are.

 

Won't know until you can get a look under there. It could be a whole new suspension system the previous owner put in, or could simply be an add on to help out the existing shocks and springs.

The latter are made for towing vehicles to level out the rear when the trailer is hooked up. My bet is that is what you have, and it was needed to deal with those replacement wheels.

Plus size wheels are paired with tires to keep the same general diameter. The width does have an impact with air flow, but the may issue is the weight.

Those wheels alone weigh as much as the stock wheel and tire combo. At around twice the weight of the OEM wheels, that is over 120 pounds added to the worse place to add weight to is car. Low profile tires have more material in the sidewalls to keep them rigid; they are going to weigh more than the stock tires too.
How much do stock wheels weigh? | PriusChat

Getting back to wheels & tires in the stock weight range will likely fix the fuel economy.

 

You're probably going to have to research the type of mod that was done when adding the air. The added air might also be a lowering mod, as that's what's usually done with mods like that one. I was thinking when I looked at tiresize.com to compare the OEM tire size 195 65R 15 with your 225 40R 18 that if the cars alignment is off with the wider tires and the air mod, that could surely be A reason for lower mpg. Any other mods to the engine ie: air intake, bored throttle body, exhaust parts, coils, etc. would also contribute to lowering the cars mpg.

Without knowing exactly or at least a bit more specifically what's been added or changed, it's really hard to guess what's the biggest culprit that's reducing your cars efficiency.

Wheel and tire weight and size as Jerry and Fuzzy mentioned above are probably a big part, maybe the biggest part, but there could also be other things, especially if the air mod was also a lowering mod and double that if the alignment is off and the wider tires are not rolling in spec.

See @RightOnTime angry panda mods for a convenient list of his mods to date
Project: Angry Panda | PriusChat

suggest: when you find out what kind of stuff was used to do the air mod, you might want to start a new thread asking if anyone has already done the mod and written it up here at priuschat or somewhere else.

 

Don't see how the air suspension can get in the way of OEM wheels. A brief search for air suspensions yields ones that swap in for the stock struts and shocks. There is other equipment, but that isn't near the wheels.

So you could keep the air suspension. It might need adjustment though. I'm guessing the current wheels threw the ride out with the stock suspension. With lighter OEM wheels it might feel off. Or everything will be fine. Won't know until lighter wheels are on.

Is there any controls for the suspension? The ride height looks normal from the photo of the wheel. If you could lower it on the highway, that would help with aero drag by reducing frontal area.