Prius Prime 2017 Low/Unusual MPG

Sorry to hear about your issues. We can't really give you any accurate advice until you crawl under it, take pictures and tell us how far from OEM this car has been modded. If you don't have an air suspension control console on your dash or center console - the air ride system isn't programmable or tunable. A moderately priced, decent system should be tunable to adjust ride quality and driving modes - that's the whole point of dropping all that cash for such a system. BTW; I don't think anyone makes a LRR 40 series tire.

FWIW; When I see non-OEM mods on a used car - I'm thinking discount; not increased value. The problem here is; who did the mods and did they really know what they were doing. Is this some shade-tree mechanic that saw something in a magazine and thought this looks good and should work great in my car? When I was younger and dumber, I bought a couple of really nicely modded cars. The euphoria didn't last long. Cheap junk that broke or threw the car so far out of spec.; it wouldn't pass smog or get even close to OEM EPA ratings. You place the car on a Dyno and find the power curve is worse than OEM; but it sounds fast. This doesn't include the hassles of stripped, broken bolts, and heli-coiled spark plug holes. IMHO: more problems than they're worth...

Live and learn.....

 

Looks like others have chimed in to point you to right direction. It seems your non-OEM mod is causing this low efficiency. While the max distance driven from a full tank is not the best indication of the fuel efficiency, especially true if you used any EV mode from wall charge, the DTE distance shown on the dash when you fill-up the tank does give you a rough estimate of the HV only mpg. The PP does not show HV only mpg on the dash, but if you divide the DTE number right after a fill-up by 9.3, you get a rough estimation of the HV only mpg from your last tank full of HV drives. In your case, if the DTE after a fill-up was showing 390, then it is ~42mpg (390/9.3) which is really low for a PP, but I can see that low if it is heavily modified and have larger wheels and tires.
See this thread: What is your DTE after fill-up?

I always wonder about those who mod a new car, why do they even bother? I kinda understand if they like to tinker with the car and if they are going to keep the car for a long time. But any kind of modification good or bad always brings the resale value down. In OP's case, it even brought the fuel efficiency down. And I have to ask to gain what???

 

Glad to hear that you found the culprit and are able to fix it relatively easily. Yeah, heavier and larger rims are known to drop the efficiency. But people go for the looks of the larger wheels with lower-profile tires. Manufacturers go with the flow and stick those larger wheels with a lower profile on the higher trim of cars they make. I really don't understand it, especially on a car like Prius (no Prime trims come with 17" wheels, but regular Prius higher trims are equipped with 17" wheels). And many PP owners like to switch to heavier and larger plus-size wheels... mostly for aesthetics.

Aftermarket Wheel Fitment Guidelines

 

yeah!!! 32 lbs is no joke.... Those wheels are thickkkkkk... But are more aerodynamic than a lot of wheels. For reference my RFP1 17 inch OEM sized wheels and tires weigh about 30 lbs total. The wheels weigh 14.6 lbs each though. So very lightweight.