I'm starting to suspect someone let the air out of my tire.

Bring it to a tire shop for proper diagnostic

 

^ ^ ^

what JC said. You haven't and probably do not possess the equipment to properly diagnose.

OR - you may have bragged to the wrong person about getting 70 MPG!!!!!
Quit it

 

I was going to suggest checking the valve core, but so many things can go wrong with a tire that it's always best to bring it to a professional. I consider it money well spent (if there was a charge) and priceless peace of mind at 75+mph.

 

OR, how about the logical step being you picked up a nail, screw or thorn on your 70+mpg run? You'd be surprised how they can hide when you look for them on the outside, but they stand out a mile once the tyre is taken off.

Just let your local friendly and trusted tyre shop take a look and fix the leak. That's the logical explanation

 

Just doesn't seem to make much sense as an act of vandalism.
Of course vandalism really never makes any sense.

The only scenario that seems to fit your results, would be if somebody decided they wanted to covertly let air out of your tire, with the sole intent of reducing your gas mileage.

That's pretty specific "malice" and pretty careful execution.

If you were magically missing your valve covers?
Or if you came out to find your tire entirely flattened...Then I might believe it was malice and vandalism.

But the idea of somebody carefully removing only "some" air, then carefully replacing the valve cover, with the intent of simply screwing with your gas mileage? Doesn't make much sense to me.

My money is on the valve stem either never was right, or accidentally got damaged or loosened...it happens.

I'd be paranoid enough to monitor for a while...and it's good to check tires monthly anyway....but I'd move on, assuming it was simply what it was.

 

I would be interested to know which side of the car wheel 4 is on. Sometimes these valve cores just spontaneously, or over time, loosen up. I think it might be some sort of torque hysteresis, which probably isn't the proper use of the word. You remember back in the day when they would put left-hand thread lug nuts on one side of the car. Same deal. But in that case, both rotations were in the same plane. These would be slightly out of alignment as the valve fitting is angled instead of parallel to the axis of rotation. It also might help that the TPMS valve body is rigid, where a flexible rubber body might absorb these transient forces. Or maybe I just think about this stuff way too much.

 

I know. It's me letting your tyre down just to mess with your head.

Me and a few guys on the forum were pissed that you were getting such great economy so take it in turns to pop over to your house to let one single tyre down.

 

Tire dunking at tire shop ruled out any nails/screws, or a hairline crack in the rim (happened to my buddy).

I would be more inclined that someone is messing w/ you on purpose or its a complete stranger hitting Prius' specifically, or random cars (one of which was yours). If you are in a position of power (say any level of management), it could be a subordinate(s) who hate(s) you, and did this to be funny and add some misery to your life. Does the garage have a video camera? Ask for footage?

 

Did the tire place remove the tire off the rim or just place it in a tank of water? If they just placed it in a tank, it was off the car and unloaded. Nails can sink in below the outer edge of the tire making it hard to spot. Also, with the tire aired up to 42psi and unloaded, makes the rubber seal tighter against the nail. It's only until you flex the tire that you'd get a leak. I see this all the time in the shop.

 

You have an object in your tire, probable a nail. Most of the time its a very slow leak. Park just right, however, and you have a less slow leak. Just have your tire shop pull it off the rim and fix it.

 

Ahh, that's why I recently was instructed by the mechanic, to turn the wheel all the way, so the tread was facing him. Then drive slowly in increments, upon command, while he sprayed a soapy water solution from a spray bottle, looking for bubbles. Eventually we found it. It was a small nail (looked like a finishing nail), buried in the tread, no visible sign of the head. Only the bubbles indicated its presence.

 

You're wasting your time guys. The obvious answer is that someone must be sneaking over and letting the tyre down.

Not a slow puncture, a nail, a screw, a thorn, a dodgy valve or a cracked or damaged alloy wheel, but some jealous hypermiler. It's the only logical explanation

 

My Prius needs to have the rims cleaned with a wire brush and black goop put on them to ensure a good seal at the bead. These alloy wheels tend to corrode and cause slow leaks at the tire bead. Sometimes you can see the bubbles and sometimes you can't so I just clean and seal the bead as well as replace the valve stem to cure the problem.

JeffD