Well, if she lives in Montana or something and often exceeds 100mph, it will have a significant impact; the faster you intend to go, the higher your pressure needs to be, because you need to minimize the amount of deflection.
The recommendation I received from a QA labby type that does testing for Bridgestone is to inflate the tires to the pressure printed on the sidewall.
Incidentally, oil change places may adjust your tire pressure to the car manufacturer’s recommendation as part of their complimentary service checks. So if you get in the habit of keeping your tires at higher pressure, check them afterward. If you keep track of your gas mileage (e.g. trip odometer reset every time you fill up) you will be able to tell when your tire pressure is wonky.
Uh, yeah. 88 year old mother who never goes on highways? 52 year old me who drives 2-3 thousand miles per year? I guess I didn’t mention the vertigo either… You know the term little old lady? It was kind of coined to describe both of us even though I don’t appear to have a grey hair in my head. (I may have some, but I’m ash blonde, so I can’t tell)
No 100 MPH. 
However, for those playing along at home, if you regularly go at much higher speeds, higher tire pressure may be desirable for you.
1010011010, that’s a very good point. Thank you.
Car Talk guys, Ray and Tom, have this on-going issue about tire inflation, as there is a guy who recommends that tires be inflated to the maximum pressure for:
Better mileage
Better traction in all conditions
Better braking (shorter, more responsive)
Tom and Ray only have this problem with the advice: Car manufacturers tell you what is best for each car, but they concede that all the evidence points to this guy being right, and Tom and Ray even agree with his tests and recommendations, but can’t come out and endorse them, because…well, the manufacturers don’t.
However, one cold hard fact is evident: You are much better off with tires that are over-inflated vs. under-inflated. Under-inlfated tires are dangerous (heat up faster, more vulnerable to tread separation and bad emergency maneuvering), waste fuel and wear poorly.
Well, Mom and I used a little cigarette-lighter-outlet-powered compressor to fill our tires today, and took it up to the low thirties. The problem is, the independent gauge and the gauge on the compressor read differently. So we’re going to check them on garage air when we next get a chance.
THanks again to all for your input. The Tappit brothers are always welcome.
Can you explain why? Is it simply because your measurements will be inaccurate and you’ll end up with too little air in the tire, or is there something dangerous about the act of bleeding air from a hot tire?
I ask because I’ve gone along on four-wheel drive club outings (as a passenger), and usual practice seems to be to drive a long distance on the highway fully aired up, then bleeding your tires down to the low teens when you get to the trails. In other words, they bleed air from hot tires. And these guys are pretty safety conscious.
(Also, the club I went with was primarily interested in rock crawling and the conventional wisdom was that underinflation also helps grip on rocks, since the tires can deform around irregular shapes to get more contact.)
WAG: The stem assembly on a hot tire can explode, resulting in little bits of valve stemmy shrapnel pelting you in the groin.
>recommends that tires be inflated to the maximum pressure for: Better mileage, Better traction in all conditions, Better braking (shorter, more responsive)
>Tom and Ray only have this problem with the advice: Car manufacturers tell you what is best for each car
Well, that’s the thing, isn’t it? These two recommendations aren’t contradictory. The mileage, traction and braking aren’t the only contributors to “best”. Big American cars have long been famous for soft, even sloppy and sluggish, rides. Soft tires would help. So, if the car maker thinks a soft ride is a priority, he’s going to compromise on the other points, isn’t he?
>drive a long distance on the highway fully aired up, then bleeding your tires down to the low teens
Ditto this practice for driving on beaches. The alternative seems to be waiting around the dunes counting mosquitos and looking for places to pee while waiting for the tires to cool.
BTW I’ve used bicycle tires inflated to 125 psi, and they do make for a vibrating ride. Things sing. The road feels noticeably rougher when the tar doesn’t surround the tops of the bits of gravel in the pavement. A pothole half an inch deep and six inches in diameter could be a catastrophe, if you hit it wrong. But the mileage is great. I’ve gotten about 50 miles per 1000 calories on these.
My car has two sets of recommended pressures in the manual - the second for speeds above 100mph. Only for the European market, I guess 
Well, it’s sure as all get out not directed at my mom or me!
First off go back and read the first paragraph of my post
OFF-ROAD under inflation can be beneficial. This is not true of driving on pavement.
What you forgot to mention is that when your 4 wheeling buddies got done off road, the used a compressor to air their tires back up to highway pressure before they drove off.
So if on the highway why do you never bleed of air from a hot tire? OK let me pull some numbers straight out of my ass here.
You car that calls for 32 PSI cold. Your tires are in fact at only 20 PSI. With tires at 32 PSI as they are driven on a hot day they will climb to say38 PSI. Your under inflated tires are running hot, and have climbed to 34 PSI. You put a gauge on your overheated tire and go “Whoops!” too much pressure. So you bleed off 2 lbs. of pressure.
Now your tires have the equivalent of 18 PSI cold in your tires. You are not helping your cause out here. 
It doesn’t unless you’re driving around with 5 lbs of air in one of your tires. Radial tires don’t collapse under low pressure like the old bias ply tires did.
I drove over an industrial staple a month ago and the 2 puntures ran the tire down pretty quick. I noticed it at the next fill-up and checked the tires. The punctured tire didn’t register on the guage so it was near 5 lbs. I lost 1 mpg from 32 to 31 mpg.
I check my tires regulary and also carry a 12 volt compressor in the trunk. I highly recommend this. You can get them for $20. It’s nice to be able to fill a leaking tire to get it to destination for repair. I use to carry a repair kit but I’m not keen on plugging a leak on the side of the road. I also keep a folding bike in the trunk and it’s nice to be able to fill the tires with the compressor.
It’s the max pressure to seat the tire when mounting it. Tires with stiff sidewalls have higher numbers. You do NOT want to run your tires at these pressures as you will get cupping (uneven wear that will make for a rough ride and make noise).
Depends on how you define “profound effect,” I guess. The stats I’ve seen (and the ones spurring this whole tire pressure idea, I assume) say, “You can improve your gas mileage by around 3.3 percent by keeping your tires inflated to the proper pressure. Under-inflated tires can lower gas mileage by 0.4 percent for every 1 psi drop in pressure of all four tires. Properly inflated tires are safer and last longer.” Your anecdote seems to fall in line with the numbers: a single tire underinflated by ~30psi (your other three were OK, I presume) should give about a 3% drop in fuel economy, which is 1mpg on a 32mpg vehicle. (Low precision data, of course, but it’s falling in the ballpark.)
If you reread my question, you can see that I obviously know that bleeding air from a hot tire will give you inaccurate results. What I was trying to establish was if your stern warning was an indication that the act itself was inherently dangerous. A simple “no” would have sufficed.
And if you want to pick nits, your previous post has two inaccurate statements:
(which you later qualified to general off-road use, not just soft dirt or sand)
and:
(which you later qualified to mean for on-highway use, so “under no circumstances” is a little strong)