Is buying only one new tire okay?

You can not use a tape measure for that. The numbers printed on the sidewall tell you everything you need to know about the tire.

Make all tires have the same pressure.

 

If you aren’t replacing all four, the next best would be replacing in sets of two. (front two same age and back two same age) i’m not sure how drastic the difference between a new tire and one with 40k miles on it, but i’d imagine it’s enough to make it worth replacing two at a time. if not, just keep up with rotations.

my tires have just over 40k miles on them too right now, so reply with anything u learn!

 

Like 185/65 15 215/45 17 all four should be same the garage should know the proper tyres per SE.

 

And that’s why he works at the gas station and not in a tire store.

 

This is a valid way to sell more tires because if your left driveshaft tire has slightly less circumference than the right driveshaft tire it can put more stress on the gearing similar to if you only drove your car in a small circle all the time. Of course cars are designed to handle this, and it's leaning too close to being BS, but it's a relatively logically defensible way to sell more tires.

But when it comes to tires, there's an incredible amount of waste and people are too quick to replace all four tires all at once for the sake of convenience rather than to slowly rotate them out one at a time based on tread wear.

 

The circumference of of a brand new tire compared to the circumference of an identical tire at same psi that's tread is worn out can indeed be measured accurately with a measuring tape. It's a significant difference...

 

The circumference of a tire doesn't equal the distance it rolls per revolution under load.

 

It's not a huge deal, don't worry about it. If possible, measure the tread and match it up with the next deepest one on the car and then keep those two on the same axle through your usage & rotation pattern.

That keeps your traction about right when driving in deep water / near hydroplane situation. That's your first concern here, so that you don't suddenly lose directional stability because of standing water.

 

40k’s getting on. Check the remaining tread depth of them, post what it is. Most automotive stores have tread depth gauges.

My 2 cents: getting close to replacement time.

 

40k is close to replacement. The tires on my new car from 2019 only lasted a little over 30k miles. So if I were you, I would ask them to put one more new tire on so you'll have a pair. Replace in pairs.

 

It won't hurt anything but it's a waste of money. So when the other 3 wear out, what are you going to do then? Probably replace all 4. I would have bought a used tire and in 20-25k miles replace them all.

 

That exact example is leaning close to being BS, but something very close to it could be what buddy Joe is thinking of for stressing the drivetrain.

If you have a circumference difference between left and right on the same axle, or drive in a circle all day, that difference is taken up by the differential. A plain differential like what's in the Prius literally doesn't care; it can do that all day. It's gears spinning in oil, doing what they were put there to do. It's fine.

At a time in automotive history there were things called limited-slip differentials, where friction elements were built in and would be involved while turning corners (or with a left-right circumference difference), and for those there would be more of a reason to avoid long periods of that.

But most importantly, back in the day, there were some vehicles built as 4WD, where the transfer case between front and rear axle did not have any differential function. My old Bronco II was like that, The front axle had a differential to allow different speeds left and right, and so did the rear axle. But any overall speed difference between front and rear would have no place to go; it would just build up twisting stress on the driveshafts that could only be relieved by a tire losing grip and sliding a little. In wintry conditions that was ok, because the tires would be sliding a little anyway. But you had to remember to disengage the 4WD before driving on clean dry pavement. If you forgot, and the tires were a little different in size on dry pavement, the drivetrain could wind up so tightly you then couldn't disengage the 4WD, unless you could release a wheel hub or jack one corner up to relieve the stress.

Happily, absolutely nothing in a Prius, even an AWD Prius, works that way. The AWD uses a separate motor in the rear; there is no front-to-rear mechanical connection to be stressed in any way.

The main thing that matters in a Prius (whether it's FWD or AWD) is to keep the four circumferences close enough to each other to avoid trouble codes from the skid ECU about wheel speed differences. It's not a matter of mechanical stresses building up, but just of the ECU reaching a limit on how much speed difference it will accept without thinking you're skidding.

There is an extra-sensitive ABS wheel-speed sensor test mode that the repair manual explains how to use, whenever work has been done, like hub replacement, that could affect the speed sensors. If you have any question about whether your new tire is near enough in circumference to the others, a short drive in test mode should answer it quickly. If the skid ECU is happy, you can be happy too.