How crucial is it to buy new tires in pairs?

The theory is that having tires with different amounts of tread wear (the new one has no wear, the old one is somewhat worn down) is inherently unsafe. It has to do with the way unmatched tires will handle—unmatched tires my pull and veer in unpredictable directions.

NOthing will happen unless she’s driving and the car doesn’t handle as well, or pulls to one side, or steers strangely.

Lots of people do what your friend did, with no consequences. But there are risks. How good a driver is she?

Critical if the tire do not match both manufacturer and size, the drive axle can and will fail because each tire is turning a different number of times per mile. The differential has to compensate and will wear out.

A tire’s performance characteristics change as it wears.

This goes from an improvement as the tire scrubs in in response to your particular car’s mass, drivetrain layout and alignment specifications to usually a very gradual decrease in performance as the tire wears normally.

If the tire opposite the new one is old enough you will lose straight line stability under braking (particularly in the wet) and a loss of traction under acceleration at the tire that is more worn.

You could also experience what we call tire pull, in other words straight line tracking will be compromised. Talk about annoying.

Unless the tire at the other side is quite new the advice to replace both was sound.

Typically, the new pair should be put on front wheel duty (especially on a front drive car).

If she decides to go ahead and replace the one on the other side as well she should do it ASAP for all the same reasons above.

Depends on the car. If you tried that with an old 4WD VW Vanagon, you would blow the transmission, but most cars are not that sensitive.

As @Tropical_Willie points out, new tires are not the same size as used tires. The difference in size puts a little wear on the differential in the transaxle. It also messes up the operation of ABS and other electronic driving aids. Some cars can handle that, some cannot. Those with 4WD/AWD will often wind up with damaged transfer cases and/or transmissions.

That said, I ran my old (‘87) Corolla with severely mismatched fronts (a 175/70 and a 185/65), but only because I knew for a fact that that thing was built like a brick shithouse. If that Corolla is an ‘88 or newer, it isn’t built nearly as ruggedly though. Simple test; drive at 35 mph and shift it into reverse; if the transmission survives, then it’s tough enough to handle mismatched tires. (That’s how I found out how tough my Toyota was; I accidentally did that!) If the transmission fails, or you just don’t have the courage to try, then buy in pairs.

@SecondHandStoke When I got a new pair, they insisted on having the worn tires on the front. Apparently it’s the law now.

@jerv

Yes, the law. Because the motoring punters of the world would shit eggrolls if they experienced anything other than understeer.

You and I both know that the typical front drive box has so much plow built in that there’s practically no chance that the car would switch ends on it’s hapless operator.

Driver license tests should include time on the skidpad.

Sigh.

@SecondHandStoke Mine actually might; stock, it’s 53/47 instead of the 62/38 of the 2014 models.

Wow; all these experts ! I think she’s a safe driver and all but she did have major foot surgery nine months ago. She just stopped using a cane.

Even the best driver in the world probably wouldn’t get an old Ford CD4E transmission (used in certain years of Ford Probe and Mazda 626) to last. Then there is the old Subaru 360, allegedly named for it’s 360cc engine, but as it (like certain Porsches) was rear-engined and RWD, “360” was more descriptive of it’s handling.

My point is, you can be a safe driver, but you still cannot overcome the mechanical limitations of your car.

The difference in tire height between a worn tire and a new tire could cause problems with a vehicle/differential that had Positrak or another locking or limited slip 4X4 type differential, like on Jeeps.
Also, tire heights do not always match between different manufacturers. A 205/70R14 TOYO might be taller or shorter than a 205/70R14 FIRESTONE. Best to buy in pairs, really. Both fronts or both rears.
I heard a report the other day that said to mount new tires (2) on the rear because the improved traction will help keep your car from skidding/sliding due to decreased rear wheel traction.
If in doubt, consult the dealer.
I have never seen or heard of a transmission or differential failure due to different worn vs. new tire wear situations. Doesn’t mean it couldn’t happen.

@kritiper “I have never seen or heard of a transmission or differential failure due to different worn vs. new tire wear situations. Doesn’t mean it couldn’t happen.”
That particular VW is the only one I know that is that touchy, but running mismatched tires (a compact spare on the front) on my 4WD Civic played hell with the drivetrain by overheating the viscous coupling in the driveshaft. It was bad enough that, within a mile, I pulled over, swapped the donut to the other end of the car, and disengaged the 4WD; something not all cars allow these days. I’m not sure what would’ve happened if I had left it, and I’d rather leave it a mystery.

I did a 180 in my Integra. Not hard to do if you get better than stock tires.

I don’t really understand @RocketGuy

Was this a first generation Integra that lacked double A arm suspension?

Was this the world beating Integra R, with the 23mm rear sway bar (USDM spec) that allowed the pilot to steer with the throttle as God intended but could actually snap oversteer if the lucky owner was not diligent at the controls?

I currently have an ASR rear crossmember brace with complimenting 24mm rear swaybar set in the less aggressive setting on my Integra.

This setup is even more rear biased than the stock R, yet I haven’t come even close to having the back step out hard without my packing it into the corner bigtime. And then it is still easily controlled.

That’s what I love about my Corollas. Even the sedans tend to be utterly controllable at all times. If you lose it, you’ll probably lose all four tires at once, but even then it’s pretty easy to control the slide. Not sure how it does that as it’s far from an Integra….

The latest Corolla:

Steal your design from Honda much?

It was a 95 Integra with stock strut front end but Yoko tires. I came into a turn too fast, and missed my shifit, so basically glided around the turn with a little bit of brake to prevent going into the iceplants. The rears could not hang on.

Ooooh! Yeah, missed shifts in/near a turn rarely end well.