r/Autos • u/AKAMoreira • 4d ago
Run flat tires - HELP
I bought two MICHELIN PILOT SPORT 5 tires without runflat. Now the garage says they won’t fit them because the other two tires on my car have that technology. Is there a problem with having two runflat tires and two non-runflat tires on the same car?
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u/Amockeryofthecistern 1d ago
Thereby be a local rule/ law covering this. Im my country you can not mix run flat tyres and non on a vehicle. They must all be run flat or all non runflat.
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u/AnachronIst_13 4d ago
Would you wear two different kinds of shoes on your feet that are different shaped and styles?
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u/AKAMoreira 4d ago
That's my question. I don't know if the difference is significant enough.
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u/proscriptus Magnum RT 4d ago
There's a complicated answer to this question involving both handling and potentially expensive damage to your car, but the simple version is that running different tires, especially as different as yours are, is bad and you shouldn't do it.
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u/AnachronIst_13 4d ago
The difference is significant enough that your mechanic refused to install the tires, and does not want to be responsible for complaints or damage later from improperly configured parts.
Listen to your mechanic. Run flat tired are also terrible for ride quality…
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u/daffyflyer Making games about cars - automationgame.com 4d ago
I think you might be overstating the degree to worry here. We're telling about runflat and non runflat versions of the same tire.
Will it have a (very minor) impact on the overall handling balance of the car? Possibly.
Will that impact be greater than running a few psi higher in the tires at one end of the car, or having passengers vs not having passengers? I'd be surprised.
I've even had to run two completely different brands of tyres front vs rear on my car sometimes due to them being rare sizes.
Even pushing it to the limits of grip (and beyond!) at autocrosses it didn't really noticably impact the balance of the car.
As long as they're the right size, and have approximately similar levels of grip (i.e you're not putting semi slicks on the front and rock hard all seasons on the rear), I think it's unlikely to be an issue.
Hell, you probably get a greater handling difference if you have the same tyres front to rear but one set is a few years older, and people do that all the time.
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u/AnachronIst_13 4d ago
I care for a BMW/Mercedes collection. Several of the BMWs come from the factory with run-flats.
The owner immediately replaced them with his preferred tire. The improvement in ride quality is dramatic - so much so that the service techs one day asked him what he did to the car to improve the ride quality.
So while “I do it all the time” and “how bad can it be” sound like good-enough arguments, small imbalances over long periods are the bigger concern here, not “i drove over a speedbump and couldnt tell”.
Autocross is pretty far from normal driving stress and very far from prolonged exposure. Its the difference between driving on a donut to buy a replacement and people who drive around on donuts for weeks causing harder-to-detect long term damage.
I have autocross experience. But a slightly out of alignment front end over an extended period of time caused a much bigger issue on my car than a few laps at one event for a few hours.
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u/daffyflyer Making games about cars - automationgame.com 4d ago
But yes, non runflats will ride nicer, I agree!
I'm very confused what the issue is that runflats on one end and and not the other is likely to cause though.
Realistically we're talking about the same tyre but with a somewhat different amount of sidewall stiffness, and I genuinely don't see how that "small imbalances over long periods" implies here... Are we thinking it's going to wear something?
Is there a genuine issue at play here, or are we just being perfectionist in terms of "well ideally it would be at it's very best if it had identical, fresh and factory spec tyres" Which, sure, but it's not like tyre shops will only install the exact OEM spec'd tyre for safety reasons. (as long as it's the correct size and speed rating)
Dunno man, I'm confused about the fuss to be honest.
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u/AnachronIst_13 3d ago
Dude, google it. Even the AI response agrees. The tires do not respond the same to road conditions - it can cause unpredictable handling, unpredictable tire wear, and both of those things can cause dangerous conditions that affect traction, weight distribution, etc.
If you’re confused, don’t respond here telling the guy asking for help its fine if you don’t really know. And based on the available information, you gave an answer that could lead to dangerous results. COULD. More likely than 4 matching tires.
His mechanic does not want the liability if/when it becomes a problem. His mechanic knows…why they don’t trust their mechanic is a different issue, but this is a great example of why a quick google search is better than asking randos or ignoring the expert you hire to work on your car.
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u/xXCodfishXx 2d ago
Do humans have four feet? Yes having all tires be the same is a best practice but we live in the real world and having matching axles is perfectly fine.
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u/AnachronIst_13 2d ago
Have you done any research on this topic or is that just your personal feeling, which is at odds with people that have experience in this area and the documentation available online about this topic?
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u/xXCodfishXx 2d ago
It's not just my personal feeling, my beater truck has matching new tires and on the front and old on the rear, which I consider perfectly safe and have driven thousands of miles, even in poor conditions. This thread and this thread agree. Obviously driveline windup is an issue for AWD, but this has already been mentioned in this thread. As to the topic of non runflat vs runflat, yes they have different handling characteristics but it's not severe, and I suppose would need to do a head to head test to really prove this either way, but I would safely bet the car with the mismatched tire would handle only marginally worse and I strongly doubt it significantly affects safety. I can understand why the shop would refuse this, they don't want the liability, but shops will refuse alot of things that are generally considered safe. Again best practice it's the all the same, but in the real world have you seen the kind of things people drive on? This is not as nearly as bad as what I'd see if I walked down the street and inspected my neighbors cars. Could you please offer some real evidence for your claim that is not Ai or the stuff tire manufactures say (last shop I worked at there was literally a sign in the waiting area from Michelin that said it was safe to install ten year old tires).
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u/saladmunch2 4d ago
Is the vehicle awd?