Mismatched brand tires on back, bad on high speeds?

Question

Wouldn't post a new question if the internet wasn't full of "Yep, perfectly fine" one one side and "Nope, you're going to hit the closest tree" on the other side.

Driving a FWD Volvo S40 2.4i. Wearing a set of 205/55/R16 Hankook Ventus Prime 2. One of my rear tires got a side bubble and no stores around want to sell only one tire.

So can I get away with buying a pair of used tires of another brand and place them on the rear (maybe even different size like 215/50) and still drive safely?

Usual city driving should be fine and I'm not a fan driving fast on busy roads, but what about the highway cases when I'm going around 70-100mph (100-160km/h), are mismatched tires going to require a couple additional prayers before I turn the engine on ?

Answer 1

If they are matched brand across the axle you'll be fine for the sort of driving you describe - assuming the new rears are of comparable quality to the fronts. If you put ditchfinders on the rear you might find it a bit unpleasant in the wet!

Regarding sizes - the OP mused in the comments about putting 225/50/R16 on the rear with 205/50/R16 on the front.

The suggested change actually provides a 2cm difference (it's 1cm in radius so double it for the diameter) which doesn't sound a great deal but it can potentially affect the vehicle's rake in a way not anticipated by the aerodynamics designers (which could be a big deal at high speeds), while an increased rake angle isn't a de facto bad thing (and in many cases can be quite positive) without seeing the changes to airflow (such as in a wind tunnel or CFD) you can't predict what will happen. The flow over both the top of the car and beneath will change and even small changes in angle can lead to quite dramatic differences.

this then also affects a) the spring rate (you've got 2cm more of sidewall), b) the circumference changes by over 6 cm so the revolutions per mile shifts by over thirty - so you are getting significantly difference wheelspeeds between the front and rear axles.

The ride height of a normal road car is expected to vary - it changes as the weight in the back changes (passengers luggage etc) but this is in the opposite direction - and these scenarios will have been accounted for by the designers of the car. It's difficult to create a scenario where the ride height at the rear will actually increase beyond the original design specs through normal use.

Of course it could be fine and you would never have a problem - personally it's not a risk I would take.

The taller tires on the rear also affects a) the spring rate (you've got 2cm more of sidewall), and b) the circumference changes by over 6 cm so the revolutions per mile shifts by over thirty - so you are getting significantly different wheelspeeds between the front and rear axles, while an FWD car doesn't have to worry about this causing wind-up in the center differential like a 4WD does this change in wheel speed is going to make any stability control, traction control and ABS systems potentially very confused.

The increased height could potentially change the angles for the rear control arms (depends on suspension layout) affecting camber at the rear giving negative handling affects and/or uneven tire wear.

Basically don't use mismatched sizes unless the car is designed for it (such as RWD BMWs, Mercedes' etc)!

Answer 2

If it is winter and you have summer on the back and winter on the front then you will definitely notice. Summer all round and not four wheel drive will be ok.

If it was 4 wheel drive then keeping the tires matched is important depending on the 4wd drive system - and Volvo is one known for that.

Answer 3

This was meant to be a (series of) comment(s) on @motosubatsu's answer, but it was getting long to the point where I didn't want to split it into 4 consecutive comments. I'll mainly be addressing the issues that he pointed out. (which are valid concerns, but are simply not significant)

I'm going to be the guy on the internet that says "Yep, perfectly fine" and argue that the difference in tire height will make a near-zero, and certainly negligible difference in suspension geometry, spring rate, and aerodynamics.

1. Simply put, suspension geometry is dictated by the position/angle of the suspension arms and the load on the system. The only difference a taller tire would make is a tiny shift in weight distribution front to rear (equivalent to driving down something like a 1° slope).
2. The same can be said about spring rates - wheel rate is determined by the spring stiffness multiplied by the wheel motion ratio, as described here. As for the "tire sidewall spring rate" itself, that can easily change with the sidewall construction of the tire alone.
3. The majority of road cars are designed to have minimal drag, or even generate lift at higher speeds. They don't rely on aerodynamics for stability. If they were that sensitive to aero, then rear suspension would be much too stiff (it's easy to load a car to lower the rear by 2 cm), and bike racks would be banned on highways.

At the end of the day, you're far more likely to feel the influence from the difference in grip level front to rear. I would simply recommend getting a tire with a similar grip level to the fronts, or slightly increased grip. This would fall in line with the way manufacturers typically default to understeer for safety. The brand does not necessarily matter. Would I run the set up forever? No. But if it's the most affordable/realistic solution to use until the next tire change, then I would say it's no big deal.