Our reader wants to fit tyres that are 245/45 R18 100W on the front axle and on the rear 245/50 R19 100W. They haven’t said what car this is for. See what the tyre writing means here.
What does the user manual say?
In general, you should stick to what the manufacturer recommends in the car’s user manual. If you are replacing like with like, then this should be fine. Some car makers (often BMW) fit their cars with different sizes front and rear as standard.
Equally, hot rods use bigger wheels at the back than the front. But generally, on modern mass market models, the tyre mixture will be specific to the model and you shouldn’t deviate from what the user manual says.
What looks good might not be right
You might think that your car is going to look great with bigger wheels at the back than the front. However, there are some good reasons why you should think twice. First of all, consider wheel diameter. You can use this handy calculator to work them out.
The diameter of the 18-inch front wheels will be 677mm. Those 19-inch wheels at the rear will have a diameter of 727.6mm. That might not sound like much but the difference is 7.5%. Experts recommend that the difference in diameter between wheels is no more than 3%. Your proposed wheel change would be more than double that.
And your car’s suspension might not be set up for the bigger wheels at the back. You need to check this out.
It could have an impact on the electronics
Modern cars are full of sensors. Arguably some of the more important of these are the anti-lock braking ABS sensors in the wheels. These enable heavy braking without the wheels locking and losing control. To do this, the sensors feed a computer that calculates how quickly the wheels are revolving.
Put different sized wheels and tyres on the front and rear axles and the wheels will be revolving at different speeds. This will confuse the sensors and vital safety systems may not react when you need them to.
Can you have different profile tyres on front and back
In order to fit tyres for 19-inch wheels, you would need… 19-inch wheels. You might be able to get the same pattern of wheel in 18 and 19-inch. But if they’re provided by car makers, it’s unlikely. So, you’ll probably need to provide four new wheels – which is going to be costly.
When you can’t mix and match
It’s illegal to have different types of tyre construction on the same axle. That means you can’t have one crossply and one radial tyre at the front. Equally, it’s not recommended that you have tyres from different manufacturers on the same axle.
And you certainly shouldn’t mix winter and summer tyres on the same car, let alone the same axle.
Tyre makers recommend that you fit the same brand and type of tyre across each axle, which means replacing at least two tyres at a time.
You can also mix tyres with different tread depths. However, tyre makers recommend that tyres with the greater tread depth go on the rear axle. But some car manufacturers will suggest fitting new tyres to the front axle. Again, check the user manual for your car.
I’ve been writing about cars and motoring for more than 25 years. My career started on a long-departed classic car weekly magazine called AutoClassic. I’ve since pitched up at Autosport, Auto Express, the News of the World, Sunday Times and most recently the Daily Telegraph. When I’m not writing about cars and motoring, I’m probably doing some kind of sport or working in my garden.