Previous owner fitted
front: 225 40 18 Dunlop SP Sport 9000
back: 225 45 18 Bridgestone Potenza RE040

would this cause any problems for me?

 

Do you have DSC?

 

yes i do :D

 

The slight difference in circumference between front and rear tires would/might trigger the DSC under certain driving conditions where it normally wouldn't have.

 

^ +1. Front tire will be spinning faster. Might make DSC go "WTF is going on!? Lost of traction!"

 

The good thing is: you probably won't get DSC when you turn and accelerate hard. That's when I get it a lot.

If you ever want to rotate those 3% smaller fronts to rear.... well just don't do it or you'll know why.

 

im not sure what you mean by if i ever want to rotate those 3% smaller fronts to rear......

 

DR.TuB,

225/40r18 are 25.1 inches in diameter approx 830 rpm per mile
225/45r18 are 25.9 inches in diameter approx 804 rpm per mile

So the 40's roll 3% faster or more than the 45's for the same speed.

 

so if i were to replace the back with the 40s i should be okay?

 

Yes, as long as the circumference is not beyond 3% difference, your DSC will not pop up.

 

The 245X40X18 are a great upgrade.