I'm about ready to put new tyres on my wife's 2006 XJL. My question is can I use the 245 50 H18 size. The car originally came with 235 50 H18 Continental Pro-Contact as OEM. This is the only size that's close to OEM if I decide to go with the Michelin Primacy MXM4.

 

According to this tire size calculator

Tire Size Calculator - tire & wheel plus sizing

The 235 tires are about a half inch smaller in diameter. So your speed and mileage will register about 1.5% lower than actual.

I would expect the ride to be improved slightly as well due to taller sidewall height.

 

Thanks for your input.