So couple months ago I purchased a nice set of Rainsport 3 275/35/20 for the rear of my XJ X351, not knowing they don't make them in the front size 245/40/20.
However they do do them in 255/40/20, I have the 9inch KASUGA alloys will going up to a 255 tyre on the front fit and look ok?

 

WHen it comes to the electronics of the car, you will have a 1.4% error between the front and rear (where stock you had a 0.4% error). So, you are adding a 1% error to the system. This is within tolerance (this is here to allow you to turn corners and not get error codes). So, from the mind of the electronics, all will be good. The only "problem" is that you will end up with a 0.5% error in your speedo (showing slower than you really are going. But, if you are doing 100 KM per hour, the speedo will show 99.5 KM per hour. So, you probably won't even notice that.

As for fitment on the car, that also should not be an issue as the 255's are the stock tire size for the XJR as I recall.

ON a side note, going with say 255/35R20's would have a bigger error than sticking with the 255/40R20's (-2.2% error vice +1.4%).

 

Thanks for the working out, most helpful. I can get 245/35/20 but I figure I'd rather gain width than loose profile plus the lower profile would create more of an arch way gap between the wheel and the car.