Tire size for this car is 255 35R 20. I am contemplating going to a 45 or 50 R to improve the ride. I know that the speedo will be affected. Is the speedo a relatively easy fix and how? Is there sufficient room for this increase diameter? Any thoughts would be appreciated.
RFG

 

RFG, there are 2 fixes to your said problem. Before you spend the money, I would be looking into whether the tires would even fit on your car. You may find that you will need to step down to say a 235 tire to get up to a 45 or 50 profile. Then you have the whole issue of whether the rims will support that narrow of a tire. But, that is a separate issue.

As for getting the speedo to read correctly, your 2 fixes are this:

1) the harder way is to install a speedo correction box. These are sold at 4x4 shops to help trucks get their speedos back reading correctly. In short, what the box does is it takes the number of pulses seen from the wheel speed sensors and based on how you have the box set up, it will output a different number of pulses. With a little bit of setup and testing, you can make your speedo read correctly. The big thing with this box is making sure that all 4 wheels are set the same. Otherwise, you may get ABS issues because the computer is going to see your wheels spinning at different speeds. This allows for unlimited adjustment of tire size. The downside is that you need to cut into the wiring for all the speed sensors to provide the input and output of the box. I have installed one of these and the install really is not that bad. The big things are getting the setup of the box correct and doing the splices where you can protect them from the weather or doing something to cover your connections so they are water tight.

2) the stock computer has a value in it that relates to the number of revolutions that you tire makes for every mile of travel. By adjusting this number up and down, you can recalibrate the speedo. The issue is that this has a very limited range and if you change the diameter of the tire too much, the computer will not be able to adjust enough to get the speedo correct. The other negative is that you have to take it to a place that has an SDD computer so they can get into the factory computers and adjust the value. This is going to come at a cost unless you have access to one of these computers.

 

You should have posted your inquiry in the X358 Forum, which is for your 2009 XJ Vanden Plas. This is the X351 Forum, for the current XJ.

 

I think it would be way easier to buy a smaller rim, like a 19” or 18” set to allow yourself to add more tire sidewall without changing the speedometer. I own a 19” set and a 20” set for my car, partially for this reason and also to have a set of spares.