My 2016 XJL originally was fitted with 20” wheels. When I purchased it, it had 19” wheels all around, with 8.5” wide rims. Will the smaller wheels have an affect on the indicated speed, or are all tires, regardless of wheel size, the same diameter? Thanks.

 

The rolling diameter will be the same regardless to the wheel size because the actual profile of the tyre will be taller with the smaller wheel.

 

Try this site: -

https://tiresize.com/comparison/

 

DonJ, the best advice that I can give you is if you question the speedo, you can check the calibration yourself. It is really easy to do. Get out on the high way and set the cruise for some speed (actually speed doesn't matter, but it is easier if you set it to a multiple of 5, 55, 60, 65, etc). Now, as you pass a mile marker, start a stop watch. Drive for a number of miles (just note how many miles you drove. As you pass the final mile marker, stop the stop watch. Convert the time to seconds. Now, divide that time into 3600 and then multiply by the number of miles that you drive.

So, using my method, lets say you set the cruise at 60 mph. YOu do a 5 mile drive and it takes you 4 minutes and 55 seconds to drive that distance. So, 4minutes, 55 seconds equates to 295 seconds. 3600 gets divided by 295 and the multiplied by 5 for a final value of 61. In this case, your speedo is off by1 mph (slow) when travelling at 60 mph. So, there is a 1.7% error in your speedo (ie, (60-61)/60). If your speedo is going faster than actual, then your tires are smaller than what came factory. If your speedo is slower than actual, then your tires are bigger than when the car was new.

 

Even easier is just about all phones have a GPS speed somewhere. Monitor that and drive around some to see how they compare.
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I think you should be good if the wheels and tires are all OEM spec. I went from 19s to 20s on Jag wheels and the correct matching spec tires with no issues at all. If non-OEM, then your mileage may vary, literally!