What's the speedo error and how about tire
 

Just a couple of questions regarding the speedometer and tire size. 1st for those who use a Garmin type GPS what have you seen as the speedometer error using OEM size tires. This would be for the 4.2 and the 5.0 cars and what size tire was your OEM and if you up sized what is the new tire size. I am not considering tire wear as this is insignificant in the scheme of things but just the various tire sizes. So is 60 mph actually 60 mph or is it 59 , 58 or what. By the way so far I have seen 285/30-20 , 295/30-20 and 305/30-20 being discussed for the 5.0's What would be the error be.

Oh I've read the charts and see the revs per mile differences but I don't have a GPS method to read actual MPH. I guess that is the 1st place I would be interested in seeing how close the speedometer is to the GPS speed.

Can't say for the XKR as I have not checked, yet. I will next time I drive it.

I did check the speedometer in both the '12 XF and the '12 XJL when I switched tires/wheels between the two cars, trying to see if the ride issues with my XJL were wheel/tire issues or not. They were not.

In the process I checked both car's speedometers using my Android phone where I have several speed app. The XJL tire diameters are exactly 1" larger than the XF wheel/tire diameters. Prior to switching both car's speedometer checked in to show 2-3 MPH faster than my GPS-driven phone apps. They both indicated 72 MPH when the GPS clocked 69 MPH. After I placed the larger wheels/tires on the XF its speedometer was dead-on accurate.

Not surprising as over the decades I found that most car makers make their speedometers read "optimistically". I had never seen one that would go the other way and show less than the actual speed. Italian exotics were the worst, often being off close to 10MPH at speeds over 100 MPH.

Mine reads about 2 mph high at motorway speeds. So, if I set cruise at 87, GPS says it is doing 85.

Here's a tyre calculator which will tell you the change in speed reading if you alter the tyre size:

Visual Tyre Size Calculator | Kouki Tech

Thanks and already had the tire calculator. By the way what size tires are you running. I just wanted to try to get a sampling of the difference of the speedometer and a GPS speed reading. 2-3 mph is pretty good .

I have the stock 295/30R20 on Vortex wheels. Until Tuesday, when I will have 305/30R20s - that should reduce the difference by about 0.9%. But I'll let you know once I've taken the car on the motorway.

Interested in hearing how the wider wheels feel and perform on your beast :D

They're only about 3.5% wider, so I doubt I'll notice anything. Unfortunately, I couldn't get MPSS in that size at all, so have had to go with Pirellis again (I need to have the car sorted before this weekend, and it is an MOT failure at present). Not that the P-Zero is a bad tyre, I just wanted to try the Michelins.

Mine shows a 6 percent error, so 90 indicated is 84 actual (GPS)

Ok, the result of upgrading from worn 295s to new 305s is: under 1 mph error. If i put 86 on the cruise, I get 85 on one GPS and 86 on another.