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Yes, wider tyres have a larger contact patch so therefore more friction thus need more engine power to overcome it.
Wider tyres have a wider but shorter contact patch but the area remains approximately the same for the same car with the same weight and tyre pressure. The degradation in performance/efficiency comes from the increased frontal area of the wider tyre and the added weight of bigger wheels with the extra power it takes to turn and accelerate them. Car and Driver did a nice article explaining quite a lot of this:

Effects of Upsized Wheels and Tires Tested

These are all small changes but fuel economy will be noticable, especially if one is a high-mileage user. Higher friction might come from the widest tyres also being softer models for extra grip (as happened in the linked test), not from the size of the contact patch itself.
 
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The speedo takes a reading usually from the gearbox and that is geared or electronically calculated to display on the instruments. The mechanical difference at the tyre doesn't change. If a tyre has a different rolling circumference, it will affect both types of speedometers in the same way. The two types might differ in accuracy but as they are designed to over-read anyway, it isn't a big deal if you use an approved tyre size.

I'm guessing that the electronic instrument is more accurate, as it is easy to monitor the speed accurately and then just add a margin at the display. The old-style cable driven ones seemed to be more inaccurate as the speed increased, possibly something to do with the way the rotating drive was converted to a needle position.
I have the Digi hud app on my phone and according to that, my X reads about 5/6 mph over at speeds above 70. 78mph is 72mph according to the GPS.

One of my old bikes would apparently go off the clock, pretty impressive when your speedo maxes out at 200mph.
 

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Yep, when I plugged my old Garmin in, it showed 67mph against a 70 on the Speedo. I recall that speedos were traditionally set a bit fast in days of old to take tyre wear into account: the more worn your tyres, the more accurate your speedo became as the rolling diameter reduced.
That said, my student-days MG Midget cable speedo bounced between 20 and 50鈥
 

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I think you'll find that the main reason speedometers read 'over' is that manufacturers don't want you suing them if you get a speeding ticket etc, the law stipulates that a speedometer must be within a certain % of accuracy and they get that very accurately in their favour. (A cynic, like me, would also surmise that it helps their 'official' MPG claims... ;) )

On the X250 the speedometer reading can be changed from (IIRC) 103% of speed to 100% of speed via SDD. The speed is calculated from feedback from the ABS sensors (two I think) on an X250.

Wider tyres have a wider but shorter contact patch but the area remains approximately the same...
Can you explain that please Neil, if the rolling circumference is the same I can't see how that could be?.. 馃し鈥嶁檪锔 Unless you're saying that shorter contact patch is because of the lower pressure per centimetre due to load spreading over a greater width?
 

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Yep, when I plugged my old Garmin in, it showed 67mph against a 70 on the Speedo. I recall that speedos were traditionally set a bit fast in days of old to take tyre wear into account: the more worn your tyres, the more accurate your speedo became as the rolling diameter reduced.
That said, my student-days MG Midget cable speedo bounced between 20 and 50鈥
Mate of mines first job, back in 1983, was to build speedos for AC Delco, to go in Vauxhalls. As back then 120mph was about all they could do, he said the accuracy went out the window the faster it showed.

I once swapped the instruments in my GL Capri, my speedo did the same as yours, I had no idea how fast I was going :LOL:
 

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As far as I remember if you go from official Jaguar 18" wheels with the specified tyre to official Jaguar 16" wheels, also with the specified tyre for that size, the outside diameter of the wheel and tyre combo doesn't change. Can't remember where I saw that though it seems to make sense.
 

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Can you explain that please Neil, if the rolling circumference is the same I can't see how that could be?.. 馃し鈥嶁檪锔 Unless you're saying that shorter contact patch is because of the lower pressure per centimetre due to load spreading over a greater width?
Because the tyre deforms at it point of contact (it isn't a rigid cylinder), the contact patch is effectively an ellipse about the length of the width of the tyre. Widen the tyre and the ellipse gets longer but thinner as the same weight is pressing on the tyre. This explains it better than me:

 

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the more worn your tyres, the more accurate your speedo became as the rolling diameter reduced.
The other way round, I would expect - the more worn a tyre, the smaler it is so the less far it goes per rotation. However the speedo still reads from the number of rotations per unit time.
 

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As far as I remember if you go from official Jaguar 18" wheels with the specified tyre to official Jaguar 16" wheels, also with the specified tyre for that size, the outside diameter of the wheel and tyre combo doesn't change. Can't remember where I saw that though it seems to make sense.
This is the usual choice. A car is designed using a particular tyre diameter. All option wheels should maintain this nominal size. Some manufactures have wider choices but it is not the rule and the biggest tyre must still not have an under-reading speedo.
 

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The other way round, I would expect - the more worn a tyre, the smaler it is so the less far it goes per rotation. However the speedo still reads from the number of rotations per unit time.
鈥.. but a smaller rolling diameter would do more revolutions per linear unit?
 

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鈥.. but a smaller rolling diameter would do more revolutions per linear unit?
This would be my maths:

A assume that the speedo is 100% correct.
With new tyres or old tyres, let's assume that 1000 rpm of the rotating element being used as the speed source equates to 100 mph on the speedo as it is independent of the actual tyre diameter.
With new tyres of the correct size for the car, when the speedo shows 100 mph, the car is actually doing 100mph.
Let's now assume that the older tyre has lost 1% circumference due to wear. Now at 1000 rpm, the speedo still shows 100mph because it is geared from the same rotating shaft. However, the car has travelled 1% less distance. In an hour, 99 miles not 100 miles. So the speedo becomes progressively less accurate as the tyres wear.

Both new and old tyres do the same number of revolutions at a displayed speed, but the old tyres would need to do more revolutions to do the same distance in the same time. As the tyres wear down, the overall gearing of the car also goes down.

Is there a flaw in my thinking?
 

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No Neil.. you were both saying the same thing as far as I could see.. the same as you said the same thing as I said..

It's part of the joys of interaction between humans using different native languages... (y)


And therefore it should be obvious that it's all @JohnW's fault... :)
 

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And therefore it should be obvious that it's all @JohnW's fault... :)
I鈥檝e been married 40 years to Missis W: I鈥檓 used to that! 馃榾

鈥︹ if in doubt about rolling radius versus road speed, take a look at an ickle trailer wheel on the motorway at 70mph. Woooooosh!
 

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No Neil.. you were both saying the same thing as far as I could see.. the same as you said the same thing as I said..

It's part of the joys of interaction between humans using different native languages... (y)

And therefore it should be obvious that it's all @JohnW's fault... :)
I believe we are speaking the same native language. I'm from the UK originally but been out here a long time now :)

But we weren't saying the same thing. John said "the more worn your tyres, the more accurate your speedo became as the rolling diameter reduced".
I said exactly the opposite. One of us must be wrong, but its not an important difference so it doesn't really matter :cool:
 

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I agree. Let鈥檚 go down the virtual pub and have a 馃嵑
 

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I agree. Let鈥檚 go down the virtual pub and have a 馃嵑
Great idea! I do miss UK beer though. Never did really get completely into German Pils. Czech Pils ain't too bad. However, the boom in craft beers and IPAs in particular has helped quite a lot :)

My daughter and I were in Aldi yesterday and against all planning (we have a lot of decent wine in the cellar) we bought some promising looking red wines to do a mini tasting. As usual, they were a little better than their prices would suggest but still pretty basic stuff - perfectly drinkable but a bit one-dimensional. Should have just gone and got a bottle from downstairs, oh well, I guess I'll never learn 馃嵎
 
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