That makes sense Neil. 👍
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:Yes, wider tyres have a larger contact patch so therefore more friction thus need more engine power to overcome it.
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.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.
Can you explain that please Neil, if the rolling circumference is the same I can't see how that could be?.. 🤷♂️ Unless you're saying that shorter contact patch is because of the lower pressure per centimetre due to load spreading over a greater width?Wider tyres have a wider but shorter contact patch but the area remains approximately the same...
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.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…
There are many online tyre size comparison calculators e.g.Can't remember where I saw that though it seems to make sense.
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:Can you explain that please Neil, if the rolling circumference is the same I can't see how that could be?.. 🤷♂️ Unless you're saying that shorter contact patch is because of the lower pressure per centimetre due to load spreading over a greater width?
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.the more worn your tyres, the more accurate your speedo became as the rolling diameter reduced.
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.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 would be my maths:….. but a smaller rolling diameter would do more revolutions per linear unit?
I believe we are speaking the same native language. I'm from the UK originally but been out here a long time now
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 lotI agree. Let’s go down the virtual pub and have a 🍺