Speedo V Satnav

[Wooie1958]

I have noticed that when I drive at 50mph on my speedo my Satnav says I am only doing 46mph.

Which is correct??

That`s the same difference as i get between my Speedo and Satnav.

I normally split the difference ( i.e. 48mph ) and assume that`s the speed i`m actually doing.

Never had a problem and more importantly never had a Speeding Ticket.

 

[Teutone]

I do understand tyres. Believe me, different aspect ratios notwithstanding, the wheels on this one (17" rims) fill the arches much more than on the last one (15" rims). Because, although the aspect ratio is indeed lower, they're also much wider, and the aspect ratio is a percentage. Sort of. And they still wear out, and the diameter can't help but reduce then, can it?

Maybe that was a bad example. Point is, the speedo can't take account of the many factors that can affect the diameter of the wheel and tyre combo. Wear, inflation pressure, load, temperature, tyre type (M&S or summer), all these things change the speedo reading.

If you replace a 15" tyre/rim with a 16" tyre/rim you will end up with the same outside diameter.
Unless you use the WRONG aspect ratio on your replacement/upgrade tyre. All the tyre sizes I have listed have more or less the same outside diamter or rolling circumference and can be used without altering the speedo reading or the gearing outside allowed tolerances.

Some manufacturers use 16" (with a larger tyre) instead of 15" on bigger engined vans to be able to use the same gearbox and diff ratios. With a correction of the speedo of course.

I agree that a speedo is not able to compensate for wear, pressure and even same size tyres from different manufacturers are slightly different. But taking the correct tyre size, will have minimal effect on the reading.
To account for all the above differences, the speedo always over read a little to make sure that one is never driving faster than the speedo shows.

So if your "new" wheel/tyre combo fills the arches out more, they sold you the wrong size.

There is a good read here under the "Error Section" Speedometer - Wikipedia, the free encyclopedia
google for "tyre

 

[johnkyte]

All comments here are good but let me add,,,
If you work out your MPG on the mileometer reading of your speedo---which is usually fast,,,then you are not getting as many MPG as you think

My old 89 Transit ambulance When doing 56 shows a speedo reading of 70 ,,,not only illegal but a nuisance

I guess the diff or gearbox has been changed at some point