Speed v Distance

oldagetraveller1

It's easy - Distanceraptor divided by timeraptor = Velociraptor.

Goudy

Mutton_Geoff said:

Goudy said:

It's become far easier to program the car with the correct wheel/tyre size, where in the past it was an expensive job to calibrate an analogue speedo for the same model cars that were available with different size wheels/tyres.

I don't ever remember a time when a speedo needed calibrating to suit wheel/tyre sizes although I've only been driving & buying vehicles since 1974. Whenever different size wheels are used on a car, the tyre size, specifically the profile is changed to compensate so the rolling diameter is almost the same (certainly within calibration limits of a speedo).

If it were any different, then cars might look daft across a model range having huge gaps in the wheel arches on the 1.1L model whilst the 3.0GT looked more buxom.

https://www.willtheyfit.com/

It's not the speedo head unit that needs recalibrating, it never did.
And it's not just the wheel/tyre sizes either that makes a difference, but we'll come back to that.

For example, your modern day Range Rover can come with either 20" 21", 22" or 23" wheels.
Depending on model, it could have different tyres, but the 23's were available with 285/40 23's.

You could also buy one with 21" wheels and were available with 275/45 21's.

That's over 4% difference in rolling distance. Times that up to 70 revolutions a minute or more and that's a considerable difference.
The smaller wheels would cause the speedo to over read outside of tolerance or the larger wheels would under read beyond tolerance, which as we know there is no tolerance for that.

With a modern car, the ECU will be programmed for the particular size and the computer sends the worked out signal to the electronic speedo head to display, simple.

Older, analogue cars used a gear on the side of the gearbox to drive a cable that turns the speedo head.
And depending on a few things, that could be altered.

In the example above, it would be likely a different speedo drive gear would have been fitted at the factory when the car was ordered with different wheels or final drive ratio (we'll come to that).

The speedo head unit would be the same, they just changed a cheap, small part in the speedo drive at the gearbox.
The smaller wheeled car would likely have less teeth on the drive gear than the car with larger wheels to catch up with the different wheel speeds.

Off hand I think they had between 20 tooth to 23 tooth drives as standard on older Range/Land Rovers depending on wheels (and axles) but as mentioned, there is something else that effects old analogue speedo drives and that's the final drive ratio.

It's an extreme example, but I'm sure most of you have come across a line of army Land Rovers on the motorway.
All doing 48 mph yet as you pass, the engines are screaming their head off.

Both the army and civilian versions have the same engine (usually either 2.5 NA or TDI 300) they all have the same gearbox, transfer box and speedo head unit, but the army version has different axles than the similar civilian version and the final drive ratio of the diffs are a lot lot lower.
The difference between final drives is something like 3.5 to 1 and 4.7 to 1 which is a massive difference in wheel speed to gearbox speed.

This would obviously effect the speedo drive from the gearbox, so the army version has a different speedo drive gear.

A not so extreme example might be something like a sporty version of a normal car, say like the Golf Gti.
It might share the same engine, gearbox and speedo head unit with other versions or other cars in VW's line up, but the ratios inside the gearbox, particularly the final drive ratio could be a lot different.

All they would have to do to change the small cog in the side of the gearbox to compensate and not build a completely new speedo head unit or completely new gearbox just because the wheels and final drive ratios are different.

Normally, most owners and drivers would never know or have to bother with the either type of speedo configurations as they are done at the factory, but there are some people that like to mess around with final drives and wheel sizes.

They might want to lift a 4x4 and fit really large wheels and chucky tyres or gear down a  car to give it better acceleration or low down torque, like the army did (at the cost of some top speed).

Then they might need to think about changing the programmed wheel size or change the small, cheap gear inside the speedo drive at the gearbox.