Technical Speedo accuracy

[alfacool]

There has been a lot of chat about this i know,
Just how far out are these speedo,s out?.
I ask cos when driving the Alfa at 80 i seem to eat up the road, and in the cinq at the same speed shown it seems longer to overtake Ect.
My Cinq has sporting 13" alloys and the correct size tyres.
So at 80 what speed am i doing??
Cheers.

 

[RuiG]

I guess a GPS reading would give an aproximate idea of the real speed. My speedo error really increases with speed but it's in kmh so I can't really tell the values in mph.

Btw https://www.fiatforum.com/cinquecento-seicento/164081-whom-believe.html

 

[fingers99]

As a rule of thumb, I think they're maybe 8 to 10% optimistic at the top on stock tyres.

Would be interesting to get someone to drive the Alfa and Cinq together and check the speedo outputs.

 

[alfacool]

As a rule of thumb, I think they're maybe 8 to 10% optimistic at the top on stock tyres.

Would be interesting to get someone to drive the Alfa and Cinq together and check the speedo outputs.

Yeah i think the concensus of opinion is that (Abarth)14" wheels and tyres make the speedo read more accurate.
So at 80 im more likely to be doing 70ish i guess?

 

[BRAV-OH!!]

In my Bravo when the speedo is reading 80mph according to my Tom Tom I am doing 72mph.

When my speedo is showing 70mph the Tom Tom says my speed is 65mph.

Every car I have ever had has been around the same % in error at any given speed.

 

[alfacool]

Right then, took the sat nav in the Cinq up to work and back.
The sat nav said 70 and the speedo said 78,
Thats why it feels slow when i sit at 70 using the speedo.
Cos it is......

 

[mrcinquecento]

iirc, doesn't sat nav under-rate your speed?, something to do with how it calculates it between geo-stationery objects? could be wrong tho, id always say it under-reads by 2mph, just to be safe and keep myself the right side of the limit or any hidden speed cameras that seem to be popping constantly up nearby, i agree it's still more accurate than the speedo though, but its always safer to over estimate the speed than under estimate it, thats why they have to read over by law

 

[GoBe]

Satnav GPS samples the positions every 1 to 2 seconds depend on the unit. So if you're going straight and level at a constant speed it'll be more-or-less acurate. However how many roads do you know that are perfectly straight or level? When the road isn't straight and level there is an underestimate of the distance traveled between the sample points so the speed reads low.

To get around the problem you increase the sample speed, 5Hz is good 20Hz is the best you can do iirc, use elevation decoding & add in a bit of software to calculate/estimate curviture. When you do that you can be looking at less than 0.5% error. All examples of car GPS speedos I know then increase the value they get by up to 3%