Calibrating a Speedo?

ironclaw

Hi Folks,

Was doing some thinking. I drive a Toyota with a digital speed read out. Now, from doing some tests with multiple GPS's and pacing, it seems the speedo is off by almost 8km/h at 120 km/h. Apparently this is done for safety which I can agree with. However, can a speedo be calibrated? Obviously the Garda Traffic Corp have it done so its possible.

Anyone know if you can flash the software / ECU to combat the safety percentage? I'm presuming the actually device for reading speed must be very accurate so the limitation would be on the software end.

Cheers!

ironclaw.

robtri

ironclaw wrote: »

Hi Folks,

Was doing some thinking. I drive a Toyota with a digital speed read out. Now, from doing some tests with multiple GPS's and pacing, it seems the speedo is off by almost 8km/h at 120 km/h. Apparently this is done for safety which I can agree with. However, can a speedo be calibrated? Obviously the Garda Traffic Corp have it done so its possible.

Anyone know if you can flash the software / ECU to combat the safety percentage? I'm presuming the actually device for reading speed must be very accurate so the limitation would be on the software end.

Cheers!

ironclaw.

Yes speedos can be recalibrated...

a lot of people get then recalibrated if the increase the size of their wheels...

ironclaw

robtri wrote: »

Yes speedos can be recalibrated...

a lot of people get then recalibrated if the increase the size of their wheels...

Interesting. Any idea of the cost? Presume its a Dyno job? Or just inputing a wheel value into the ECU?

Dilbert75

If you do it, make sure you do it with brand new tyres or make allowance for wear. As the rubber wears, the wheels turn more often for any given speed - if you calibrate it with worn tyres, the speedo will under-read with new tyres - makes it easier to pick up penalty points.

ironclaw

Dilbert75 wrote: »

If you do it, make sure you do it with brand new tyres or make allowance for wear. As the rubber wears, the wheels turn more often for any given speed - if you calibrate it with worn tyres, the speedo will under-read with new tyres - makes it easier to pick up penalty points.

I'm going to have to call you out on that. Your talking millimeters in wear which even mathematically wouldn't make sense to make such a dramatic change. Maybe adding under-inflation you'd get a 1km/h or two, at most. I'm looking at almost 10 km/h under reading at 120 km/h with the same diameter tyre's as stock.

Anyone work in this field who could give a definite answer?

robtri

ironclaw wrote: »

I'm going to have to call you out on that. Your talking millimeters in wear which even mathematically wouldn't make sense to make such a dramatic change. Maybe adding under-inflation you'd get a 1km/h or two, at most. I'm looking at almost 10 km/h under reading at 120 km/h with the same diameter tyre's as stock.

Anyone work in this field who could give a definite answer?

worn tyres can effect your speedo by up to 3% in bad cases, depending on tyre size orignally....

rcdk1

Since I'm bored waiting for the Playstation to do a system update, I decided to do the maths....

So lets assume I have 205/55 R16 tyres which according to Bridgestone should have an overall diameter of 632mm when new and normally loaded.

Lets also assume that my unworn tyres have a thread depth of 9mm and when worn they're down to 2mm (i.e. still legal).
So my worn tyres have a diameter of 632 -7 -7 = 618mm (that's 7mm off the top and bottom).

The circumference of the worn tyres is (pi * Diameter) = 3.14 * 0.618 = 1.941m.
To do 120kph the tyre is going to have to revolve (120,000/1.941 61,808 times in one hour.

Now the unworn tyre's diameter is 632mm and its circumference is (3.14 * 0.632 1.985m.
So if the unworn tyre revolves 61,808 times in an hour it will cover (61,808 * 1.985 122,718m in one hour i.e. 123kph.

So in theory, there could be 3kph of a difference between worn and unworn tyres at around 120kph.

Playstation is finished so I'm off to play AC Brotherhood

Dilbert75

ironclaw wrote: »

I'm going to have to call you out on that.

rcdk1 wrote: »

So in theory, there could be 3kph of a difference between worn and unworn tyres at around 120kph.

So I was right then?:D

rcdk1

Dilbert75 wrote: »

So I was right then?:D

I wouldn't be betting my driving licence based on those calculations!