Speedo Calibration or Re-Calibration

[s2_wot]

I've read a few threads on here about the speedo.

I was just wondering would it be possible to use a wheel & tyre combination with a larger rolling diameter AND recalibrate the chip in the instrument cluster to give the ECU the correct figure and also to have the speedo reading accurately?

[Error404]

by my calculations with something like a 225/40 18 the speedo reads about right - under-reads normally

[s2_wot]

I've read that previously on here about the enthuiastic reading on the console.

By calculation then you've gone for a tyre that has a rolling circumference 0.84% larger; certainly well within the generic 3% rule. I was thinking more along the lines of 5-10% increase in circumference.

Benefits being better cruise MPG, larger wheel & larger brake with a pothole proof sidewall. Obviously power consumed during accerleration would be more meaning a trade off.

[newsh]

You can get a device called a speedo healer or a yellow box, I have one of these which I've used on bikes with gearbox speedo takeoffs when changing the final drive sprockets. Goes in line with the speedo sensor and generates a percentage shift in the reading programmable over a wide range, )enough to correct from kph to mph in fact). http://www.yellr.com/

[johne]

The chip in the instrument cluster that drives the speedo is a UAF2115 and connected from its pin 4 to ground is a resistor that controls the current through the speedo - i.e. its reading so changing this resistor would appear to let you recalibrate the speedo by a small amount. It wouldn't change the odometer, only the speedo so not ideal.

Paul N @S2central.net gives more detail on the speedo workings in this post

An Engine Simulator is Born - Part 1

John

[s2_wot]

You can get a device called a speedo healer or a yellow box, I have one of these which I've used on bikes with gearbox speedo takeoffs when changing the final drive sprockets. Goes in line with the speedo sensor and generates a percentage shift in the reading programmable over a wide range, )enough to correct from kph to mph in fact). http://www.yellr.com/

Thanks newsh, a mine of information as usual. That'd be one way to "create" impressive looking acceleration videos for youtube wouldn't it

The chip in the instrument cluster that drives the speedo is a UAF2115 and connected from its pin 4 to ground is a resistor that controls the current through the speedo - i.e. its reading so changing this resistor would appear to let you recalibrate the speedo by a small amount. It wouldn't change the odometer, only the speedo so not ideal.

Paul N @S2central.net gives more detail on the speedo workings in this post

An Engine Simulator is Born - Part 1

John

I was reading that thread earlier and hence the question really.

If it could be adjusted by changing the signal to the chip as per newsh above or by adjusting the resistor that'd make for good options in relation to tyre selection. There is room for a bigger diameter tyre inside the arch rather than lowering the suspension and running a very low profile tyre.

[Error404]

If you get a road angel or decent satnav you will see roughtly how much the speedo overreads, its about 5% IIRC

[Dan]

Mine seems to be more like 15%

I mostly ignore it and drive by the sat nav.