Inaccurate Speedometer

So my speedo isn't accurate, ive used a gps speedo and have learnt whats 30, 40, 70 etc. But I would like it to work properly. Is there a fix for this? im running wider 18"s and wouldn't mind getting it sorted. My speedo wiring has been played with before. How do I go about getting it to work accurately, wouldn't mind getting new speedo faces if need be to. Just fed up now...

I think the only way it can be done is to take the needles off, drive at 10mph via GPS speed, pop the needle back on and see how it goes.

Usually the same deal once you've took the needles off to fit new dials.

That dosent work dude, the needles never match up or match the real speed its doing, urghhh

But thats why the needles are off, the spindles will still move (be in the position it thinks it is) but you'd position the needle where it actually is. Then it should always be the same

could be your after market wheels

If your Supra is a Jspec then isn't inaccurate speedo readings normally to do with the KMH -> MPH conversion (black box) being faulty

If your Supra is a Jspec then isn't inaccurate speedo readings normally to do with the KMH -> MPH conversion (black box) being faulty

This could be it, but my speedo has KM and MPH, but I know nothing about this box

Digital speedos basically work on pulses per mile travelled. But you have added a mechanical unknown by removing the speedo needle, which operates against a helical spring. It is preloaded against the minimum speed stop, and now it will have a different preload, and they are very picky about that being bob on. If you'd not fiddled with that it's a (pretty) simple change in how many pulses per mile the speedo sees, despite how many the tailshaft speed sensor is emitting. Modern digital electronics would have sorted that out.

But once yo lose the preload on the needle spindle, set in manufacture, if you get the needle right at say 70 MPH it won't be right anywhere else, as it's still getting the wrong pulses per mile. This is because you have presumably changed the rolling circumference of the stock rear tyres by changing sizes? Fiddle with speedo and rev counter needles at your peril.... Getting the PPM right is enough to contend with, without trying to get the preload right, that's an instrument maker's job.