M35 Speedo re-calibration to compensate for wheel size

[Tumble]

I would like to fit a wheel tire combo with a lager than stock diameter. Is it possible / is there a product out there... that would let you either make and adjustment somewhere in nissan data scan, or intercept and adjust the signal coming from the speed sensor - similar to a kph to mph converter, to compensate for the change of wheel size and not affet your speedo or overall trip km...

[KiwiRS4T]

Jaycar used to sell a speedo correction unit - a box inserted in the signal to speedo. But by how much are you going to increase the rolling circumference? Speedos read fast anyway. A small increase may just result in your speedo reading more accurately.

[Tumble]

Thanks, I'll have a look at Jaycar. Just to confirm, that's an electronics/gizmos shop?

I've not done the actual math, but gaining 2" od would help with ground clearance issues, and arch gap ?

It would be more to keep the overall kph accurate, I tend to use gps for mph if I need to know the speed I'm traveling

[GTSBoy]

2" rolling diameter is a huge change.

You can't do it in the CU data, because the speed signal starts at the speed sensor as a +/-1 volt AC voltage and the speedo head converts it to the pulsed square wave 0-5v signal that everything else in the car uses. Thus you need to fix it in the signal coming from the sensor to the speedo.

Yes, Jaycar is an electronics chain in Australia. Their speedo correction kit should work, but there should also be somewhat freely available equivalents out on the net (in terms of circuit design) and I would imagine that there would have to be Arduino type projectes out there too.

[Tumble]

Thanks mate, surprised I've never heard of them, but looks like they are in the UK too

Will look into Arduino projects as well.. new wheels are quite a long way off but good to know there's a solution(s) out there..