Speedo Difference

[Teiresias]

Got new tires today.

I understand that if you get a higher profile tire than what's stock it changed the speedo's reading slightly, it'll be a bit off.

I was wandering of there's a way I could calculate that mathematically, if there's a set formula? Or should I just get someone with a GPS that tracks speed?

It's an R34 25GT so, stock is 225/45/17, I've got 235/45/17s on there now (just the back for now).

Thanks!

Teir

[GTSBoy]

Of course there is a formula. Not one that's all written out nice for you though. Your new tyres are 45% of 10mm taller on each side of the centreline. (that's 45% being the 45 profile, 10mm being the difference between 235 and 225) and that difference turns up twice in the diameter of the wheel+tyre together. Total increase in diameter is therefore 9mm. Old diameter was two tyre heights + 17" = 634.3mm. Round off the decimal because nothing is that precise with this stuff anyway. So 634mm. New is 634+9 = 643. That's a thumping 1.4% difference. Given that your speedo error before was probably 6-10%, your new speedo error is probably 5-9% or possibly 7-12% depending on which way your old error was, + or - compared to actual speed.

[Teiresias]

lol awesome. I was thinking it would be - compared to actual speed, thinking that the speedo uses rpm of the driveshaft to calculate the km/h? Or am I completely wrong? I really don't know, I'm quite curious.

[sultanaz]

I use this http://www.miata.net/garage/tirecalc.html

[Teiresias]

I use this http://www.miata.net...e/tirecalc.html

Nice! Told me everything I need to know.

Though GTS boy is right in the end, 1.4% makes lil diff considering the 7-12% off that comes out of factory (I'm assuming the speedo is 7-12% faster as to save their asses if the speedo is that tad off)

[GTSBoy]

Nissan speedos used to be terribly optimistic. My mate's '84 Bluebird used to tell him that he was doing 180 when it was barely nudging 160 (as judged by a couple of Holden speedos that were at least owned by people who knew how to check the accuracy).

By contrast, my R32 was only 3% optimistic until a few weeks ago. With a new gearbox and diff ration in it, I'm going to have to work it out again. But anyway, the 3% error I had was a combination of my original diff ratio, a gearbox from a car with a slightly different diff ratio, plus slightly different sized tyres from standard. All added up to 3%. Could just as easily have gone the other way if I was using the parts in the opposite way.

[sultanaz]

Nice! Told me everything I need to know.

Though GTS boy is right in the end, 1.4% makes lil diff considering the 7-12% off that comes out of factory (I'm assuming the speedo is 7-12% faster as to save their asses if the speedo is that tad off)

No problem mate it helped me out when i was deciding

[R31Nismoid]

Stock speedo's are generally pretty close @ 60km/h buy by 110km/h they are quite badly out. Over 200km/h, forget it. Can be upto 30km/h wrong.

So calculate all you want, but you've still gotta account for the fact the stock one could be +/- 10km/h @ 100-110 to begin with.

If you have a PFC or similar look @ the read out. It will give you a much better idea than the speedo I've found.

[Teiresias]

On the way back from the GOR cruise, (Think it was on the princess) I went past one of those speed indicators that show what you're doing in big LEDs, and it was pretty much on the money, maybe 2km under at most. I was doing 100 and the sign said 102

[GTSBoy]

Those things are not always reliable. One day I went past the one near Geelong heading back to Melbourne. 3 cars abreast, all doing exactly the same speed. The 3 displays were +/- 2km/h different from each other.

[R31Nismoid]

They clearly state on the sign just near the sensor - that they aren't accurate

[Teiresias]

bollocks.

ok, what's a PFC?

[blind_elk]

They clearly state on the sign just near the sensor - that they aren't accurate

Yet they supposedly use the same technology that radar guns use.

Probably why they have shut done most of them.

If you want to double check the speedo, use a GPS.

[dlai]

To fix the issue, order a yellow box. Wire it up and tune it using the manual and gps speed.