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Speedo Difference

Teiresias
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Teiresias Community Regular

Teiresias

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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

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GTSBoy Grand Master

GTSBoy

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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 Community Regular

Teiresias

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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 Grand Master

GTSBoy

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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 Proficient

sultanaz

Posted
Posted

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 Grand Master

R31Nismoid

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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 Community Regular

Teiresias

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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

blind_elk Grand Master

blind_elk

Posted
Posted

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

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.

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