Speedo

[Flash89]

hey guys

my speedo is about 10ks slower then it actually says. well its 20ks under when doing 100ks. any idea??

example. speedo say 70 speed is 60

speedo say 120 speed is 100

[QWK32]

what size wheels and tire profile are you using? and did your car go from a auto to a manual?

[adam-__-]

Can tyres really do that much? 20km difference at 100kmh?

[QWK32]

hmmm.... true, tire size wouldn't affect it that much, 5 km/h mabye, not 20.

did this just happen or have you done some work to the car? changed gearbox? swapped diff?

[BNR32t]

Na tyre size change could, i was out by 10 km per hour will just 2 size smaller profile (same rim)

So if you are increasing/ decreasing the out side diamitor of the tyre you are changing you speedo reading...

You will also be putting more or less km on the car.

[Russman]

Mine is about the same. about 18% all through the range. about 120 is 100. My problem is R33 gearbox in my R32. I might fix it one day.

[Flash89]

havnt done anything to the car to do with the gearbox or diff. have now got 18s but it was still like this with the stockies. i reckon i could have a R33 gearbox but can you tell if it is??

[adam-__-]

I would say it's something to do with your gearbox. Not sure on how to check if it's a rb25 one though.

[thrtytwo]

no chance it could just be speedo getting old ?

[tacker]

yer tyres can make that difference.

RPM / gear ratio / diff ratio x tyre circumfrence in metres x 60 = KPH

GTR 4th gear is direct so easier to calculate

18 x 240 x 40 tyre is 2.05 metres in circumfrence

17 x 235 x 40 tyre is 1.95 metres

7000 / 1 /4.11 x 2.05 x 60 = 209.4 KPH

7000 / 1 /4.11 x 1.95 x 60 = 199.27 KPH

[_Ryan_]

http://www.miata.net/garage/tirecalc.html

[Flash89]

i dont think it will have anything to do with my tyre size cause my speedo will say 120 and i will actually be doing 100so if the speedo was saying 100 i will prob be doing 80-90

[adam-__-]

Did you get your gearbox checked out?

If your doing 100kmh, but speedo says 120kmh, just get larger tyres than what you've got on and cancel the difference out maybe?