Speedo reading under.

[Baphomet]

g'day

the speedo on my missus car(vp 3.8 sedan) is out by about 5km's so if it reads 80, its really going about 85.

what could be the possible causes of this and how do you go about fixing them?

[calais_cruzin]

What rims are on hte car????? big onse?

also how did u determine that its out by 5km?

[mr calais 5.0]

Tires can make a difference.. How did you test it?

[Baphomet]

its got 17's, with 225 wide tires

i havn't actually tested it, my girlfriend said she saw on a radar setup to tell you how fast your going that it was out.Dunno how reliable they are?

[MrNutter]

What size tyres exactly? Without the height of the sidewall it is impossible to calculate the rolling distance of the tyre. I'd nearly lay money that you have 225/55-17's on it, which would put your actual speed at 104kmh when your speedo is reading 100kmh. Remember that the second set of numbers in a tyre size is the height of the sidewall compared to the width of the tread, not a measurement in millimeters. For example 225/55-17 is 225mm tread width, 55% of the tread width is the sidewall height & the 17 is obviously a 17" rim.

If you already have 225/50-17's on it then I would be looking to the testing method being inaccurate or the speedo itself being out & needing recalibration. Get it checked properly before you decide what you need to do.

[Munz]

Don't rely on those radar setups..... most probably a radar for roadworks
you can have speedo tested on a dyno for next to nothing

[304runner]

All cars are over calibrated from the factory, so if your looking at your store bought GPS and its saying your out by usually +4 kph then this is normal.