Very handy if your speedo is correct in the first place, which it seldom is. Tyre pressure is also a factor and will vary (become higher) as you heat up the tyres.
Only one really good way to do it: Get on the freeway to an odometer check. I think it's a blue sign. It starts with 0 KM. For every kilometer you travel there will be a new blue sign saying 1 KM, 2 KM up to 5 KM. Check your odometer at 0KM. You can zero your trip counter at this point. Read the trip counter again at 5 KM and note the distance. Subtract the distance from 5 KM and divide the result by 5 KM. Multiply by 100. This will show how many percent your odometer shows too much and thereby how many percent your speedo shows too much as well. It's normal for Soarers on standard wheels to show 2-5% too much.
Mike Triggs Goo Roo Western Australia 3.0GT G-Pack
Friday, February 15, 2008 - 02:05 pm, by: Mike Triggs(Mikeandimah)
It's easier, and better, to check against a GPS. I've found that the Rexton speedo is 3% optimistic, and that the odometer under-reads by the same amount. This equates to 0.15 km over a 5km check length, which is very hard to read accurately.
I didn't have a chance to check the Soarer with the standard tyres, but with the 16" Lexus wheels I've found that the speedo error is no more than 1 km/hr right up to 120 km/hr, so the error is now less than 1%.
Since the new tyres' rolling circumference is at least 2.5% bigger than the old 15" tyres', it follows that the error range Benny quotes is reasonable.
I never understood how those 5km freeway checks worked, unless the entire distance was straight and you don't change lanes then wouldn't the bends in the road mean two cars driving next to each other would travel slightly different distances? Plus people take different lines in corners
Mike Triggs Goo Roo Western Australia 3.0GT G-Pack
Thursday, January 01, 2009 - 12:13 pm, by: Mike Triggs(Mikeandimah)
Benjamin Anderson wrote on Thursday, January 01, 2009 - 11:07 am:
I never understood how those 5km freeway checks worked, unless the entire distance was straight and you don't change lanes
All the speed check lengths I've seen are on straight sections of motorway, Benjamin. You can check distance against odometer (although 5km is rather short- you need to do it several times). In my experience if the odometer under-reads the speedo is optimistic by same percentage.
You can also time your 5km stretch, but you need to stick resolutely to your chosen speed- 100km/hr is easiest. Forget how many seconds 5km at 100km/hr is but you get the idea. Lane changing shouldn't make too much difference- as long as you don't do it too many times.
I've found the check lengths are useful to about 0.5%, if several checks are made. It can be difficult to gauge where you're at when the km markers flash past in the blank between the tenths of a km on your odometer.
These days GPS are cheap and much more useful than check lengths for determining speedo/odo error (and a lot less distracting!). I've noted the Rexton go from ~3% out to 3.5% as the tyres wear using GPS, and of course GPS are great for knowing your true speed in cop country.
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