Ls400 Tire Size Speed Descrepency

[Slobbo]

Guys I am wanting to buy some winter tires soon but have changed the wheels to 17" IS200 wheels with the incorrect size tires on it for an LS so the speedo reads wrong. 62 on the clock is 70 real speed according to the gps. What I was wondering is if the speedo is out with stock 225/60/16 wheels and if so by how much? I want to buy a size that is as close to true speed as possible.

Has anyone done a gps comparison?

[steve2006]

Try putting your tyre sizes in the calculator below (the normal standard fit and the proposed winter replacements)

http://tire-size-conversion.com/speedometer-calibration/

[Slobbo]

Thanks Steve. Not got a problem finding an alternative size. Most cars speedos are inaccurate. They can be up to 10% out. What I am wanting is the difference between the speedo reading and the gps reading. so if the speedo reads 70 what does the gps say. I am after this for the standard LS wheel/tire size. This will then allow me to tweak the tire size calculator to find the closest size to suit a true speed reading.

[Calders]

I was told some years ago that all car manufacturers speedos are inaccurate so as to bring the service intervals around quicker.

[Bluesman]

Thanks Steve. Not got a problem finding an alternative size. Most cars speedos are inaccurate. They can be up to 10% out. What I am wanting is the difference between the speedo reading and the gps reading. so if the speedo reads 70 what does the gps say. I am after this for the standard LS wheel/tire size. This will then allow me to tweak the tire size calculator to find the closest size to suit a true speed reading.

If my car is showing 70mph the TomTom Satnav shows 64/65 mph. When Satnav shows you doing 70mph car speedo shows 75/76mph. Mike.

[Malc]

......... this is the reason I'm told one can do 70mph + 10% + 2 = 79mph before you will get stopped for speeding on the motorway.

The Highways guys allow you 2mph over the " real " speed limit before they worry about catching you.

Malc

[Cotswold Pete]

Thanks Steve. Not got a problem finding an alternative size. Most cars speedos are inaccurate. They can be up to 10% out. What I am wanting is the difference between the speedo reading and the gps reading. so if the speedo reads 70 what does the gps say. I am after this for the standard LS wheel/tire size. This will then allow me to tweak the tire size calculator to find the closest size to suit a true speed reading.

I think you have to do it for your car. I have found that with current and previous LS, the speedo versus gps gap was different between cars,and with my current one the gap is not linear.

So gps reads 70 when speedo says it is doing 75 but gps says 73 when speedo says 80 and gps says 80 when speedo says 89.

What I need to do is the work out whether the gap between speedo, gps and average speed number is different, but probably need a nice clear road and too much spare time on my hands

[Spacewagon52]

This is worth checking so that you know you are under the REAL speed limit. It is just remembering what the difference is!!

[alien1974]

mine seems very accurate at 30 mph as those lamp post speed displays say 30 too

[plastic orange]

I fitted 235/55 x17 tyres to my DHP rims instead of the extremely expensive original spec 245/50 x17's and found the speedo reading to be slightly more accurate with an indicated 73mph being a true 70 as per sat nav.

In Scotland from 1st August for a 6 month trial, Police will be stopping speeders who are only just over the limit and issuing a formal warning. A subsequent offence will get 3 points and fine. We all see where this is going.

What I can't understand is that any car I've hired in the US, the speedo matches the sat nav exactly - even Toyotas, so why not here?

Pete

[ColinBarber]

I was told some years ago that all car manufacturers speedos are inaccurate so as to bring the service intervals around quicker.

No, one because the odometer is accurate, and two the speedos are more accurate in other countries such as Australia, it is set up purposely to be over in the UK. The over reading is due to legalities.

You will always be out to some extent. Tyres wearing down from 8mm tread to 2mm will give you up to 2% error