Speedometer Over-Reads

Nothing new there of course but the error is more than in my other cars.

I drove today with a GPS based speedometer in the car. With the speedo reading 83mph, the GPS device (which is much more accurate) read 78mph.

Checked mine and its 1.5 to 2mph above actual speed
I am on 19" wheels withh 55 ratio tyres
Do 20" wheels with 45 ratio tyres have the same circumference ?
Same as my last Audi funnily enough

Yes, should have mentioned mine is on 20" 45 profile tyres.

(24-05-2012 12:54pm)speary Wrote:  Checked mine and its 1.5 to 2mph above actual speed
I am on 19" wheels withh 55 ratio tyres
Do 20" wheels with 45 ratio tyres have the same circumference ?
Same as my last Audi funnily enough

There are on-line calculators for this (rolling diameter is the critical bit) - you feed in the parameters and they will indicate potential speedo deviation. There is one here Rolling diameter calculator link

The 245x45 size chosen by LD for the 20'' rims are actually about 6-7mm lower in profile then should have been,in order to maintain the overall average circumference of all the other rim size/tyre combo's in the range. Thats about 14mm smaller overall circumference compared to all the other stock rim/tyre size combination range. This would account for some of that speedo error i would think.
255x45 or 265x45 would bring it closer to the average.
I payed extra to get them to fit 255x45's on my 20'' style 7's for this reason, and for that ever so slightly beefier look.

All cars speedo's over read,.

They only have to be accurate to +/- 10% therefore manufacturers make them overread slightly.

If they under read that people would complain that they were getting caught speeding when the speedo was telling them they weren't.

This is not a fault, its perfectly normal.

Same here Mark, I have the 20" wheels as well, and with a Garmin GPS, the speedo reads exactly 5km/h more than the GPS.

At 78mph the speedo must read a minimum of 81.2mph as I believe the requirement is a minimum of 4% overread. Plus your inflation level of the tyres will also affect the accuracy.

(24-05-2012 02:23pm)Ezandania Wrote:  Same here Mark, I have the 20" wheels as well, and with a Garmin GPS, the speedo reads exactly 5km/h more than the GPS.

What model of garmin gps you are using ?
Not to hijack the thread as this is unrealted to speedometer, but I need to find a gps that works ( for navigation ) and would be nice to know what model you use.

I don't know why, but there was always a part of me that never trusted my TomTom and therefore any GPS read out device simply because the map would always show me slightly behind where I actually was in real life. Therefore, I thought that my GPS was calculating my speed based on a (slightly) incorrect location and therefore a slightly slower read out would result. I guess the rate at which the location changes (speed) would still be the same though.