Speedo

[Guest oshiney2]

can anyone tell me what is more accurate your speedo in your car either mechanical cable or magneto OR GPS through your phone app ??????

[Guest Ian & Carole]

GPS

 

Speedos are set to read around 5% higher than actual speed.

[Bob Tucker]

+1 for GPS.....I used it to setup my speedo for SVA,

& allowing for my deliberate overreading it matched the SVA rollers speed.

[agent_zed]

As Ian & Carole says speedo is set to be off a bit. It can only read fast not allowed to read slow.

Personally i prefer it saying i am doing more than i actually am as i am less likely to get caught speeding if my speed has crept up a few mph.

 

think from memory at sva it was something like you could be reading 70mph and min actual speed was 57mph. But not allowed the other way.

 

The other problem we have though is often we have lower profile tyres which make it even slower as increased gearing.

 

Not sure i fully trust gps enough to use as only speedo

[Guest jhfozzy]

I work with speedos and GPSs, and the only answer is they are both accurate at certain times and inaccurate at others.

 

Your car speedo is set up to comply with C&U regs which allow a 10% over read and 0% under read. They average about 5% over read. So at 100 mph you are generally doing about 95 mph.

 

All GPSs read the speed across the ground, but only some take into account altitude for their speed calculation. This means that when you drive up or down hill, the actual speed across ground drops off slightly. E.g. 100mph on the flat may appear as 97mph going up hill.

 

As a general rule, on a flat surface, your GPS will show the most accurate speed.

[Guest 2b cruising]

I would not mind if mine was reading over. Unfortunately, as I thought it was reading under causing me to speed.

Had it re calibrated using present tyre circumference and calibrated lazer read tachograph rollers.

I know it will be slightly out when my near new tyres wear but I will actually be going slower. So safer.

Edited August 26, 2015 by 2b cruising

[megadodo]

GPS speed readings are very accurate as they are a differential change in position over time so the very small positional inaccuracy is negated. I agree that previous vehicles I have owned along side a stand alone alone GPS device show their speedos do read around 10 % higher. However, my BMW E36 & 2 E46 BMWS have speedos that read the same as a GPS speed! I set my 2B speedo to read the same as GPS. Not that I speed any more but I still have to remember not allow 10% speedo error in speed restricted areas!

[Guest oshiney2]

Thanks for all your help interesting