speedo readings

[hotrodder13]

when i put my alloys on im going to put lower profile tyres on will this make the reading wrong

if so will it be far out thinking about 55-60 rather then the orignal 80

thanks tom

[bmcecosse]

Run a tape round the tyres and compare the dimensions.

[mike.perry]

It depends on how far out the speedo is in the first place, reckon on at least 10% overread. My 1098 speedo is within 1mph running on a 3.9 diff and slightly oversize tyres. The only reliable way is to check it with a Sat Nav.

[rossrox]

The only reliable way is to check it with a Sat Nav.

but sat navs arent fully reliable, my driving instructor has a brand new ford fiesta and on his side of the car he has a satnav so he can tell what speed im doing, but the satnav is about 4MPH out

[bmcecosse]

Doubt it! Sat navs are deadly accurate!!

[mike.perry]

Doubt it! Sat navs are deadly accurate!!

Not quite true:-
When the sat. nav. instalation of 24 satelites (+3 spares) was established the general public was only given access to the course aquisition mode which had a built in time error in the signal transmission and gave an accuracy of about +- 100metres.
There were ways of overcoming this by relating to known bases, but it was not an instant position fix. (in simplified terms)

[bmcecosse]

Position perhaps - but mph figures are extremely accurate.

[rossrox]

then why would my instructors be 4 mph out??? new car and the satnav isnt a cheapo as its tom tom, and satnavs are really delayed in results, normally a couple of seconds behind

[Jefftav]

AS BMC says and my understanding is that satnavs a the most accurate piece of equipment you can use. How are you working out that the satnav is 4mph out?

[bmcecosse]

Exactly - 4mph 'out' - against what! ?

[alex_holden]

When the sat. nav. instalation of 24 satelites (+3 spares) was established the general public was only given access to the course aquisition mode which had a built in time error in the signal transmission and gave an accuracy of about +- 100metres.

Selective Availability was disabled a decade ago; civilian GPS receivers now give a fair bit better than 100m positional accuracy, plus speed calculations are very accurate because positional error drifts very slowly. I think it's something like 0.1MPH error.

[rossrox]

4MPH out against the car speedo

[oilypaws]

The car speedo is out. Virtually all car speedos over-read even when new (I think the car companies are worried you will get a ticket (or worse) and blame them). GPS is very accurate when it comes to speed, although as you mentioned there is a slight lag in response.

[mike.perry]

I believe that S/A was switched off during the 1st Gulf war when there were not enough military GPS receivers available and civilian receivers had to be used.