Speed Gun

irishgal23

I am just wondering i was on my way to work this morning and their was a Gardai with a speed gun behind an unmarked car on the dual carriageway. The speed zone was 100km/h i think i was doing between 104-108km/h. Do the guards send out the penalty points and fines or do they have to pull you over? Also how long would it take for the fine to arrive in the post?

Gonad

Should be fine .car speedometers usually 7-12 % over the actual speed . If you were just over you should be ok

Also with soeed gun they need to stop you here at the time

Melendez

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loyatemu

Melendez wrote: »

This post has been deleted.

It's not a standard 10% and most don't over-register by that much. It's related to the size of the wheels/tyres AFAIK.

I know in my car when the speedo says 105 the satnav says 100.

coylemj

loyatemu wrote: »

It's not a standard 10% and most don't over-register by that much.

+1 it's not a 'standard' 10%, another urban myth. My speedo shows +3 kph in the range 100-120 and +2 kph from about 70 to 100 based on the GPS reading.

loyatemu wrote: »

It's related to the size of the wheels/tyres AFAIK.

It's not 'related' to the size of the tyres and wheels, it's set by the manufacturer based on nothing in particular but the displayed speed is always higher than actual.

A Micra could read over by 8%, a Porsche by 2%, the excess of displayed vs. actual speed has nothing to do with tyre size.

johnciall

coylemj wrote: »

the excess of displayed vs. actual speed has nothing to do with tyre size.

Assuming a perfectly calibrated speedometer that displays a true speed, if you were to replace the factor fitted tires with a lower profile tire the speedometer will be reporting a higher speed than true, and inversely if it was replaced with a larger profile tire then it would report a slower speed than true

coylemj

johnciall wrote: »

Assuming a perfectly calibrated speedometer that displays a true speed, if you were to replace the factor fitted tires with a lower profile tire the speedometer will be reporting a higher speed than true, and inversely if it was replaced with a larger profile tire then it would report a slower speed than true

No arguments there, the speed displayed is calculated based on the assumed circumference of the tyres and if that element changes, the displayed speed will change.

My point is that the discrepancy between displayed speed and actual speed has nothing to do with the size of the original tyres fitted to your car. The manufacturer can decide to overstate the speed by 1% or 10%, it has nothing to do with the tyre size.

C3PO

If I'm understanding the OPs post correctly, the Garda was holding a "hand-held" radar gun? If so, I don't believe that they have any way of recording the details of your car unless they actually pull you over at the scene - the radar gun merely records the speed of the car! In my opinion the OP has nothing to worry about!

seagull

The speedometer is typically set so that it should be accurate with the largest manufacturer recommended tyres for the model. From the manufacturer's perspective, it's legal for it to overread, but not for it to underread, provided the car is fitted with recommended size tyres. If you fit larger tyres than recommended, it's your problem that the speedometer underreads.

In my case, one car overreads by about 5%, the other by about 7%.

Peregrinus

Speedometers are a good deal more accurate these days than they were with older technology, and the manufacturers no longer have to allow for deviations and discrepancies of the size that they did in the past. Twenty or thirty years ago it might have been true that you could safely assume an overreading of 10% or so, but certainly not now.

seagull

My father got a nasty shock once when he was pulled over at a speed trap, and found his speedo underread by about 20%. It was a company car - bog standard with no modifications. He was lucky that he met a nice policeman on the day. Car back to dealer for recalibration.

9935452

seagull wrote: »

The speedometer is typically set so that it should be accurate with the largest manufacturer recommended tyres for the model. From the manufacturer's perspective, it's legal for it to overread, but not for it to underread, provided the car is fitted with recommended size tyres. If you fit larger tyres than recommended, it's your problem that the speedometer underreads.

In my case, one car overreads by about 5%, the other by about 7%.

Another thing to take into account that no one thinks about is worn tyres versus new.
My old car at 120kmph was over reading by 5kmph on tyres at the legal limit .
On new tyres it was 2-3 kmph of an over read

L1011

Digital speedometers usually overread by 1% at most assuming vehicle is unmodified.

The analogue / mechanical overread used to be to allow for the spring to weaken as the vehicle aged, obscure technical hangover that exists still

coylemj

L1011 wrote: »

Digital speedometers usually overread by 1% at most assuming vehicle is unmodified.

The analogue / mechanical overread used to be to allow for the spring to weaken as the vehicle aged, obscure technical hangover that exists still

My vehicle is unmodified, has plenty of rubber on the (bog standard) tyres and at 100 kph (GPS) will show 103 kph on the digital speedo.