WrX 03

How does your speedo reading compare to road angels reading, my MY03 tends to be 9% faster than RA.

supertouring

I've recently got a GPS Sat Nav and have been suprised at how much out it can be, especially at motorway speeds. 8 or 9% sound about what I get, with slower town driving only out by a few mph.

Nice to know that I was never actually over the limit on those occaision where I have passed speed cameras in the past.

WRX_BLUES

Yes, but generally the speedo overreads rather than underreads anyway. I would have thought the RA is probably slightly more accurate.

T4molie

Road Angel against MY02 WRX speedo eg 90 mph = 85 RA reading

I kind of thought it might be to do with the acuracy of the tracking on the RA....?

Andy

Beemer_Deano

Me too - I always thought GPS speed readings were pretty innacurate.

Deano

misty

Speedo's are not allowed to over read by law! My classic is about 3% fast compared with the R.A. My old bugeye was about 8% out. Think the difference may be down to the 18inchers on the classic?
dave

AvalancheS8

It'll be the car speedo that's inaccurate, the GPS should be very close if you are cruising at a constant speed. Expect your car speedo to be around 10% fast.

DanTheMan

Mine is about 9% out too, MY99 TypeR, no wonder I havent been clocked speeding

WRX_BLUES

Misty,

I think you'll find that the speedo isn't allowed to under read. It makes more sense because if they caught you and you think you were doing 33 in a 30 and you were found to be doing 36 you could be up for points. But if you were doing 33 and your speedo overreads then you are probably doing only 30.

dr_ming

Legislation for car speedos is +10%,+2MPH high, -0%, -0MPH low. So at 100MPH true speed your speedo is in spec if it reads anywhere from 100 to 112MPH.

macka

Yep my speedo is 9% over reading and I'm sure road angel is spot on because it reads the same speed as my portable sat nav only thing to watch is a slight delay on sat GPRS devices.

macka

H7

@30mph on speedo, RA shows 27
@40mph on speedo, RA shows 39
@50mph on speedo, RA shows 50
@60mph on speedo, RA shows 60
@70mph on speedo, RA shows 70
@80mph on speedo, RA shows 80
@90mph on speedo, RA shows 91
@100mph on speedo, RA shows 101

Pretty accurate on my car, but it's not a Scoob

what would scooby do

My Type R shows 165MPH when @ 150mph /8250 RPM

Gangsta Smurf

Your true speed - i.e. that shown on a GPS unit (such as a road angel) will be around 10% different than what is shown on your speedo. This is because (I think i'm right in saying this) speed within the car is measured by the speed your wheels are turning, however a GPS unit is tracing your speed by your position in relation to the turning axis of the earth!

This is why Police Speed cameras allow the 10% difference before they take a photo - your true GPS speed is 10% less than what is shown on your speedo.

Jap Imports

So hang on. The GPS Speed like Road angel, Snooper S6 and Sat nav, are the right speed then

H7

GPS needs to be accurate to be effective, although there may be some (small) time lag in displaying your position. At a constant speed, though, your progress across the planet will be tracked by the GPS system and, as speed is a function of distance/time, your speed as measured by a GPS unit will be accurate.

The speedo in your car may not be accurate because the feed from the gearbox, or whatever, is subject to variation from tyre size, tyre wear, gearing, etc.. Police traffic car speedos are calibrated daily (I believe, but don't kick me if I'm wrong) to take account of that. In order for car manufacturers to ensure that the products they sell comply with the law, they will artifically change the gearing of the speedo component to ensure that it ALWAYS overstates your speed, regardless of tyre wear or other variation that may have an effect during the life of the car. The figure of 10% has been generally accepted, but will clearly vary from car to car.

The difference in my car, where the speedo reading is pretty closely aligned to the GPS reading, is that mine had an electronic calibration fitted when converted from kph to mph and this was set closer to actual speed than any manufacturer would do. At the top end of the scale, my speedo underreads the GPS speed which could be deemed illegal, but so is 100mph in this country

Brian

compshack

From the road angel web site
"Road Angel is showing a different speed to the speedometer in the vehicle.
Road Angel uses GPS to locate its position and does so once every second. The speed will lag under acceleration and deceleration. In addition to this all speedometers display a higher speed than the true speed of the vehicle, so you may find that Road Angel is a few miles per hour slower than the speedometer."

AvalancheS8

Nope, simpler than that, your speedo is deliberately set to over-read. They could easily make them accurate, but they don't.

misty

My apologies! I did mean under read, DOH!!!
dave

DanTheMan

YEAH but thats under acceleration, and its the same as a digital speedometer, its going to lag behind a little....

But we are on about constant motorway speeds here, where the GPS RA shows the true speed and reveals exactly how much the cars speedometer is out !!!

RB5SCOTT

157mph indicated, 151.5mph through speed trap

ThrustSSC

If you want to be really sure, use the same calibration markers the police use each day in the calibrated cars. You'll find them on motorways and good A roads, and they're a series of disks with quarters progressively coloured in marking the quarter miles.

Just keep a constant speed and time yourself, then work out the real speed from that.