Accuracy of speedo (and GPS for that matter)

[NoelWatson]

Paul (NSX2000) were at VMax today (where he got 165mph), and we were attempting to see how accurate GPS was. Mine showed 162 when the timing beams showed 164, while Paul's phone GPS was showing 170. The speedo was approaching 180mph. So I reckon at a genuine 170 in the NSX, the speedo would be off the end, unless my car is the only one that overreads.

[SILVER BULLET]

It is only since recently changing my first Tom Tom for a newer model showing speed that I have realised just how much the speedo's of our various vehicles differ. I now find that I am able to go about 3mph faster through motorway roadworks controlled by average speed cameras than I did before:no:.

Now I know why my journies took longer than they should have given the apparent speed indicated by the car speedo!!!:rolleyes:

[NSXGB]

Paul (NSX2000) were at VMax today (where he got 165mph), and we were attempting to see how accurate GPS was. Mine showed 162 when the timing beams showed 164, while Paul's phone GPS was showing 170. The speedo was approaching 180mph. So I reckon at a genuine 170 in the NSX, the speedo would be off the end, unless my car is the only one that overreads.

My GPS shows about4 MPH slower than the speedo. I thought all production cars were made with a bias to showing slightly faster than actual speed?

[TheSebringOne]

I agree that most production speedos read a couple/few mphs more than the actual speed. Thats with the OEMs wheels and correct tyre pressures (PSI). May be thats why when you get your speedo up to the limit at speed cameras, you are always safe as your actual speed is 2/3 mph lower.

[greenberet]

Here in Europe, EU Directive 75/443/EEC requires of speedometers that, “The speed indicated must never be less than the true speed.”

The true speed is impacted by:


How worn down your tires are. If the tread is gone, the tire may have lost 3% of its circumference, so your speedometer will display 3% higher than when the tires were new.
How exactly the manufacturer made the tire in the first place. If you put tires with the same dimension (say, 255/40 17) from different manufacturers next to each other, you can see that the tires don’t even start out being the same exact size when new.
What tire pressure you’re running. This impacts how much the sidewall flexes and therefore the effective tire diameter as you’re driving.
How much weight your car is carrying. This also impacts sidewall flex and therefore the effective tire diameter.
etc.
Given all those variables, Directive 75/443/EEC goes on to state that the indicated speed on the speedometer must be between 0% and 10% + 4 km/h higher than the actual speed. If Honda made its speedometers to meet world market regulations, they should display more than the actual speed when you're running stock tire sizes.


Regarding GPS accuracy, GPS speed measurements aren’t accurate under all conditions.

If you’re driving through a turn, the GPS will display too low a speed. It triangulates the distance between two points and the slower the refresh rate of the device, the more it will think you “cut” the corner and travelled a shorter distance in a given amount of time than you actually did.
If you’re accelerating, the slower the refresh rate of the device, the more its display will lag behind your actual speed.
If the GPS receiver doesn’t have a good signal, it won’t know exactly where you are so it can’t calculate the speed between two points very accurately, either.
Due to the slight inaccuracy built into non-military GPS position measurements, the indicated speed will jump around more the slower you drive.
However, if you’re travelling on the highway in a straight line at a constant speed, you have a decent GPS device mounted on the windshield where the antenna has good reception, and you average the speed for a couple of seconds, it should show your true speed quite accurately.

[NoelWatson]

Here in Europe, EU Directive 75/443/EEC requires of speedometers that, “The speed indicated must never be less than the true speed.”

The true speed is impacted by:


How worn down your tires are. If the tread is gone, the tire may have lost 3% of its circumference, so your speedometer will display 3% higher than when the tires were new.
How exactly the manufacturer made the tire in the first place. If you put tires with the same dimension (say, 255/40 17) from different manufacturers next to each other, you can see that the tires don’t even start out being the same exact size when new.
What tire pressure you’re running. This impacts how much the sidewall flexes and therefore the effective tire diameter as you’re driving.
How much weight your car is carrying. This also impacts sidewall flex and therefore the effective tire diameter.
etc.
Given all those variables, Directive 75/443/EEC goes on to state that the indicated speed on the speedometer must be between 0% and 10% + 4 km/h higher than the actual speed. If Honda made its speedometers to meet world market regulations, they should display more than the actual speed when you're running stock tire sizes.



Regarding GPS accuracy, GPS speed measurements aren’t accurate under all conditions.

If you’re driving through a turn, the GPS will display too low a speed. It triangulates the distance between two points and the slower the refresh rate of the device, the more it will think you “cut” the corner and travelled a shorter distance in a given amount of time than you actually did.
If you’re accelerating, the slower the refresh rate of the device, the more its display will lag behind your actual speed.
If the GPS receiver doesn’t have a good signal, it won’t know exactly where you are so it can’t calculate the speed between two points very accurately, either.
Due to the slight inaccuracy built into non-military GPS position measurements, the indicated speed will jump around more the slower you drive.
However, if you’re travelling on the highway in a straight line at a constant speed, you have a decent GPS device mounted on the windshield where the antenna has good reception, and you average the speed for a couple of seconds, it should show your true speed quite accurately.

Tyres are pretty new, checked pressures before I left. I still reckon that 180mph on the speedo equates to around 166-67 on Paul and my cars, as confirmed by laser measuring equipment.

[eclipse1501]

One strange variable I do know affects the NSX speedo is also engine bay humidity. When my header tank started leaking, it was just like a kettle venting a little pressurised steam to start with from a crack just above the min fluid level into the engine bay.

The speedo used to go haywire. Started by showing a lower speed, then nothing at all, then doing loop the loops! Once I had the header tank changed it went completely back to normal. Does not give you much confidence in the accuracy if the system can be corrupted like this.

[Lankstarr]

I think my speedo is about 5% too high; I have tomtam and roadpilot gps which are consistent and show approx 5% down on the speedo all the way up.

I dunno how Leighs car got to 186 on gps on the autobahn!

My bro was there in a 700 bhp rs6 yesterday. Shame to hear about the ruf crash, sounds awful.

[greenberet]

I still reckon that 180mph on the speedo equates to around 166-67 on Paul and my cars, as confirmed by laser measuring equipment.

That sounds completely plausible. The EC Directive calls for 180 mph on the speedo to be 161 to 178 mph in reality.

NSX speedometers can be calibrated (procedure here (http://www.nsxprime.com/wiki/Speedometer_Calibration)) and I’ve been toying with the idea of doing that. But as soon as the tires wear or you change brands, the speedo will be off again. And at top speed your tires could be generating a percent or two of slip, also making the speedo inaccurate. Laser measuring equipment, a stopwatch, or decent GPS equipment (not the built-in GPS receiver in an iPhone!), are probably a better bet than even a calibrated speedometer.

[NSX 2000]

I think my speedo is about 5% too high; I have tomtam and roadpilot gps which are consistent and show approx 5% down on the speedo all the way up.

I dunno how Leighs car got to 186 on gps on the autobahn!

My bro was there in a 700 bhp rs6 yesterday. Shame to hear about the ruf crash, sounds awful.

Hi Luke, yesterday was great fun as usual, with some very nice cars including a Mercerdes MClaren 722 (or is it a 772?). But the car that crashed big time :( was a 9ff, I have a picture of it with my car which I'll post later.

Paul.

[NSX 2000]

That sounds completely plausible. The EC Directive calls for 180 mph on the speedo to be 161 to 178 mph in reality.

NSX speedometers can be calibrated (procedure here (http://www.nsxprime.com/wiki/Speedometer_Calibration)) and I’ve been toying with the idea of doing that. But as soon as the tires wear or you change brands, the speedo will be off again. And at top speed your tires could be generating a percent or two of slip, also making the speedo inaccurate. Laser measuring equipment, a stopwatch, or decent GPS equipment (not the built-in GPS receiver in an iPhone!), are probably a better bet than even a calibrated speedometer.

You'll be pleased to know it wasn't an I Phone but a Sony Ericsson:D

[greenberet]

You'll be pleased to know it wasn't an I Phone but a Sony Ericsson:D

Ah, that makes all the difference in the world!

Actually, I think Sony Ericsson use the same “Assisted GPS” chipsets that Apple use. Those can quickly determine your location to within 100 meters or so based on signals from cell phone towers, even if you’re indoors. However, the actual GPS portion of the receiver that sees the satellites is somewhat, shall we say, handicapped. It seems to me that even a cloud passing overhead can cause it to go blind.

[JQD84983]

One of the weekly car magazines, can't remember which one, always used to post the accuracy of the speedo in its road tests and there was always a discrepancy of a few %.

I always thought thats why the police gave a %age leeway on the speed limit.

[havoc]

Assuming that GPS is ~1mph accurate, then speedos still seem to be all over the place:-
- My old Mk1 Focus was 2-3mph over at 80
- The ITR is 4-5mph over at 80
- Becs' Golf is 7-8mph over at 80!!!


Bad news about the RUF though...glad Jan's OK.

[Silver Surfer]

http://i86.photobucket.com/albums/k94/trax_x/IMG_3701.jpg

VMax

SS

[NSXGB]

I quite like the GTR in that colour. That plate belongs to a well known motoring jouro IIRC?
The NSX looks tiny in comparison.
What did the GTR max out at?

[NoelWatson]

I quite like the GTR in that colour. That plate belongs to a well known motoring jouro IIRC?
The NSX looks tiny in comparison.
What did the GTR max out at?

192mph. Belongs to David Yu

Not standard

http://www.auto-journals.com/journals/Nissan?model=R35&journal=111&entry=1798

[Minch]

Ahhh love the old Lotus Carlton. My dad had one very low mileage example back in '99 . . . a little fragile and twitchy in the wet but deceptively quick!

[jaytip]

I quite like the GTR in that colour. That plate belongs to a well known motoring jouro IIRC?
The NSX looks tiny in comparison.
What did the GTR max out at?
The car is part of the long term fleet in Evo magazine.It has recently been wrapped in that colour.

[NSX 2000]

Assuming that GPS is ~1mph accurate, then speedos still seem to be all over the place:-
- My old Mk1 Focus was 2-3mph over at 80
- The ITR is 4-5mph over at 80
- Becs' Golf is 7-8mph over at 80!!!


Bad news about the RUF though...glad Jan's OK.

Sorry to pick people up on this but it was a 9ff not a RUF.

Pictures taken on the morning of V Max, 2nd picture I think is Jan cleaning the car.

[markc]

So a lightly "optimised" (circa 300hp) facelift NSX will run 164-165mph and shade a similarly optimised E46 M3 with circa 350hp. All hail small frontal area, as also demonstrated by the 9FF GT850. Another 100hp, courtesy of a S/C, should see an NSX run to 180+ at Brunters? :)

I looked at the pics on the Pistonheads thread, there was some seriously nice metal, and carbon fibre, in attendance :) Slightly surprised how awkward the Aston Rapide looked in those pics though!

http://www.pistonheads.com/gassing/topic.asp?h=0&f=133&t=823949&mid=0&nmt=VMAX%20Armageddon%20-%20The%20picture%20thread%20-%20post%20your%20shots%20here

Cheers

Mark

[NoelWatson]

So a lightly "optimised" (circa 300hp) facelift NSX will run 164-165mph and shade a similarly optimised E46 M3 with circa 350hp. All hail small frontal area, as also demonstrated by the 9FF GT850. Another 100hp, courtesy of a S/C, should see an NSX run to 180+ at Brunters? :)

I looked at the pics on the Pistonheads thread, there was some seriously nice metal, and carbon fibre, in attendance :) Slightly surprised how awkward the Aston Rapide looked in those pics though!

http://www.pistonheads.com/gassing/topic.asp?h=0&f=133&t=823949&mid=0&nmt=VMAX%20Armageddon%20-%20The%20picture%20thread%20-%20post%20your%20shots%20here

Cheers

Mark

Paul and I were discussing S/C but I am too much of a girl to do the upgrade on my own. Brakes would definitely have to be upgraded.

My favourite car was the Murcielago - it sounded like the air was being torn- similar to Eurofighter.

[Senninha]

........I looked at the pics on the Pistonheads thread, there was some seriously nice metal, and carbon fibre, in attendance :) Slightly surprised how awkward the Aston Rapide looked in those pics though!.......

Cheers

Mark

I happened across a Rapide at the services on the M40 last week and had a good chat with the driver and a good look around the car.

I would say that it is definately more appealing in the metal than in pictures.

IT rounded of good AMV12 day as earlier in the day I'd had a run alongside the V12 Vantage .... well until he smiles and changed up to 3rd :laugh: but what a glorous noise :eek:

regards, Paul

[NoelWatson]

Paul and I were discussing S/C but I am too much of a girl to do the upgrade on my own. Brakes would definitely have to be upgraded.

My favourite car was the Murcielago - it sounded like the air was being torn- similar to Eurofighter.

Looking at the vid here

http://www.youtube.com/watch?v=YwyLbjkm5nk

we changed into sixth at around 22 seconds, with the timing beams at 34 seconds. In that time we gained around 4mph.

[havoc]

Looking at the vid here

http://www.youtube.com/watch?v=YwyLbjkm5nk

we changed into sixth at around 22 seconds, with the timing beams at 34 seconds. In that time we gained around 4mph.

Which shows how good the basic package and the aero is, but how the lack of BIG power hampers the car in that sort of company.

Still...how often do you want to top 150?!?

[markc]

Paul and I were discussing S/C but I am too much of a girl to do the upgrade on my own. Brakes would definitely have to be upgraded.

I'm man enough... but too poor. I'd have to upgrade the brakes to a minimum of the Racing Brake Oversize (324mm/330mm) kit first but that's the best part of £2k just there :(


I happened across a Rapide at the services on the M40 last week and had a good chat with the driver and a good look around the car.

I would say that it is definately more appealing in the metal than in pictures.

IT rounded of good AMV12 day as earlier in the day I'd had a run alongside the V12 Vantage .... well until he smiles and changed up to 3rd but what a glorous noise :eek:

regards, Paul

Aston's just don't do it for me, especially a 4 door with cramped rear seating... I don't get it? It could be argued (so I will :)) that the Rapide is less interesting and almost as ungainly as the Panamera. The Panamera looks a bit like a stretched 911 but the Rapide IS absolutely a stretched DB9!

Astons do make a nice noise though :)

Cheers

Mark

[TheSebringOne]

Mark, if you had the money and wanted a four/five seater, what would be on your list? The problem with the Aston & Porsche, is that they have stretched the coupe version of the car and this spoils it for me!

I know Maser is getting on a bit and is due a replacement, but that would be my choice as its elegant and understated, plus its quite a bit cheaper too! Theres a good write up in last month's Evo on big saloons.

[markc]

Mark, if you had the money and wanted a four/five seater, what would be on your list? The problem with the Aston & Porsche, is that they have stretched the coupe version of the car and this spoils it for me!

I know Maser is getting on a bit and is due a replacement, but that would be my choice as its elegant and understated, plus its quite a bit cheaper too! Theres a good write up in last month's Evo on big saloons.

Well I'd shun all the German stuff, IMHO they're all too firm and fast, and lack real class as does the Lexus LS even though it rides much better than the Germans.

Actually I still quite like the Masser too, its properly big and spacious but even with the torque converter auto, rather than snatchy robot clutch, the engine is wrong (too peaky) for such a big arse luxobarge.

As I'm an old git, and therefore qualify for Jag ownership, I might just plump for the new Jaguar XJ in supercharged petrol form. If money was more of an object it be Citroen C6 every time :)

Cheers

Mark

[dan the man]

purple wrap is ace. Ive missed my EVOs since being in Canada. All still being delivered tho so will have some catching up to do!

[Alf Tupper]

I wonder if anyone has any ideas on this one?

Collected my NSX today and drove 120 miles mainly on the M4 and M25 so should give pretty accurate gps readings as discussed on earlier posts.

At an indicated 30mph the tom-tom reads 30-32mph.
At an indicated 40mph the tom-tom reads 40-42mph.
At an indicated 60mph the tom-tom reads 68-73mph.
At an indicated 70mph the tom-tom reads 79-80mph.

I also thought indicated speed should never be higher than the true speed. This is the case with my other car - at an indicated 70mph, I know that I am doing a true (gps) speed of about 65mph.

[Alf Tupper]

Oh, forgot to mention that the car has original OEM alloys, has correct sized tyres, tyres have decent tread all round and is the original gearbox.

[NoelWatson]

Oh, forgot to mention that the car has original OEM alloys, has correct sized tyres, tyres have decent tread all round and is the original gearbox.

If you live around M4/M25, you aren't too far from a number if us, so come along to the next mini meet.

[Alf Tupper]

If you live around M4/M25, you aren't too far from a number if us, so come along to the next mini meet.


Sounds good to me, would like to meet other owners, see other cars and talk NSXes!

[greenberet]

I wonder if anyone has any ideas on this one?

It sounds like there’s something wrong either with your speedometer or with your TomTom - a flash of genius, I know :). If you drive along in your NSX with your TomTom and have a friend drive behind you in another car with another sat-nav, you could call each other and compare speedometer and sat-nav readings. That should help narrow down where the problem lies. Maybe your speedometer just needs to be re-calibrated or maybe you need a new sat-nav.

[Alf Tupper]

It sounds like there’s something wrong either with your speedometer or with your TomTom - a flash of genius, I know :). If you drive along in your NSX with your TomTom and have a friend drive behind you in another car with another sat-nav, you could call each other and compare speedometer and sat-nav readings. That should help narrow down where the problem lies. Maybe your speedometer just needs to be re-calibrated or maybe you need a new sat-nav.

Thanks greenberet. I'm pretty sure it's not the sat-nav as it gives consistent results with other cars but not the NSX. Didn't realise speedometers could be recalibrated, I thought I might need a replacement unit so 'll ask a garage to do this when I get the chance.