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MG MGF Technical - Speedo accuracy?

Sorry if this is old ground.
Long story so i'll cut to the quick:-

I was following a 54 reg HGV on the motorway (which are I thought were electronically governed to 56mph) I was doing about 2.5k rpm, which according to the info I have is pretty much bob on at about 11mph/500rpm. However the speedo was saying about 62/64mph.
Over a 300 mile journey the tripometer is accurate to to within about 2 miles - based on distances from multimap.
What is likely to be more accurate? revcounter or speedo?
And why, if as I suspect my speedo is way out, is the tripmeter so accurate?
Thanks
G
PS
Can you get speedos recalibrated? is it expensive?
g hampson

I have checked my 2000 VVC MGF against a couple of different GPS systems and it tends to read accurately up to about 30mph then over reads above that eg speedo says 60 when GPS says 56, I also get similar results from my work audi quattro.
R Allan

Most speedo's will read about 10% over actual. ie when your speedo says 70mph, you are probably only doing 63-64mph.
It is an homologation regulation for the speedo not to under read for obvious reason, so most manufacturers design the speedo to be 'innacurate'.

Dave
Dave

The rev counter is more accurate. I also agree with you - the trip on my '96 MGF is pretty much spot on.

As Dave and R Allen say, the speedo will usually under-read by 10% at around 70mph which is permitted under homologation rules. It is normal, and nothing to worry about.

It also means that when you travel at 80mph on the motorway, you are probably only doing a genuine 72mph, which is within tolerance that the Police specify on speeding penalties... But I wouldn't advocate speeding, obviously.
Rob Bell

I actually asked a police motor-cyclist (after he'd stopped me for angrily gesticulating at him for driving too close to me on the M2 and then pointing to my speedo when he overtook me at about 85+) if he could drive at exactly 70mph so I could check my speedo (not scientific, I know) and, assuming he was actually doing that speed, my speedo read about 73mph.
S R Kenyon-Edwards

Thanks guys.
So from now on when I overtake a bobby on the m/way if I keep it around 3.2k revs, he won't scowl and shake his finger at me?
Like the one did yesterday?
Ta
G
g hampson

I have passed police cars on the motorway many times at an indicated 75mph or so as the marked cars seem to drive at about 65mph. Whats always amusing is that there is usually a cluster of cars stuck around the police car, I presume because they think they can't go faster with out being done.
D Billington

Is it possible/affordable to get a calibrated Speedo?
Russell Parslow

Russell
From the lack of response to this question in my original post - maybe not?
I'm thinking of calling at the local rolling road to see if they can give a true read out of speed and just making a mental (or on back of sunvisor - might actually be of some use afterall!!) note?
Anyone know if rolling roads give a true speed read out?
G
g hampson

A cheaper way would be to find a motorway where you can drive at a constant speed then compare your speedo reading with the time taken to pass, say, 10 of the 100m markers. I can't remember the metric conversion factor but 1m is equivalent to 3 feet, 3&3/8 inches and there are 5,280 feet in a mile.

Happy calculating.

Chris
Chris

Chris
An equally cheap but far less mentally taxing alternative, would be to do do this excercise using the Km markings on the speedo!!
Now how much are stopwatches?
g hampson

It used to be that every mile on all major roads in the UK 2 metal studs about 4" square were inserted in the road surface to be picked up by detectors in a vehicle. When you know they are there you can look out for them. I'm not sure if they are still present as I haven't looked recently, with the advent of GPS they may have become redundant..
D Billington

A friends wife was recently fined with penalty points for doing 31mph in a 30 zone and after complaining was told that there was zero tolerance, this was in Doncaster.
mike

Mike, I would doubt that any speed sensing device used by the Police had an accuracy of 3% @ 30mph - so I think that that judgement is somewhat harsh :o(
Rob Bell

I was under the impression there was a threshold of +10% + 2mph?
i.e. 35 in a 30?
It does seem a little heavy handed.
g hampson

I tried a quick test whilst driving into work this morning. My car is a TF135 with 15" wheels, so according to this page
http://www.mgcars.org.uk/mgtf/mgtf05.html
should do 20.8mph/1000rpm in 5th gear.

That means that at 60mph the rev counter needle should be halfway between the 2750 and 3000 rpm markers as 1000*60/20.8=2885. In fact, when the rev counter reached this point I was doing more like 65mph. This suggests that the speedo reads about 7-8% under.

I also checked what's happening at lower speeds. Looking at the gear ratios on the mgtf spec page (as above) I reckon that in 4th gear I should be doing 20.8 *0.765/1.033 = 15.4 mph/1000rpm. This means that 30mph should be at 1948rpm but I was doing no more than 1800rpm when my speedo said 30mph. This is again consistent with a 7-8% error.

I can fully understand why speedos are set to read high but wonder whether has always been the case. If not, this would certainly explain why when I had an MGB if someone followed me in a modern car they always reckoned I was going over the speed limit. Maybe it was them who were driving well under the limit??

Chris.
Chris Rudge

Hi all,

I own an MGF 1.8i and look at the BBS regularly.

Surprised to hear about the 'zero tolerance' situation in Doncaster. Even here in North Wales, where the police have a reputation for being hard on speeding motorists, they apply a 10% plus 2 mph tolerance.

I passed a mobile camera in a 30 mph zone in my Peugeot 406 last year when my speedo read 37/38 mph but I didn't receive a ticket. The only explanation I could think of at the time was that my speedo doesn't show my correct speed. Your comments would seem to confirm this is the case with all cars.

Keith.
Keith

I am running 16" wheels Front 195/45. When the speedo said 100mph the GPS said 95mph.
Steve Ratledge

This thread was discussed between 04/01/2006 and 06/01/2006

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