Speedo error and fuel economy?

[croc]

Hi all
Just reading some posts re fuel economy (mpg) and indicated speed v GPS speed.
Question is:- My hitop, 1.9DG, standard steel wheels, indicates 10 mph faster than GPS at about 60 ish. I thought it was only mine but apparently many others have same issue. Is there a known reason for discrepancy?
Also, does this mean the odometer showing miles travelled is also in error by the same proportion?

Re fuel economy, we did 2700 miles on an EU trip last year and overall averaged 27 mpg, this at max 60 indicated on speedo and included towns, motorways and the Picos in Spain.
Now wondering if this mpg is in error as 60 miles travelled by speedo may really only be 50 miles ?? This would make mpg much better than it really was!!
Any thoughts or facts here??
BTW thoroughly enjoyed our, slow, trip and am so impressed with our T25 for being a fun, nice driving and convenient camper. Wouldn't change for quids!!
Cheers jon

[Aidan]

all T3 speedos overread speed by 10%, that was a legal requirement, most vehicles overead on the speedo, but the odometer will be accurate provided the vehicle is running the original tyre size or an equivalent radius tyre, if you fit smaller diameter tyres then your odo will overread and the speedo overread by even more, if you fit bigger tyres your odo will underread, but the speedo will appear more accurate

[croc]

Thanks Aidan
That explains some of the error which is 6 mpg at 60 but there is still an extra 4 mph.
Did the T3 come setup for crossply which would have a bigger rolling radius than radial?? I run 185 14C radial.
This would fit with your comments about changes in tyre size.

[Aidan]

185R14C is standard tyre size, they were radials, there was a change to the speedo set up from 1985 certainly for the american market which is covered in the Bentley manual which is where the only known published information regarding the speedo calibration comes from, obviously UK market speedos in mph may not be exactly the same but we'll never get that technical information from VDO the manufacturer, the earlier speedo may be more innacurate in speed reading, all you can do is your own calibration using gps but be aware that gps isn't totally accurate apparently.
But as an aside I drove a syncro back from germany in company with a golfmk2 and the difference between the odos over 500+km was only 1.2km, so they were calibrated pretty similarly back in the day and the swedish sika I have which has the swedish market only printing odo under the seat has a difference of only 47km on 267,000+ recorded km between the dash and the special one so pretty well calibrated, both made by VDO of course
I run 205R14cs on my van with the speedo drive (syncro only) for 185R14C fitted to front diff, my speedo only overeads by 7-8% and my odo undereads by 4%, and if I put the 215/75/15 MTs on the speedo is almost bang on at 50, but odo is underreading by closer to 10%
a caravelle running 205/70/14 will appear to be faster than a van running standard tyres as the 205s are a nats smaller in circumferance

[HarryMann]

That explains some of the error which is 6 mph at 60 but there is still an extra 4 mph.

Aidan right I think

But you should do the correction from the actual speed not the indicated (really), so at genuine 50, 55 indicated would be allowed, 56 not.

In practice, I think those tolerances are to reduce speeds a bit (working on the safe side), but the usual ex-works accuracy is more like + 6~ 8%

On a long motorway section you could get a passenger to count the miles on the posts and compare against your Odo. Do this over ten miles accurately enough should give you a fair idea. (100 yds over 10 miles is about 0.5%). You'd need to clock the odo closely on the tenths reel moving

Some of the mpg figures we see on here , I must say, do raise eyebrows (some almost theoretically impossible!)

[California Dreamin]

Hi all
Just reading some posts re fuel economy (mpg) and indicated speed v GPS speed.
Question is:- My hitop, 1.9DG, standard steel wheels, indicates 10 mph faster than GPS at about 60 ish. I thought it was only mine but apparently many others have same issue. Is there a known reason for discrepancy?
Also, does this mean the odometer showing miles travelled is also in error by the same proportion?

Re fuel economy, we did 2700 miles on an EU trip last year and overall averaged 27 mpg, this at max 60 indicated on speedo and included towns, motorways and the Picos in Spain.
Now wondering if this mpg is in error as 60 miles travelled by speedo may really only be 50 miles ?? This would make mpg much better than it really was!!

The last statement would be the other way around...worse not better.

You also contradict yourself a little as you say 10mph faster at an indicated 60...so your speedo says 60 but your GPS reads 50? thats 20% over reading..which is a pretty high over read compaired to the average T25.

Then later you say 6mpg (I'm sure you mean 6mph?) which is a 12% over read....a little higher than most which tend to over read be just below 10%.

As for miles per gallon....27mpg is very good, even exceptional so I would be pretty pleased about that.

Martin

[Plasticman]

I got a friendly hofficer to to follow me for a few miles and I know that mine says 65 when its 57-58.
mm

[croc]

Thanks all for your comments.
This is not a critical issue, more curiosity but it is good to have confirmed that the speed discrepancy does not risk fines ... or worse.

Martin, I must take issue with your comments!!

This would make mpg much better than it really was!!

To clarify; I sould have said "This would make the calculated mpg better than the actual as the vehicle has not travelled as far as indicated by the odometer for x amount of fuel" Thus worse actual mpg than calculated.
I also said :

indicates 10 mph faster than GPS at about 60 ish

and at least meant:- ie 'at an indicated 60 the GPS says 50 thus over reading by 10 mph!' not %

If, as Aidan has stated, my 60 mph max speed on the trip was actually more like 50 mph in fact, given 10% speedo over-read error from factory, then 27 mpg becomes more credible also as the odometer should be correct on standard wheels/tyres.
I do admit that that the way I phrased my comments may have left possibility for some ambiguity though.
Cheers
jon

[jamesandtheopenroad]

So in the handbook, it says that fuel consumption at 56mph is around 27mpg - is this speedometer speed with the 10% increase (so 50.4mph), or actual land speed?

[California Dreamin]

As 56mph is one of the 'standard' speeds for ascertaining MPG I would have thought it has to be accurate across makes and models. Therefore, I'm guessing this would be done with an accurate speedo although not with satelite navigation as that wouldn't have been around at that time.

Martin

[CovKid]

Even with all these calculations, at the end of the day you drive it the way you have to and it does what it does to the gallon. Besides, with petrol seeming on a continual rise, this is all academic. Its what it costs you in your pocket that really matters.