Fuel economy/speedo accuracy

Doc · Post by **Doc** » 07 Jun 2004, 05:43

Anyone out there know how much XM speedometers/odometers overead by?
Just worked out fuel consumption of my XM 2.1 SED at 45.14 miles per gallon.[:)]
On the 2nd June I filled up with 77 litres after driving 76 miles with the fuel light on. (I know its illegal driving on a motorway knowing there may be insufficient fuel but I suspect the fuel light is a little over zealous) Tank hold 81 litres so still had 4 litres in reserve plus whatever is in fuel lines.
Today with 395 miles showing and still over half a tank I got in 39.36 litres in using the same pump as before. 39.36 litres = 8.75 gallons. Hence the figure of 45.14 mpg. If the speedo and therefore the trip meter are overeading at say 5 percent then the real mpg would be 42.8mpg - still a respectable figure bearing in mind the size and weight of the XM. I generally keep car at 75-80mph (where conditions allow) as I don't like the idea of 44 ton trucks driving a few feet from bumper. A common occurrence on M11/A14
I suppose a 20 percent error would drop actual economy down to 36.1mpg
If my maths is wrong I appologise in advance.
What could one expect from a ZX diesel and a ZX turbo diesel in average miles per gallon? I am still considering changing to a smaller car.
Thanks in advance
Doc

drkstuart · Post by **drkstuart** » 07 Jun 2004, 11:30

as i drive a "44 tonner" i don't think my xm is a big car but any of the fuel figures you mention are respectable for the speeds you travel.

Kered · Post by **Kered** » 07 Jun 2004, 13:14

I recently bought a GPS navigation system for my Xantia.
It uses a PDA hand held computer and Tomtom navigation.
It is superb as it gets me to where I want to go - and she doesn't nag if I go wrong!!!!
All for £400.
It offers many things apart from the navigation. One is a speedometer. As it's computed from the GPS satellites it is accurate. My Xantia reads a consistent 5 mph over. Another Xantia did the same. A Renault Laguna was 2 mph over.
These navigation systems have no connection with the car apart from power from the cigar socket. So if you know anyone who has one - then trying in your car is your answer.
Derek

Doc · Post by **Doc** » 07 Jun 2004, 17:38

Thanks gents, I may keep the XM after all. GPS SATNAV for £400 sounds good and as it must use the same satellites as the military it must be as accurate as any corrected police speedo.
Doc

TomH · Post by **TomH** » 08 Jun 2004, 00:13

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Doc</i>

What could one expect from a ZX diesel and a ZX turbo diesel in average miles per gallon? I am still considering changing to a smaller car.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
I mainly do town driving with the odd blast up the motorway at quite high speed (80-90) and my ZX diesel (n/a) gets usually about 44-48mpg, never below 42. The last tank, which was a lot of A road driving, got me up to 55mpg. But I do have a heavy right foot. I suppose a more careful driver could get more out of it!

James.UK · Post by **James.UK** » 08 Jun 2004, 00:15

arry_B gets 65 mpg from his ZX and covers a LOT of miles.

JohnD · Post by **JohnD** » 08 Jun 2004, 00:16

I've had a BX, and now a Xantia, and they have both had inaccurate dials. When doing long journeys following a route taken from Microsoft Autoroute, the figure on the Xantia's mileometer is always around 5% more than the Microsoft distance. Also, if I calculate speed by rev counter using 27.3mph/1000rpm (the 2.1TD's engine spec), the speedo also shows a 5% inaccuracy. It therefore follows that my 42mpg is really only 40mpg.

Richard Gallagher · Post by **Richard Gallagher** » 08 Jun 2004, 01:44

John, don't rely on rev counters being accurate, they are no better than speedos.

AndyP · Post by **AndyP** » 08 Jun 2004, 21:23

According to the GPS my Xantia 1.8i is almost spot on all the way up to about 85mph. Never more than about 1mph out ! I was amazed ....