On Mon, 24 Feb 2003, at around 09:25:50 local time, Richard Gosling
<richard.gosling@exprogroup.com> wrote:
>Mike,
>
>You seem to be suggesting (correct me if I got this wrong) that you are
>getting an accurate speed reading but an inaccurate odometer reading - so
>the gauge is disagreeing with itself?
Dear Richard,
Yes, but this is because I have freaked the speedo readings by moving
the needle up by about 9 mph. Taking road speed readings from the rev
counter (rpm x 0.018), and comparing them to my speedo, I get:
Road Speed Old Speedo Readings Freaked Speedo Readings
0 0 9
30 26 33
40 34 42
50 43 51
60 51 60
70 60 70
In other words, the speedo, when it was first fitted to the car (by me),
read about 15% low across the board. By advancing the needle, I have
made it spot on at higher speeds, and within 10% at lower speeds.
Quite why the odometer should read 9% low, and the speedo 15% low, is
beyond me, but presumably has something to do with the position of the
needle at rest. If I change the drive gear such that it gives me an
accurate odometer reading, it will increase the speedo reading as well,
meaning that if I set the needle at 3.5 mph at rest, the speedo readings
would be:
Road Speed Freaked Speedo Readings Reset Speedo Readings
0 9 3.5
30 33 30
40 42 40
50 51 50
60 60 60
70 70 71
Which looks pretty good to me!
>
>I'd be more inclined to believe that the odometer within the speedo is
>working correctly, since it is powered just by a set of gears - so many
>revolutions of the speedo cable must always give a certain number of
>revolutions of the digits, regardless of speed. That only leaves the input
>to the speedo cable being wrong - and that is just a function of the ratio
>between the speedo drive and the gearbox output shaft, the diff, and the
>tyre diameter.
>
>The speedometer mechanism is more likely to be wrong, as the mechanism has
>more scope for inaccuracy than the odometer. The fact that you had to tweak
>it so much adds strength to that arguement.
>
>This really leaves only the diff and the speedo drive, since you are running
>standard tyres. Since you have checked the diff ratio, I suppose it can
>only be the speedo drive - which is the conclusion you already seem to have
>come to!
Glad to hear that my reasoning makes sense! Thanks for the
confirmation.
>
>There's another reason (beyond the fact that all 4-spd boxes have a 1:1
>final output) that the problem can't be the gearbox ratios - the speedo
>drive is connected directly to the OUTPUT shaft of the gearbox, so it
>doesn't make any difference what the gear ratios are, because the bit you
>are measuring is always what is being fed directly to the wheels.
Understood (now!).
>
>Richard & Daffy
>
>P.S. what you seem to have hinted at, but totally omitted any details on, is
>that you just took Carly on holiday to Cornwall and back! Tell us more -
>how was the trip?
Not quite. The four of us went for a week's holiday to Somerset in two
cars (isn't it amazing how much luggage a baby needs?), and I went off
on my own one day down to the Duke of Cornwall's Light Infantry Museum
in Bodmin. Bedford to Somerset was about 165 miles, and Somerset to
Cornwall about another 125 (plus 9% for the dodgy odometer!), so I
reckon Carly did about 650 miles. The trip was fine, and Carly behaved
perfectly, apart from that little trouble with the manifold/downpipe
gasket which you will have seen mentioned elsewhere. I was
particularly pleased to note that on the two occasions that we got stuck
in long traffic jams, the engine temperature needle remained in the
middle of the gauge. :-)
My wife is now convinced that I and Carly are happiest cruising at about
90 mph on motorways, since her speedo appears to be set to read nearly
15% high!
ATB
--
Mike
Ellie - 1963 White Herald 1200 Convertible GA125624 CV
Connie - 1968 Conifer Herald 1200 Saloon GA237511 DL
Carly - 1977 Inca Yellow Spitfire 1500 FH105671
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