The last 4 digits written on the face of the speedo dial after the makers part
number is the number of times the speedo cable has to turn for the car to cover
one mile. So the gears in the gearbox tail shaft or overdrive have to give the
correct number of turns for the mph and odometer to read correctly. The gears
in the gearbox have to be selected based on what differential ratio you are
using, the gearbox or overdrive has no effect since the output shaft from the
transmission is of course turning at the prop( drive ) shaft speed. On my MkIV
no overdrive, 3.89 diff ratio, US Spitfire the speedo count was 1184. When I
acquired a J type overdrive and single rail box from Spitfire grave yard in the
UK I purchased the correct gears for the gearbox from Rimmers to be compatible
with the 3.89 diff used on US Spitfires. When I got the car on the road the
speedo was reading way too low. Knowing the speed that it should have been
indicating from the technical info in the ROM relat!
ing engine speed to road speed I realized I needed a speedo with a 1000 count.
Since there are places in the US that will adjust the count of the speedo to
the count you need I purchased a used Speedo off E Bay here in the US. When
this speedo arrived I was astonished to see that its count was indeed 1000!. So
I installed it and eveything is fine.
Hence it would seem that at different points in the Spitfire build cycle
speedos with different counts were used. So the first thing to do when you
modify the diff ratio or tire/wheel size etc is to establish what count you
need. This is easy to do. Take the car to a nice straight local street and
disconnect the speedo cable and using a piece of wire wrap it around the end
of the inner cable so it makes a little pointer like a finger on a clock dial.
Put a chalk mark on the road directly under a rear wheel centerline . Get
yourself a measuring tape and mark another line on the road say 100 yards up
the road. Sit in the car and get someone to slowly push you along as you count
the number of turns the little pointer makes. The guy pushing the car has to
bring car to a standstill when the second mark is directly under the
centerline of the wheel and you have to note the number of revolutions of the
pointer plus the fraction of a turn when you stop. Since there are 176!
0 yards in a mile you can then work out the count required. A 1000 counts per
mile is 56.8 counts for 100 yards for instance. Do it a couple fo times to
make sure you get consistency and you are all set to get your speedo modified.
Of course the gear in the speedo can only be adjusted to certain combinations
for which the parts are available unless the guy doing it has the ability to
make a gear with any tooth count or, if you are not bothered about the odometer
reading correctly , he can adjust the metal plate which is dragged around by
the rotating magnet inside the speedo to virtually any count you want - if he
has the skill and knowledge that is. Otherwise he will make you a little
gearbox which fits between the gearbox and the cable to get the correct count.
sagreenwood@earthlink.net
EarthLink Revolves Around You.
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