Speedo drive

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Speedo drive

Postby Neilyman » 23 Jan 2005, 13:34

Any body know how to change the speedo drive?
I thought my cable was knackered `cos the speedo stopped working, it jumped about a bit before it died, so I`ve just had a look at it.
The cable is ok, if I spin it at the gearbox end, it spins at the speedo end.
With it connected to the g/box, if I drive the car, the cable doesn`t spin, but if I press it & `waggle` it it starts to spin a bit but it`s very jerky suggesting the drive gear`s taking time out!
Good old Haynes only mentions `drifting` the drive gear back in position when rebuilding the g/box.
Anyone got a g/box lying around that they could have a look at for me `cos it`s impossible to tell laid on your back with the cold wind blowing up yer trouser leg. :)

I could also do with a rubber oil seal that fits at the g/box end of the cable if somebody has one to sell me. They`re still available from the dealers, but only in a pack of 5!, why 5 ?? I only need 1 !
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Postby Neilyman » 27 Jan 2005, 00:46

Some one must have a gear box stood in the garage that they can have a look at!....Pretty please, OR tell me how many revs equates to 60 mph.
That`s the speed limit on the by-pass I travel on most nights about 11.30 when there`s no other cars about. I don`t want the blue light brigade after me!:)
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Postby Shellysowner » 27 Jan 2005, 01:17

You can work it out.

Gear 4 is 1:1,

Rear axle ratio is 4.11:1

And to get the overall diameter use width x aspect ratio x 2+rim height (I found this formula on a forum)
so for 155/70x13 it would be:

155x70x 2+ 13 x 25.4 = 547.2mm

2 x pi x radius = 1719.0795

at any rpm gives = (1719.0795 x rpm) / 4.11 mm per minute.

x 60 (mm per hour) = 1719.0795 x rpm x 14.5985

1 millimeter = 6.21371192x10^-07 miles so multiply by this.

= 1719.0795 x rpm x 14.5985 x 6.21371192x10^-07

= 0.01559392 x rpm

i.e 4000rpm = 62.4 mph
so times the rpm you're at by 0.0156 for a standard car although you'll need to account for your different wheels neily and do the calculation in full. Obviously the final value is gonnna be affected by tyre pressure, wind resistance, gradient etc. and many other factors but this will give you a rough guide. :D
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Postby shuvit-tim » 27 Jan 2005, 05:52

ZOIKS! :?
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Postby Neilyman » 27 Jan 2005, 08:55

Sod it!, I`ll risk a speeding ticket!..... :P :lol:

As it `appens Ralph, I`ve got `normal` 13" Rostyles on for the winter, so you`ve answered my question, keep it just under 4k & I should be ok.
8)
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Postby Shellysowner » 27 Jan 2005, 12:48

Sorry, got a bit carried away there :roll:

Yup 3846 is 60 dead so anything under 4000 should be fine.
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Postby Doive » 27 Jan 2005, 16:52

My rev counter shows an indicated 3500rpm exactly at 60, but then there may be errors in rev counter or speedo. I'm afraid Neily if the old bill pulled you for speeding they would do you twice, as its illegal to drive a car without a working speedo. Sorry mate!
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Postby Shellysowner » 27 Jan 2005, 17:33

Well the speedo on a standard chevette reads about 2 1/2 to 3 mph slower than the actual speed if I remember correctly (fortunately I have free access to some very complicated GPS + datalogging equipment for measuring that kind of thing). I've altered mine so that 60 is dead 60 although error on cheapie production speedos is not linear, you'll have most error low down, getting more and more accurate towards about 50 and then getting less accurate again above that speed.
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Postby Doive » 27 Jan 2005, 18:09

I never venture much above 50 anyway so I don't have to worry about that one! However, a lot of the speed cameras in edinburgh seem to be set to 31mph, so I'm a bit wary about going through them at anthing like 30 in fear of being nabbed. Hasn't happened yet to be fair.
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Postby Neilyman » 27 Jan 2005, 18:36

Thanks Doive & Ralph,
I know my speedo has to work, but If don`t tell them they won`t know:)
I think the drive is made of nylon, so I`m assuming it`s stripped a tooth or two, but I`ve no idea how to change it without seeing a g/box out of a car.
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Postby shuvit-tim » 27 Jan 2005, 19:04

the speedo threshold is set at 10% of the speedlimit plus 2 above the speed limit. ie:

35mph for a 30
46mph for a 40
57mph for a 50
68mph for a 60
79mph for a 70

These are the thresholds recommended by the government as being a reasonable allowance to take into account inaccuracy of speedos and the fact you cant look at the speedo 100% of the time you have to look at the road. If you are caught at below any of these thresholds, take it to court and you'll get off. :D
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Postby Shellysowner » 27 Jan 2005, 19:45

Which means you can do indicated 80 on the motorway without worrying!
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Postby Doive » 27 Jan 2005, 19:55

Not convinced I could do 80 anywhere!
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Postby Shellysowner » 27 Jan 2005, 20:06

You could y'know :D
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Postby shuvit-tim » 27 Jan 2005, 22:01

absolute top end out of my 1256 when it was still in was 86mph over flat ground. ie not including down a big hill.
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Postby Doive » 27 Jan 2005, 22:43

I think mine would probably burst, not to mention accelerating the wear already present in the valve stem oil seals. It sounds rough enough at 4000rpm, never mind 6000!
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Postby Shellysowner » 28 Jan 2005, 03:06

You always make it sound like you're fighting a losing battle against your motor doive - what with rust, brakes and smokey engines. Did the previous owner run it on diesel oil and pack it in damp salt every night?
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Postby shuvit-tim » 28 Jan 2005, 03:45

nah that's just leanne in an attempt to get rid of it! :wink: :lol: :lol: :lol: :lol:
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Postby Doive » 28 Jan 2005, 12:21

I'm struggling to keep up with all its needs on a student overdraft, for the last year it has seemed to be one thing after another. Fix brakes - 300 quid. Prepare for MoT - 250 quid. Repair floorpan thats fallen out - 300 quid. Tax and insure the beast - 400 quid. Replace worn stem seals - £unknown.

I suppose these kind of costs are to be expected with a car thats been standing for three years following a hard life as a farmer's car. It's not a losing battle, the man who fixed the floorpan declared her to be in very rude health indeed - its just trying to keep up with the bills!
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