speedo

[cadetchris]

many moons ago, my brother destroyed the diff on the moggy by doing donuts. dad replaced it with a wolseley 1500 diff. however the speedo was optamistic to say the least.
having acted on advice of many fine people on here, i installed a mini mk 1 speedo with the vital number 1348 tpm.
however, this didnt work and the speedo was still up to its own tricks. this has been doing this for many weeks and its doing my nut it.
how can i fix this?

[PSL184]

What is the question?
Is it "why is the needle bouncing up and down"?
If so it will be the cable that is sticking and will need greasing and checking for kinks and damage...
If that was not the question then let us have some more details.....???

[cadetchris]

for clarification

i have a wolseley 1500 diff, standard engine and gearbox and inorder to get a "correct" speedo reading, i have a mini mk 1 speedo head with a 1348 tpm.
this in theory should all work, but it certainly dosent on my car.
why does my speedo not read anything like the correct speed when i have all the right bits, in the right order and correctly oiled and connected?

[bmcecosse]

Because -1348 is NOT the correct tpm to match that diff! Theoretically - you want a 1236 tpm speedo for a 3.73 final drive. I found a 1280 tpm Mini speedo - and it's PERFECT! Both miles and speed are absolutely spot-on (better than standard spec!) - that's with my 155 x8o x14 tyres on standard rims.

[cadetchris]

may i quote from my previous post on my speedo
"PostPosted: Sep 19, 2009 - 10:08 PM, bmcecosse
You need a Mini 1348 tpm speedo - plenty on ebay last time I looked - mine cost me £5 or so. Ask vendors to confirm that tpm figure before bidding"

hence i bought a 1348 one

[bmcecosse]

Did you make it clear that was for a 3.7 diff? I really can't remember now - but if I advised that then sorry - must have been mixed up with something else. I'll check again later what's actually in my car - I lifted that 1280 tpm figure from an earlier post - I had better look and see what I ended up with - I did go through a couple of speedos until I got it just right. Your post may have been from an intermediate posting. However - even so - your 1348 speedo can't be that far out. If mine is 1280 - yours is only 68 out - which is ~ 5% different - under reading at that! The standard speedos are only accurate to about 5% (if lucky) - so really it can't be that far wrong. How do you think it compares in the car against other cars/flashing speed signs etc ??
And - are you absolutely sure you have a 3.7 diff in the car ?

[cadetchris]

its ok, one of those things i suppose.
my speedo is actually quite a bit out, i can be doing approx 20mph and it says 50. novel, but the speed camera on the a52 is waiting for me to forget that the speedo dosent work.
as for the diff, i am pretty sure its a wolseley 1500 one, as we bought if from wolseley 1500 spares and the nice man said it was deffenently from a 1500.
is their any way of checking for sure without taking the whole thing apart? presumably its stamped on the case somewhwere?

[PSL184]

It sounds like the speedo has been messed with and is faulty - It won't be that far out with that diff or any other for that matter. The id is usually on the inside of the diff so the easiest way to check would be to jack up the car and turn the wheel whilst looking at the propshaft and seeing how many times it rotates for one full wheel rev. You will get a more accurate figure if you turn the wheel say 10 times then remeber to divide your answer by 2
ie Turn wheel 10 full turns = propshaft spins 74.5 times
divide 74.5 by 10 then by 2 = 3.72:1 ratio

[MarkyB]

Stamped into the ally on the top to one or other side of the fin.
After a bit of a clean you could try taking pictures with a mobile phone camera, unless you have a very thin head

[bmcecosse]

I haven't managed to double check what the speedo is in my car - it's not easy to get at, at the moment! But 50 mph when doing 20 is very definitely not right! And not caused by tpm differences.

[cadetchris]

fudge, well i am putting a new engine in in a few weeks, so i my as well do the gear box as well (the syncro has gone on all gears) and check the diff again. if its not any of those, i and many an expert are stumped

[bmcecosse]

What tpm speedo was in the car before - and how did it read true/indicated speeds?

[cadetchris]

it had the standard morris 1000 speedo, because of the change in diff, this was also wildly out. i could be happily going approx 60mph and it would read 100,

[cadetchris]

but before we changed the diff, and had the standard one, the speedo was bang on

[PSL184]

With everything standard and a 3.7 diff the speedo should under read shouldn't it ??? Are you sure you didn't put a 5.8 diff in !!!

[bmcecosse]

Yep - sounds very strange - the speedo should indeed have been reading low compared to the true speed you were doing. I think you need to check that diff!

[bmcecosse]

Ok - checked my speedo today - and it IS 1248 tpm which is perfect for a 3.7 diff. I can only apologise for having stated 1348 in that earlier thread - a simple mistype - although in my defence another poster had stated 1350 earlier in the thread - and that may have influenced my finger!
However - from the way this thread reads - you DO NOT have a 3.7 diff anyway. I suggest you check it carefully. Meantime that 1348 speedo would be ok with a 3.9 diff!

[PSL184]

Yes, it was me who said 1350 ish as I have a 3.9 diff and that's the speedo I have - I can't imagine the difference would be so great esp with 155 tyres fitted anyway? As your speedo is reading about 60% high I'd be checking the diff too as suggested earlier....

[cadetchris]

would tyre profile affect the speedo at all? i have slightly lower profile ones, but still the standard diameter, but having said that they didnt affect it before when i had the standard diff.

so inorder to check the diff, i have to look for what stamped on the diff casing if i am going to check it?

[billlobban]

The simple answer is yes but obviously not by a huge amount. There is a way of calculating your axle ratio if you dont know/ cant find the info but the exact method escapes me (I'll remember later) but Roy will explain.