Another speedo question!

Post by **ani** » Tue Jun 23, 2009 5:52 pm

This is what the previous owner has said about th trav/pickup "The Speedo will never be accurate as it is a Triumph Speedo, married to a Sierra gearbox. It used to show ~18mph faster than actual with occasional warp to 110mph... I just accepted it as 'character' and it didn't fail MOT on that basis so nothing more was done about it. "

Whilst I can sort out the greasing, tightening or even get a new cable, 18mph faster than actual is a bit irritating.

What's the best way forward - get a Sierra Speedo?

(Its a 1275 Midget engine)

Post by **d_harris** » Tue Jun 23, 2009 5:56 pm

Or send it off to JDO to be recalibrated......

Post by **wiglaf** » Tue Jun 23, 2009 6:11 pm

After really good service and advice from JDO last week - I'd incline to agree with Dan. Tell John at JDO all the details of gearbox/diff/wheels etc. etc. and see what he says!

http://www.jdo1.com/

Post by **PSL184** » Tue Jun 23, 2009 6:17 pm

Have you checked the speedo against a Sat Nav since you changed your wheels - I think you had 13" wheels before and you are now on 14" so that will make a difference.....

Post by **ani** » Tue Jun 23, 2009 6:48 pm

This is for the traveller conversation that my son bought How do you check what the diff is.

http://picasaweb.google.com/willy.jaun/MorrisMinor?#

Post by **PSL184** » Tue Jun 23, 2009 7:45 pm

ani wrote:This is for the traveller conversation that my son bought How do you check what the diff is.

Jack up one back wheel. Mark the wheel rim with tipex and the diff flange also with tipex. Rotate the wheel 10 full turns and at the same time count how many turns the dif flange rotates. Divide the number of diff rotations by 5 and you have your diff ratio...

ie 10 wheel rotations turns the diff 21.1 times. Divide by 5 equals 4.22:1 ratio (diff will probably be a 4.22 ratio) or if it turns 22.75 times its a 4.55:1 ratio.

Post by **aupickup** » Tue Jun 23, 2009 7:50 pm

there will be markings on the top of the diff for example 9 41 9 - 38 etc divide the numbers and this will give you the diff ratio assuming its a moggie type diff so will be 4.55 or 4.22 ratio

then u can possibly match a speedo , is it a 5 speed box

Post by **ani** » Tue Jun 23, 2009 7:54 pm

Its a 5 speed Sierra box

Post by **PSL184** » Tue Jun 23, 2009 7:54 pm

aupickup wrote:there will be markings on the top of the diff for example 9 41 9 - 38 etc divide the numbers and this will give you the diff ratio assuming its a moggie type diff so will be 4.55 or 4.22 ratio

then u can possibly match a speedo , is it a 5 speed box

True but that assumes that you can read the figures on the casing as they are in the most inaccesible of places with diff fitted in car and also that no one has changed the internals of the diff over. Also the filler plugs are in diferent places on 4.22 and 4.55's but best not to assume anything and check it for yourself, esp if you are about to fork out £50 to have the speedo recalibrated on the strength of it

Post by **aupickup** » Tue Jun 23, 2009 8:12 pm

was not suggesting it was easy to read the numbers, i have had 4.55 and 4.22 diffs that appear the same on the casings ie fillers, so that is not all ways the case

maybe a mirror can used to read the numbers

Post by **d_harris** » Tue Jun 23, 2009 9:25 pm

My diff is a 3.9 in a 4.22 casing....

Post by **ani** » Tue Jun 23, 2009 9:42 pm

Its beginning to sound easier to live with a wonky speedo!

Post by **d_harris** » Tue Jun 23, 2009 9:57 pm

You are bringing this thing to the national right?

We'll have a look at it then and work it out for you!

Post by **LouiseM** » Tue Jun 23, 2009 9:58 pm

My speedo seems to be reading faster than the car is travelling as well. The previous owner mentioned something about a midget diff but I wasn't paying too much attention at the time, being not particularly mechanically minded

But if a midget diff was fitted and the original speedo not changed how far out would the speedo be?

(sorry for the slight hi-jack of your thread Ani but it's sort of the same topic

)

Post by **ani** » Tue Jun 23, 2009 10:05 pm

That's ok Lou and thankyou Dan. We will be forming an orderly queue outside your tent

Post by **MColes** » Tue Jun 23, 2009 10:18 pm

If we're having a 'speedo surgery' I'd appretiate it if someone took a look at mine. Although I think I know the problem

Post by **d_harris** » Tue Jun 23, 2009 10:37 pm

A midget diff (assuming 3.9 here)

For 1000 turns of a standard 4.22 diff the wheels will spin 238 times
For 1000 turns of a 3.9 diff the wheels will spin 256 times

meaning the speedo will be about 8% fast (theoretically) however as most speedos read about 10% fast anyway it means you likely have a pretty accurate speed reading

Post by **LouiseM** » Tue Jun 23, 2009 10:52 pm

Thanks Dan. The speedo doesn't seem that accurate but perhaps it's just the comparison between driving my modern that's making the difference.

Post by **d_harris** » Wed Jun 24, 2009 12:26 am

Check it against a GPS?

Post by **bmcecosse** » Wed Jun 24, 2009 11:22 pm

The spedo will be out in direct proportion to the difference(!) in the ratios ! So - if the car originally had a 4.22 final drive (1098 engine) - and now has a 3.9 final drive - the speedo will be 'out' by 4.22 / 3.9 X 100 %. Best way is to find the final drive ration - the wheel turning method above is fine - then search for a speedo that matches - this is MUCH less expensive than re-calibrating a speedo!! Standard 1098 engine car will have a ~ 1408 tpm speedo - fitted with a 3.73 final drive, a ~ 1244 tpm speedo from a Mini fits perfectly and is exactly correct on both miles covered, and speed. I'm not sure if there will be a Mini speedo to suit a 3.9 final drive - but it's worth asking! Just search ebay - and contact the speedo sellers and ask them what tpm each speedo is ! You will be looking for something like 1300 tpm.
As for the Triumph/sierra combination - again, look for the existing 'tpm' on the speedo, and find out exactly how much the speedo is wrong. It should be a linear change - not a fixed '18 mph' difference. If you don't have access to a GPS - just compare with roadside signs - or follow some trucks on a flat open road - they will be doing 56 mph!