3.7 Diff

Post by **bmcecosse** » Fri Aug 29, 2008 10:53 am

Hopefully, I will soon be replacing my 4.22 diff with a 3.7 version. This I feel is far more practical (and certainly much less expensive) than a 5 speed conversion. Question is - what to do about the speedo ? Does anyone know if there is a Mini speedo that has near enough the correct 'turns per mile' to give reasonable results with this 3.7 diff ?

Post by **Peetee** » Fri Aug 29, 2008 2:40 pm

Dismantle the speedo face, respray and apply some Letraset.

Post by **dunketh** » Fri Aug 29, 2008 2:43 pm

May I be the first to congratulate you!

Safe to assume you didn't pay the earth for it either?

Post by **Kevin** » Fri Aug 29, 2008 5:29 pm

Question is - what to do about the speedo ?

You could always borrow a sat nav that would at least show how far it was out and if its miles you could use stickers on the speedo glass until you have saved up to get it recalibrated.

Post by **aupickup** » Fri Aug 29, 2008 6:21 pm

hmm i still have a few left

Post by **bmcecosse** » Fri Aug 29, 2008 7:59 pm

Well - I know my existing mileometer reads almost exactly 1% over, and the speedo agrees with the flashing signs within about 1 mph - and agrees with 56 mph behind trucks ! So - I'm assuming that I just apply the ratio 4.22/3.7 = 1.14 to the speed (and mileage) displayed to find what I am really doing! I had a very early sat nav device some years ago - didn't find it all that much use, and when it stopped working I didn't miss it! However it would be handy if a Mini speedo had near enough the right tpm to read correctly !
Edit - seems it is actually a 3.73 diff - so the correction factor is 1.13.

Post by **Kevin** » Fri Aug 29, 2008 11:23 pm

Well current sat nav's give very accurate speed readouts.
1% over isnt much, even with my 165`s mine more over than that.

Post by **paulhumphries** » Sat Aug 30, 2008 8:27 am

I've got an new electronic speedo that I bought cheap off eBay - around £10-15
Not on my Minior but another vehcile, I was going to swop out the "guts" so the original looks but electronic internals.

Paul Humphries

Post by **bmcecosse** » Sat Aug 30, 2008 10:04 am

Hmmm - interesting Paul - keep me updated how that goes along please!

Post by **Packedup** » Sat Aug 30, 2008 10:19 am

Can't you just change the speedo drive in the box? I was under the impression there's different "colours" available for the ribbed box, each one with a different ratio.

Post by **bmcecosse** » Sat Aug 30, 2008 10:23 am

Ahhh - there is a system like that for the Mini gearbox - not heard of it for Morry box.

Post by **Packedup** » Sat Aug 30, 2008 11:37 am

http://www.mgcars.org.uk/midgetspritecl ... atios.html

Not sure if it helps or not, but it claims there's different pinions available

Might be worth trying the MG specialists to see if there's any truth to it.

Post by **bmcecosse** » Sat Aug 30, 2008 12:33 pm

There's an idea then. Anyone one here got a Spridget with 3.7 diff - could you tell me the tpm number on the speedo please ? I realise they had 13" wheels - but it will be a starting point.

Post by **bmcecosse** » Sat Aug 30, 2008 12:47 pm

Finally got the link to load - excellent find Pup! - tells me speedo in that car with 145x13 wheels would be 995 tpm. And indeed seems speedo drive gears are available in a range of ratios. I wonder if they are same as Mini speedo drives - will have a look later at Mini box. Thanks for the link!
Edit - My speedo is 1504 tpm at the moment, so changing the diff from 4.22 to 3.73 means I now need a speedo close as possible to 1331 tpm. Hmmm - off to dig out some old Mini speedos!

Post by **mike.perry** » Sun Aug 31, 2008 12:15 am

Speedy Cables, tel 01639 732213 will recalibrate your existing speedo.

Post by **bmcecosse** » Sun Aug 31, 2008 5:18 am

The only Mini speedo i've found so far is 1406 so that's some way towards the required 1331, compared to the 1504 in the car at present. It's not so much the speed that I need accurate - it's the mileage for doing 'reliability run' where average speed must be held very accurately with hidden controls just round the corner!

Post by **wanderinstar** » Sun Aug 31, 2008 11:42 am

Roy try http://www.jdo10.pwp.blueyonder.co.uk/page2.html for recalibrating speedo. They are in Keighley.

Post by **bmcecosse** » Mon Sep 01, 2008 9:57 am

Thanks for the link Ian - but... the cost!!!!!!!!!!!!!

Post by **wanderinstar** » Mon Sep 01, 2008 11:50 am

I know Roy, but sometimes you have to bite the bullet and dig deep. Apparently for my setup, 3.7 plus 185/70 x 14 tyres I would need 1214 TPM on speedo head. I have an 1100 head that is 1376 but that is a long way off 1214.

Post by **bmcecosse** » Mon Sep 01, 2008 11:55 am

Quite near my 'wanted' 1331 tpm - but I don't fancy an '1100' head in my Minor. I'm surprised your 185/70 tyres against my 155/80 tyres make all that much difference to the tpm. Mind you - I could be doing with better tyres - was sliding all over the place yesterday in the wet!