Final Drive vs Speedometer Spit 4 Mk1 Non-Overdrive

[Neil Clark]

 I'm confusing myself badly, not unusual!

My car has the later swing axle fitted and per my GPS the speedo under reads by about 10% - at an indicated 30mph I'm actually doing 33mph. The speedometer is marked SN6121/16A 1248.  I think that's 1248 turns per mile.  But there are variables to original spec on my car which can affect what I see on the speedo. 

Apparently there are two swing axle final drive ratios which are different to the Mk1 Spit - do both fit my early car and can anyone help with what they might be?

The car came to me with 13" wheels with 155/80 R13 tyres fitted.  The wheel supplier MWS recommends 155/70 R13 on their website so I wondered if it is the taller aspect of my tyres affecting the speedo - larger rolling diameter.  Per Google, the 155/70 R13 tyre will be 67.8 inches circumference and the 155/80 R13 will be 71.5 inches.  So the 70 aspect tyre is 94.8%of the 80 aspect if my maths are correct.  My foggy brain says that at 1000rpm I'm travelling 5% further on my tyres than on the 70 aspect tyres so 5% faster as well.  Can that be right?

Once I know if this is the correct speedo and the possible final drive ratios along with the tyre size I guess I can work out if the speedo needs changing, servicing or is faulty.  My wife's dodgy back is not keen on pushing the car for a fixed distance on the road while I count the turns of the wheel!

[Pete Lewis]

then drive it dont have to man handle a push 

check out   mintylamb.co.uk/gearspeed/  gives some clues 

to check the diff final ratio you need to count the prop turns for one wheel rotation 

is best to jack up and count the prop turns for one full rotation of both rear wheels together ( 

if the sun and planet ratios are fixed on some you can do with only one wheel raised/counted 

I prefer to have both turned together to avoid any errors .

 the basic spec would be 4.11 diff ratio  and 155/80  tyres 

but you can play around on minty and see if you get a match

Pete

 

[NonMember]

As Pete says, the 155/80 are the correct tyres - MWS are quoting what they carry, not what you want.

If the speedo is 10% under then it's likely a combination of things, because the later diff ratios are 3.89 (which is 6% out) or 3.63 (which is 13% out). It is also perfectly possible to fit a swing spring to a 6-stud (original spec) diff, just by plugging the two unused stud holes.

However, Spitfire speedos were never terribly accurate. If it's steady and reads within 10% I'd say you're doing quite well.

[DanMi]

The spring spring conversion is irrelevant and 1248TPM is correct. It could be that someone has put a later diff in or the speedo magnets could have become weak. original tyres would have been 5.20*13 crossplies or 145*13 radials as an option. WQhat speed do you get at 3000RPM in 4th should be 50MPH

 

[Neil Clark]

Brilliant website Minty.  Thanks all.  Plenty to look at now.  I've just been out for a run but the traffic on the A3, where I can normally keep a steady speed, is horrendous and without a passenger to check I couldn't keep my eyes on the rev counter and speedo long enough.  At 3000rpm it seemed to be about 40mph but don't trust me.  It's just nice to have had a dry day to take the car out for pleasure.

[Neil Clark]

Sorry it was 3000rpm looked to be about 50mph, not the typo of 40mph..

[johny]

Its really checking revs against the GPS speed thats the most important to tell you if the diff/tyres are all standard so then youll know if the speedo is wrong. Course this does depend on the tacho being accurate😩

[Neil Clark]

It's all making me wonder, so many variables!  I suppose the only speeds that really bother me are 20, now that almost all the suburbs around here are 20, 30, 40, and 50 and it's only because of the vast number of speed cameras we have now.  Perhaps in the short term I'm just better getting a passenger to look at the revs / GPS speed at each of these and pay attention!

[johny]

yes important to know where the error is coming from so you can make the best decision on how to correct it. You could always look at the serial number stamped length ways along the underside of the diff front casing to see if its non standard (its unlikely the internal ratio has been changed)...

[DanMi]

1 hour ago, johny said:

yes important to know where the error is coming from so you can make the best decision on how to correct it. You could always look at the serial number stamped length ways along the underside of the diff front casing to see if its non standard (its unlikely the internal ratio has been changed)...

easier would be to look at the flanges, if the have 1/2" bolt heads it is likely the original 4.11 if 9/16 it is likely a later 3.89 or 3.63

[Pete Lewis]

the accuracy of the rev counter is  having exaclty the same inards as the speedo  so they give an indication not anything remotely accurate 

if you have a battery drill of known rpm then multiply the rpm by 3.55:1 and thats what the needle might show 

my  Smiths calibrated drive unit went in the skip when they closed the factory    it was too big to take home 

Pete

 

[Neil Clark]

I think I'll take my daughter to the pub a few miles down the A3 on a quieter traffic day and get her to note down the revs and mph at steady GPS speeds!  Then contemplate them over a pint or two.  I've insured her on the car so she can drive home again after the beer.

[Neil Clark]

I think the pub methodology has a lot going for it.  When I was still at school (1969) I had driving lessons in the evening after classes.  On the way home from the last lesson before the test which was the next day, the instructor said that I had to understand the changes in our driving after alcohol.  To my amazement he promptly steered to a country pub and we went in for two pints.  I thought I drove wonderfully on the way back.  This was in Rutland so a real backwater then but I was still in uniform and would have been instantly expelled if caught.  But the pub psychology worked - I passed the next day first time!