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Diff ratio silly question
Posted: Wed Mar 11, 2026 4:58 pm
by michael4
Having jacked up one corner of the car to adjust a rear brake it occurred to me to try and check the diff ratio. (I have wildly over reading speedo discussed in a previous post.)
I'm expecting the diff to be 4.55 but with no history to the car and 'enthusiastic' previous owners who knows...
So I marked wheel and UJ on the prop shaft and found that a turn of the wheel resulted in around 2.25 turns of the shaft.
Have I missed the point entirely? Have I managed to get into my mid seventies without understanding diffs? Should I be jacking up both wheels?
Re: Diff ratio silly question
Posted: Wed Mar 11, 2026 5:14 pm
by mike1864
I expect the answer to the "both wheel" question is yes, especially given your figures.
But without my checking your previous posts, is the milometer reading correctly? If so, the speedo issue can only be its magnetic "clutch".
Re: Diff ratio silly question
Posted: Wed Mar 11, 2026 5:39 pm
by Bill_qaz
michael4 wrote: ↑Wed Mar 11, 2026 4:58 pm
Having jacked up one corner of the car to adjust a rear brake it occurred to me to try and check the diff ratio. (I have wildly over reading speedo discussed in a previous post.)
I'm expecting the diff to be 4.55 but with no history to the car and 'enthusiastic' previous owners who knows...
So I marked wheel and UJ on the prop shaft and found that a turn of the wheel resulted in around 2.25 turns of the shaft.
Have I missed the point entirely? Have I managed to get into my mid seventies without understanding diffs? Should I be jacking up both wheels?
If you only turn 1 wheel you double the figure you got due to driving through the differential spider gears.
2x2.25 = 4.5 : 1 so yes 4.55 : 1 diff allowing for slight error in measuring.
Re: Diff ratio silly question
Posted: Wed Mar 11, 2026 5:40 pm
by Bill_qaz
Ratio of axle is pinion and crownwheel. Differential is not being used when both wheels turn at the same speed.
One wheel stationary and one turning differential is maximum the turning wheel is turning the sun wheel via the half shaft, the planet wheels are rotating and turning the crownwheel via their carrier at half speed. So your prop only rotated 2.25 turns.
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Re: Diff ratio silly question
Posted: Wed Mar 11, 2026 5:59 pm
by geoberni
Or you could just use your phone to photograph the ratio stamped on the top of the Diff....
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.
Up around here....
.
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Re: Diff ratio silly question
Posted: Wed Mar 11, 2026 6:22 pm
by myoldjalopy
Often hard to find that 'cause its usually so dirty up there!
Re: Diff ratio silly question
Posted: Wed Mar 11, 2026 10:27 pm
by michael4
Thanks for the clear description of what is going on. I reckon I now understand.
I used the wheel turning technique to find the ratio since my knackered frame won't allow me to go under a car anymore, well actually the problem is not the getting under, it is the getting up afterwards. If you are wondering, I marked the diff and shaft using a Tippex brush on the end of the a bamboo cane!
This old thread explains why I am doing this:
viewtopic.php?p=712150#p712150
My speedo reads way over reality and I just wanted to confirm that the diff was standard. Unless I have misunderstood I now realise that I do indeed need a speedo that is 1408 TPM rather than the 1376 TPM one that is in there. From the old thread it sounds like the part number is not so critical, 4423/06 being right for a very late car.
Re: Diff ratio silly question
Posted: Wed Mar 11, 2026 11:50 pm
by Bill_qaz
michael4 wrote: ↑Wed Mar 11, 2026 10:27 pm
Thanks for the clear description of what is going on. I reckon I now understand.
I used the wheel turning technique to find the ratio since my knackered frame won't allow me to go under a car anymore, well actually the problem is not the getting under, it is the getting up afterwards. If you are wondering, I marked the diff and shaft using a Tippex brush on the end of the a bamboo cane!
This old thread explains why I am doing this:
viewtopic.php?p=712150#p712150
My speedo reads way over reality and I just wanted to confirm that the diff was standard. Unless I have misunderstood I now realise that I do indeed need a speedo that is 1408 TPM rather than the 1376 TPM one that is in there. From the old thread it sounds like the part number is not so critical, 4423/06 being right for a very late car.
Know the feeling well
knackered old joints, going under the car on a wheeled crawling board seems a good idea until when you come out you can't get up
Glad you found the diff explanation useful, brought back memories from previous career as a technical trainer.
Re: Diff ratio silly question
Posted: Sat Mar 14, 2026 1:15 pm
by michael4
Confused I am...about speedo serial numbers
The excellent MMOC Potteries page lists the following
4423/04 for Travellers only
4423/06 for Saloon, Convertible and Traveller
The specs for both look the same, 4.22 diff, 1408 tpm
I'm assuming Travellers had the same size tyres as the rest?
I wonder, what could be the difference?
Re: Diff ratio silly question
Posted: Sun Mar 15, 2026 5:24 am
by don58van
I have wildly over reading speedo
I do indeed need a speedo that is 1408 TPM rather than the 1376 TPM one that is in there
Hello Michael4
The small percentage difference between 1408TPM and 1376TPM will not correct a
wildly over-reading speedo (unless you are exaggerating
).
Don
Re: Diff ratio silly question
Posted: Sun Mar 15, 2026 4:08 pm
by michael4
Hello Don,
Yes I worry about this but am a bit short of options.
The tyres are 155 so that would make a small difference.
Finding a correct speedo will help a bit and is not very expensive so could be seen as a 'next step'
Michael
Re: Diff ratio silly question
Posted: Tue Mar 17, 2026 12:02 pm
by taupe
michael4 wrote: ↑Wed Mar 11, 2026 10:27 pm
My speedo reads way over reality
If thats the case changing the speedo TPM by 2.5% will make no noticable difference?
How much faster is it showing than the speed you are actually achieving?
How accurate is the recorded mileage against an accurate measured mile?
T
Re: Diff ratio silly question
Posted: Tue Mar 17, 2026 3:16 pm
by michael4
It over reads by approximately 10 mph at 30mph. Gets worse as you go faster.
I have not found an opportunity to check the mileometer.
Re: Diff ratio silly question
Posted: Tue Mar 17, 2026 5:09 pm
by svenedin
michael4 wrote: ↑Tue Mar 17, 2026 3:16 pm
It over reads by approximately 10 mph at 30mph. Gets worse as you go faster.
I have not found an opportunity to check the mileometer.
Fair enough. That's a big error.
Around my area we have a lot of those digital displays that display your speed as you drive past. My speedo tallies with those displays.
Is your speedo needle swinging around wildly or fairly steady?
Stephen
Re: Diff ratio silly question
Posted: Tue Mar 17, 2026 5:41 pm
by Bill_qaz
michael4 wrote: ↑Tue Mar 17, 2026 3:16 pm
It over reads by approximately 10 mph at 30mph. Gets worse as you go faster.
I have not found an opportunity to check the mileometer.
Is it against the pin when stationary ? Are you sure its not starting with a speed because the needle is fitted wrong.
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Re: Diff ratio silly question
Posted: Wed Mar 18, 2026 10:22 am
by don58van
Are you sure its not starting with a speed because the needle is fitted wrong.
I agree that this can result in incorrect readings and could be part of the problem. However the OP says that the error increases with increasing speed. An incorrectly fitted needle would cause a fairly constant error.
Old speedos tend to lose magnetism and therefore under-read, not over-read. So we can rule that out as a factor. There is the possibility that someone had a go at remagnetising the magnets and they have been over-magnetised.
(FYI I service Smiths speedos with my own test and calibration equipment. Unfortunately for the OP, I am in Australia.)
Don
Re: Diff ratio silly question
Posted: Wed Mar 18, 2026 11:33 am
by oliver90owner
Re the speedo error - has nobody considered the possible sources, other than the speedo head?
Some thought might be given to the drive train beyond the speedo drive wire? Could be a plethora of options making a difference.
The poster only says it increases with speed. He/she does not provide any figures/data.
If the error is directly proportional to the road speed, it is likely a ratio change either at the gearbox, differential, wheels or tyres. All errors can be ‘plusative’ or negative to the final outcome.
Just considering or blaming the speedo head is just plain folly, or a (likely) wrong guess, without considering all the specific details of the case.
Re: Diff ratio silly question
Posted: Wed Mar 18, 2026 11:39 am
by Bill_qaz
oliver90owner wrote: ↑Wed Mar 18, 2026 11:33 am
Re the speedo error - has nobody considered the possible sources, other than the speedo head?
Some thought might be given to the drive train beyond the speedo drive wire? Could be a plethora of options making a difference.
The poster only says it increases with speed. He/she does not provide any figures/data.
If the error is directly proportional to the road speed, it is likely a ratio change either at the gearbox, differential, wheels or tyres. All errors can be ‘plusative’ or negative to the final outcome.
Just considering or blaming the speedo head is just plain folly, or a (likely) wrong guess, without considering all the specific details of the case.
If you read the whole post most of those causes have been discussed already
Re: Diff ratio silly question
Posted: Wed Mar 18, 2026 2:26 pm
by michael4
I mentioned previously but did not emphasise that the error increases with speed.
By this I mean that:
approximately
at 30 mph the speedo reads 40 mph
at 50 mph the speedo reads 70 mph
The needle is fairly steady
On another topic the forum seems very slow and hard to use. Is this just me?
Re: Diff ratio silly question
Posted: Wed Mar 18, 2026 3:34 pm
by Bill_qaz
Fast speedo, slow forum. Lets hope they both get sorted