Speedo drive gear? Unobtainium??

[alexblack13]

Hi Guys,
You all doing ok I hope. Looking forward to Xmas ??


Anyone point me in the direction of where to obtain a BROWN drive gear with 22 teeth? Its the item that lives in the gearbox and drives the speedo cable. Need it to go with the new 3.54 diff I have just fitted.


I have a black 23 teeth item for disposal. Anyone need??

Alex B....

[GrUmPyBoDgEr]

Alex,

have you asked Paul Matty?
I remember them advertising the availability of different ratios only about 3 years ago.
Another possibility would be TTR. They had a Black one to go with my 3.54:1 Diff', bought as "Job lot"
To be noted my car has 175/60 x 14" Tyres.

John

[alexblack13]

Hi John, All,
Yes..Tried Paul M... I have a black one john! It has 23 teeth. I am told 22 teeth is correct type. Is it??

Thanks again...

[bcmc33]

Alex,

Brian Buckland's book says:
Blue..... 3.9:1 diff
Green... 3.77:1 diff
Black.... 3.55:1 diff

From my notes, my 3.54 diff car has a black gear with 22 teeth and a green one taken out of the Lotus 5 speed box I plan to install over the winter has 24 teeth.

The speedo on my car was very accurate from 20 - 110 mph compared to the rolling road read-out at Aldon Auto this year.
Based on this information, I plan to put the 22 tooth black gear into the 5 speed box.

For additional information, as per John Pelly, the tyres are 175/65R14 with a rolling radius of 278mm.

[GrUmPyBoDgEr]

Hi John, All,
Yes..Tried Paul M... I have a black one john! It has 23 teeth. I am told 22 teeth is correct type. Is it??

Thanks again...

I don't know how the Ratios are calculated but the Rolling Radius of the Tyres fitted also plays a role.
TTR put the Diff & Tyres on my Car & according to his magic book of words the Black gear was selected as most suitable.
I have never checked the accuracy of the Speedo but one day I will do all of the checks required for Speedo Calibration.
I feel certain that Calibration will be able to take your 1 Tooth difference into account.
The checks you need to do will depend on the Calibrater. It usually involves rolling the Car for 4 rev's of the Wheels & counting the number of turns that the Speedo Cable makes.
The best way to check your Speedo's accuracy is to drive at varying speeds behind that Z4 of yours.
Much as I dislike saying it, BMW Speedo's are very accurate.

Cheers
John

[types26/36]

The best way to check your Speedo's accuracy is to drive at varying speeds behind that Z4 of yours.
John

.......or just use a Sat-Nav

[alexblack13]

Hi Guys,
Done some checks and bcmc says black. So does his book. So does my book! BUT.. My black drive has 23 teeth. I have checked the workshop man'... it says black 23teeth.

I have me one of those ... I will fit the black 23teeth and check it with the sat nav as suggested. Thats how I checked the Quite accurate Lambretta.

Stand down on the brown...

Many thanks for the help again guys. Superb!

Alex

[msd1107]

The downloadable spreadsheet at elan-f15/updated-spreadsheets-t18445.html will allow you to determine the optimum speedometer drive gear for any tire size, differential ratio and speedometer turns/mile.

If you cannot obtain the best speedometer drive gear, the spreadsheet allows you to determine the two gears in the Smiths speedometer to get the closest odometer calibration. Most speedometer rebuild shops can rebuild your Smiths speedometer with the correct gears for the proper turns/mile.

After you get the proper odometer calibration, the speedometer can be callibrated. A speedometer with incorect odometer can have the speedometer callibrated for correct speed readings, but the odometer reading will still be wrong.

David
1968 36/7988

[GrUmPyBoDgEr]

Just as a point of interest; how far must you travel at constant speed in order to make a Sat Nav / Speedo comparison?
I assume that driving on a straight Road is a necessity?
John

[JJDraper]

re Sat Nav accuracy - most have around a 3-5 second lag, so you need to be at a constant speed for that period. If you are really picky, the road should be flat as well..

Jeremy

[msd1107]

John,

Different Sat/Navs have different update rates, depending on their chip set and how many satelites it sees. Older units have slower update rates. But as long as you can stabilize the speed for more than a few seconds, the Sat/Nav speed display should be accurate.

The mechanical speedometer is an instantaneous reading device, so theoretically it is displaying the current speed. So if you are travelling at a constant speed, its display will be as accurate as it can be.

So once you stabilize the speed, you can correlate the two readings. Straight or curving roads should not have any effect. The sat/Nav is looking up in the sky and the wheels are turning the speedometer.

David
1968 36/7988

[GrUmPyBoDgEr]

Thanks Guys,

I didn't realise that Sat Nav's could react so quickly.

Cheers
John

[bcmc33]

Well wadaya know?.........................not a lot!!!!

Having already spouted superior knowledge about the speedo drive gears - I have to eat my words.

The 5 speed box I am preparing to install came with a green (24 tooth) drive gear so the box came from a 3.77 +2 not a 3.54 as the seller stated. Not to worry as my 4 speed box is accurate with the 3.54 diff and must have the required black gear.

No, no, no, no, no - the gear in the 4 speed is GREEN.

So what do you make of that then???????????????

Maybe a previous owner had the speedo changed to suit?

[GrUmPyBoDgEr]

Are you trying to help here Brian
(Sorry I couldn't help that)

Cheers
John

[rgh0]

Sounds like the classic Lotus change control problem- what the factory say versus what they do is always debatable depedning waht was in the parts bin on the day and then what a DPO may have done adds to the confusion and we are all just guessing

cheers
Rohan