Speedometer calibration

[John Larkin]

Can anybody explain what the number "980" on the face of my speedometer means? I believe that I read somewhere that it is the number of wheel turns per mile that the speedo is calibrated for, but this puzzles me because the speedo take-off is from the rear of the gearbox and is BEFORE the reduction gear of the differential, so the calibration would have to be based on the diff ratio as well, and not just on the number of wheel revolutions. My car has a 3.55:1 diff. If anybody on this forum has an Elan with an original 3.77:1 diff still with its original factory speedo, cad you tell me what number is written on your speedo?

Regards,

John Larkin.

[Frank Howard]

John,

Mine says 980 also however I have the 3.77 dif. I got my speedometer repaired and calibrated by Nisonger a few years ago. It's better than it was before although when it indicates 60 mph, I know I'm actually going around 54.

[elansprint71]

I could be wrong but.... surely the speedos will all be the same, the drive gear at the other end is where the difference is, different numbers of teeth being colour coded to match the diff ratios.

Incidentally, when I removed my 3.77 diff and fitted a 3.55 I could not get at the drive gear because the Spyder chassis prevented easy access. I left the "wrong" gear in and checked the speedo against my Sat-Nav GPS read-out. Surprisingly it reads the true speed right across the range; presumably the "correct" gear is designed to over-read the speed?

[robcall]

980 revs/mile
see this link
http://homepage.ntlworld.com/johnstonrh ... peedot.htm

[John Larkin]

Thanks Nigel for that link --- you have turned heat into light! The "980" figure is the combination of the speedo's internal worm x gear x ratchet ratios. This makes much more sense than my wheel revolutions per mile line of thinking. All I have to do to get my speedo calibrated is to find out the revs per mile of the speedo cable core.

Thanks also to Gary Anderson for the pm.

Regards,

John Larkin.

[elansprint]

John if you want it recalibrated send it to speedy cables
Ian

[John Larkin]

Hi Ian, I used Speedy Cables for my tacho rebuild rebuild recently, and found them to be good.

John.

[peterako]

Hi John,

Spoke to Sue Miller about this when I was having my speedo woes last year.
(Now fixed, but different story)

She poined out that there are different colour drive wheels for the right angle drive at the gearbox. The colour indicating the number of teeth on the gear wheel and these are matched to the final drive ratio.

Green (mine) is for the 3.77 final drive.....

A quick call to Sue or supplier of your choice will give you the correct one for rteh 3.55.

I think Se?n Murray had a 3.55 in his +2 originally and may know off hand what gear is needed.

T?g go bog ?!
Peter

[oldelanman]

The drive and driven gear info is detailed in the workshop manual - see page 18 of the Technical Data section. It also gives the Ford part numbers. Basically all vehicles use a 7 tooth driving gear. Driven gears are as follows:-

S1, S2 and early S3
3.90:1 - BLUE 25 teeth

Late S3 and S4 with standard tyres
3.55:1 - BLACK 23 teeth
3.77:1 - GREEN 24 teeth

S4 with optional 70 series tyres
3.55:1 - BLACK 23 teeth
3.77:1 - BLUE 25 teeth

If your tyre profiles are different from the original spec then these may be a bit out but probably not enough to worry about.

[John Larkin]

Thanks Roger. I missed that information when looking in the workshop manual.

John.

[msd1107]

The SN 6207/08 was used in S4 and later cars and is 980 TPM. It has speed marks to 130 MPH (although there is more angular distance to rotate so that you could get to 140 MPH).

The SN 6136/00A was used in the earlier cars and is 1000 TPM. The speedometer marks go to 140 MPH.

As usual, you may have something different in your car.

Once the correct speedometer TPM, gearbox gears, diff ratio and tire size are established, the the speedometer can be calibrated. The calibration is somewhat independent from the odometer reading, since the calibration is an analog process adjusting spring tension and rate. Thus you could have a speedometer that indicated speeds correctly, but registered distance incorrectly, and vice versa.

The enclosed spreadsheet has a worksheet called "DiffSpeedo". This allows you to enter the tire size and speedo TPM and determine the gearbox drive and driven gear for minimum odometer error for any diff ratio. Don't sweat the details too much, the tire changes its rev/mile figure by as much as 3% as it wears from new to worn. You can experiment with slightly different tire rev/mile figures to see if it would make much of a difference.

Although the file has an extension of .XLS (because LotusElan.net does not allow a .ZIP extension), it is actually a ZIPped file. After downloading the file, rename it to the .ZIP extension and extract the spreadsheet.

Every speedometer recalibration service has their own method of collecting the above information. Sometimes they have you do a lot of legwork, pushing the car a specified distance while counting cable turns, and other things. But if you give them the above information, they should have the capability to correctly calibrate the speedo.

David
1968 36/7988

[elansprint71]

Adopts invisibility cloak and wonders wtf he bothers.

[Foxie]

Hi John,

I think Se?n Murray had a 3.55 in his +2 originally and may know off hand what gear is needed.

T?g go bog ?!
Peter

I don't know, at all, at all

Originally I had a 3.9, then 3.55, then 3.7, and now I'm back to 3.55. Also started with Lotus 4 speed, then Lotus 5 speed, now Voigts T9.

I never bothered to change the speedo drives. Yes, it was more accurate with some combinations than with others, and in those days I was good with the mental arithmetic.

Then came the Terratrip, and now the Sat Nav, so the speedo has become a bit redundant.

Sean

[jimj]

With the Voight box my speedo was miles (!) out. Speedy cables fixed it but needed info; number of turns of the cable for so many revolutions of the wheel, distance covered, radius etc. They then calibrated the speedo head itself and it`s spot on. Changing the cable drive gears is only ever going to be approximate.
Jim