Lotus Elan

Slow Speedo

PostPost by: greyhound » Fri Oct 23, 2009 6:30 pm

Hi, Having finally got my +2 back on the road after 10 years, I am peeved to find one of the problems I had when it was originally being used is still there------ namely the speedo reads approx. 10 mph too slow. The speedo has been professionally rebuilt, the needle is rock steady at all speeds.
According to the works manual there were two gearings for the speedo drive according to the diff ratio. I have lots of history with the car and no sign of a diff change. It's a S130 running on standard 165 x 13 tyres 80 profile.
Any help please? thanks John.
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PostPost by: john.p.clegg » Fri Oct 23, 2009 6:52 pm

John

If it's 10 mph slow at all speeds you could take it apart and re-position the needle...

John :wink:
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PostPost by: bcmc33 » Fri Oct 23, 2009 9:07 pm

The speedo drive gearing is related to the diff ratio:

Blue gear = 3.9:1
Green gear = 3.77
Black gear = 3.54/3.55:1
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PostPost by: 1964 S1 » Sat Oct 24, 2009 1:04 am

Drive ten miles an hour faster?
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PostPost by: bcmc33 » Sat Oct 24, 2009 8:07 am

John,

I should have said:

Blue gear (25 teeth) = 3.9:1
Green gear (24 teeth) = 3.77
Black gear (22 teeth) = 3.54/3.55:1

So it's likely you have a 3.54/3.55 dif and a 3.77 (green) speedo gear.
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PostPost by: mikealdren » Sat Oct 24, 2009 9:43 am

Hi John,
is it 10mph slow at all speeds or progressively slower i.e. reading further out as speeds rise?

If the discrepancy rises with speed, it's a gearing issue and you need the correct speedo gears as per Brian's note. You need to work out how far it is out as a ratio (e.g. 10 mph error at 70 mph is about 14%), you'll then need a combination of speedo gears that at 14% different from what you have. Bear in mind that it isn't an exact science, tyre diameters alone can vary by a few % depending on speed, temperature, load, pressure and differences between makes. Also, speedos are usually set to read a few mph high so that people won't get caught for speeding as a result of speedo error, 5% is not at all unusual.

If it's a constant error, the needle's in the wrong place.

Mike
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PostPost by: msd1107 » Sat Oct 24, 2009 7:01 pm

elan-f15/updated-spreadsheets-t18445.html (latest version at the bottom of the post) has a spreadsheet that allows you to determine the appropriate transmission speedometer drive gear, or alternatively, the appropriate gears in the speedometer.

One thing to look at is the speedometer turns per mile, which is often a small number on the face of the speedometer. Common numbers are 1000 (early Elans), 980 (S4 and later Elans and most +2s), 960, and 940. The speedometer turns per mile figure has to be matched to the tire size, differential ratio, and transmission speedometer drive gear for proper mph readings.

Shops that calibrate the Smiths speedometers usually have a convoluted procedure to have you push the car and count the turns of the speedometer cable. From that, they can determine the gear pair in the speedometer to produce the appropriate turns per mile figure for your tire size, differential ratio, and transmisson speedometer drive gear.

But if you have the information, you can determine what transmission speedometer drive gear tooth count is proper, or, what the speedometer turns per mile figure should be.

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PostPost by: greyhound » Sun Oct 25, 2009 10:38 am

Thanks to everybody for their info. How easy is it to remove the speedo drive from the gearbox to check the number of teeth, if you remove the clip does it just pull out?
I have a works manual.
Cheers John.
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PostPost by: 512BB » Mon Oct 26, 2009 8:21 am

Yes, just remove the bolt and wavey clip and it will pull out. There is an O ring in there somewhere to prevent the oil leaking by. Replace it as a matter of course. Oh, and drain the gearbox first!

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