Incorrect speedo reading BS1 spider

[alfredhamstra]

Fixing a 1972 BS1. It has a 220km/h speedo in it (140mph)
But when driving it reads about twice the speed.
So driving 50km/h it reads almost double.
My question: is the speedometer wrong, does it need a newer speedo from CS1 which goes to 80 mph
Or does it have a wrong gear on the transmission, where the cable goes in.
If so, I have a gear laying around with 15 and 17 tooth.
What is the correct wrong, do they both fit actually?
I like the speedo 220km/h better that the 80mph
Somebody knows?

Greetings from the Netherlands
Alfred

[baltobernie]

Hello Alfred,

As far as I know, the only difference between the 140MPH/220KPH speedometers and the 80MPH speedometer is the cable fitting on the speedometer end. I believe the internal gearing is identical.

Three items are responsible for the accuracy of the speedometer in vintage Spiders; tire size, the speedometer drive attached to the transmission, and the rear axle ratio. Manual transmission Spiders came with three different axle ratios. The following data is courtesy of Csaba Vandor:
1968-70 4.1:1 rear end, speedo drive has 9 teeth, drive gear has 4 teeth, drive housing has a blue mark (markings often faded over time)
1971-78 4.3:1 rear end, speedo drive has 14 teeth, drive gear has 6 teeth, drive housing has a red mark
1979-85 3.9:1 rear end, speedo drive has 15 teeth, drive gear has 7 teeth, drive housing has a white mark

The correct speedo drive/axle ratio combined with tires 23.4 inches in diameter will result in a very accurate odometer and a slightly optimistic (3%) speedometer. The problem of course is that many (most?) Spiders have had transmissions and/or rear diffs replaced over their lifetime. Not to mention that 165/80R13 tires no longer exist. Also complicating your situation is the fact that the speedo gear assembly cannot be removed in situ. You must drop the transmission in order to replace this tiny gear assembly.

Your solution can be as crude as a grease pencil marking on your speedometer face with two or three popular posted speed limit numbers, or as elegant as a supplemental gear assembly spliced in to your existing speedo cable. We have several companies catering to the vintage car hobby making such assemblies here in the US.

At least your speedometer is indicating double the actual speed. If it were the opposite, you might see one of these in your mirror

[alfredhamstra]

Thanks for the answer
The drivegear, thats the gear inside the gearbox, that connects to the speeddrive?
So changing the speedo to a Cs1 version dont solve the problem... it will just give the same " double" speed...
Whats the drive housing?
Maybe i try to change just the face inside the speedo...
anyone more ideas?

thanks
alfred

[baltobernie]

Thanks for the answer
The drivegear, thats the gear inside the gearbox, that connects to the speeddrive? The "drive gear" in this context is the small exposed worm gear in the image below. This image shows the three different speedometer drives used over the years, and the mating gear internal to the transmission.
So changing the speedo to a Cs1 version dont solve the problem... it will just give the same " double" speed... correct
Whats the drive housing?The entire assembly shown in the image below. It is on the outside of the gearbox, left-hand side. The speedometer cable attaches to it.
Maybe i try to change just the face inside the speedo...The speedometer gauge is easily removed along with the entire nacelle. The glass is removed by gently prying the underside of the bezel free from the case. You could foreseeably print a new dial face with your unique speed numbers. But that's a huge amount of work, and the result would most assuredly look amateurish. The resolution of the modified speedometer would also be very poor, with virtually all of your driving in the bottom third of the dial.
anyone more ideas?If you are running a tire size closely approximating OE, the cleanest solution is to procure a speedo drive corresponding to your axle ratio. Of course, this means dropping the transmission, which is why I suggested a custom adaptor.

thanks
alfred

[alfredhamstra]

Thanks very much for the help.
I cannot see the pictures, but I get the idea.
Best thing indeed is to change the speedo drive.
I was in the past able to loosen the gearbox a bit and prying it to the right so that the drive just came out.
I hope there is 1 9 teeth inside which I can change for 14 or 15

I let you know how it goes (needs some time, I need to put it on a ramp)

alfred

[spider2081]

Somebody knows?

I don't know. I'm guessing that the difference in speedometer drives would not cause a speedometer to display a speed that is twice what the car is traveling. I don't know if the combination of the rear end ratio, tires/wheels and speedometer drive can cause that big of a difference. I would try another known good speedometer first. I think most speedometers that are cable driven use magnets and springs that require calibration when manufactured. As they age they can require repair or recalibration.