Talk Morgan Forums Morgan Sports Cars Morgan Aero 8, 2012< Plus 8, SuperSports, Coupe & Aeromax Over enthusiastic speedo

Hi Mike, I had a 2006 Jag, its speedo was very accurate as well, when viewed alongwith a Tom Tom sat nav.

Boxsters (986) are renowned for over-reading speedometers. My S was virtually 10% over from a few mph up to, well, certainly 70.

As I said, can't comment on the electronic speedos, but certainly the mechanical are two independent instuments. The mileage recorder is a fixed gear drive (usually of the worm type) from the end of the speedo cable, where the speed reading is taken from a magnetic pickup (how they worked was covered in an edition of The Morgan World a few years ago). The only way to tell whether your mileometer is correct is to cover a fixed distance and compare distances. My +4, which had an electronic speedo, used to give different mileages between trip meter and odometer - so the measurement can be altered.

As far as the rules go, I understood over read only was allowed - up to 10%, but a google search on the subject is a nightmare. The info should, I guess, be in the Construction & Use regs, somewhere... TG did a test on a few cars some years ago - Mondeo read 29 at a true 30 and was declared illegal, but that's TG... Part of the 10% over read is to allow for tyre wear, between new and full worn, though with today's thinner tread I doubt it is necessary to allow so much.

Cheers,

I know that Mike Duncan adjusted my speedo from a reading of 36000 to 39668 but I have no idea how the figures are calculated. Can anyone cast light on what the figures actually mean?

My speedo still reads around 10% fast, I'll do a bit of fine tuning of the figures sometime although it will be complete guesswork!

It's probably something like the number of impulses per mile (or km) of the electronic pick-up mounted on the end of the gearbox - i.e. (tyre revs per mile) x (axle ratio) x (no of teeth on exciter wheel). I think you can often get the revs per mile of a particular tyre from the manufacturer's website.

Fjemog

Viper

My S1 was the same. I recalibrated it using a Satnav and then reset speedo so that it now reads just 1% fast.

If you can't find it on the forum, pm me and I'll play with my speedo again to give you the figures and how I did it.

Cheers

Thanks Guys.

I have a knight in shining armour that has come to my aid.

I'm very lucky to have the best Morgan Dealer in the Country just 3 miles down the road.

It is a case of recoding the speedo and its also true that the mileage would have been affected.

Philippo......Interested to know how you managed to find the right numbers to input to match it to your sat nav.

lee

Hi, My series 1 also reads about 10% high and the distance from A to B is also incorrect and reading high.
Any advice to fix greatly appreciated.

Bill

Lee

Basically I measured the car speedo indicated speed against a couple of satnav devices. Then I entered into the speedo calibration mode, done by holding done odometer reset button then turning ignition on. Basically I then increased the speedo pulse number by close to the percentage of the over read. After a bit of trial and error I arrived at a speedo that reads roughly 1% fast, as opposed to 10% before. The final number of pulses can be + or - a bit since you're looking at a high number.

Mine is now set at 24670 but I have a km/h speedo so the number needed in your case may be different? I'm sure your dealer will know

Cheers



I'm sure your local dealer will

So you had to increase the number to bring it down?

All very clever stuff

Thanks

That's because the speedo works on pulses per mile based on wheel revolution. It takes and induced signal from one of the rear wheels probably.

You could also work out the distance traveled by one wheel rotation (probably less than the circumference due to tyre compression) and then multiply by the number of revolutions in one Mile or KM as desired.. Then you only need to know the number of pulses per wheel revolution and you can calculate as desired.