speedo reading

[ethel]

It seems as though my speedo is under reading, maybe up to 8 or 10mph at 40mph. I checked today at one of those electronic flashing speed limit warning signs that only lights up if it thinks your going too fast, it's a 40 sign and my 'modern' cars usually trigger the sign at anything over 38ish but the stag speedo was hovering around 30-32 (OK, I know I can't really be that accurate).

My car was converted from auto to manual at some time and looking at possible rear axle ratios I wonder if it was possible that the box was from a 2000 manual which had a 4.1:1 axle - or would that have involved too many changes to make it fit to be worth while doing? If the speedo drive in the box was originally intended for a 4.1 ratio and 13 inch tyres then it would read low.

Where abouts is the gear box serial no and how easy is it to see?

Thanks, John.

[flying farmer]

imported post

My stag has also been converted from auto to manual, and I was expecting the speedo to be slightly out as it has a 3.45 diff, I checked the driven gear for the speedo and it is the right one, so I suspect the driving gear is the wrong one. my speedo reads 40 when I'm doing 50 and the mileometer is out by the same amount.

I believe it is possible to get the speedo recalibrated, which may be the cheaper and easier option

Hopefully someone on the forum can enlighten us

Neil

[Andrew S]

imported post

Speedy cables can calibrate them

http://www.speedycables.com/

[flying farmer]

imported post

Andrew S wrote:

Speedy cables can calibrate them

http://www.speedycables.com/

Thanks Andrew, looks like a job for this winter, a lot easier than pulling the box out again

Neil

[marcusclive]

imported post

mine seems to go the other way, i go through one of those speed signs doing 40 and they show 35. i tend to run off the rev counter. ie 2000 in fourth is 40. seems to be roughly right.

[Rouseyboy]

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If you have a decent Sat Nav, they often have a "speedo" which I have always found to be very accurate.

(If you don't have one, maybe you could borrow one?)

....Dave

[404ROD]

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My speedo reads slightly low at 30mph. When driving past the speed warning indicators I get a sign of about 3mph faster. My tyres are the original Michelin 185x14 specification. However, what I do have is a BMW diff. My old Stag diff, despite being an exchange replacement when I hada thorough overhall, was objectionally noisey. The BMW diff transformed the car. These diffs come in various ratios but I chose the standard ratio which should be 3.7:1. I trust that the BMW is accurate. Engine revs. to speed should read 4th. gear1500rpm = 29.85mph. At the speed limit end 4th. gear 3500rpm = 69.65mph. With J type overdrive in 4th. gear 3000 rpm = 72.9mph. So 4th. gear speeds are 1000rpm = 19.9mph. 1500rpm = 29.85mph. 2000rpm = 39.8mph. 2500rpm = 49.75mph. 3000rmp = 59.7mph. 3500rpm = 69.75mph. 4000rpm = 79.6mph. 4500rpm = 89.83mph. The speedo is driven from the gearbox but the tachometer operatesby electrical pulses from the coil distributor circuit.