Topic: Brand new speedometer doing weird things

[Crisis]

Hello,

I've a '79 TA with a 403ci. I've replaced my old bouncing speedometer by a new one... which do strange things :
When i drive Under 45mph, the speedo is under the real speed. Beyond 45mph, the speedo is upper the real speed. The more I accelerate, the upper he becomes. When I drive at 70mph, it shows 80mph. the needle don't bounce but looks free, moving to right and to left when i Accelerate and decelerate fast and quickly.
And the miles counter is very fast. When I drive on 5 miles, the counter adds 30 miles...

Wires good and speedogear is the good one (regarding my wheels, the speedgear into my transmission and the ratio of rear axle).
Is my speedometer is dead ?
Is anybody has an idea about this ?

Thank you all

Chris

(sorry for my english)

[Wallington]

That's very odd. I can't even think how it is possible to have one of those slightly out and the other many times out.
Does your car have cruise control? And does it have a ratio adaptor box in the speedo cables or just one from gauge to transmission?
Have you seen what speedo gears are in use and know the current diff ratio?

[Crisis]

Thanks for your reply.
Yes, I've a cruise control which is new too. But the problem was already there with the old that i've replaced because of this problem.
No ratio adaptor. Yes, I know  speedogear and ratio, I've checked all and all are the good ones.

Miles counter is OK, my mistake. Only the crazy and floating needle problem remains. I keep searching but don't find any cause for now.
I've wrote to Classic Industries which has sold it to me but i've got no answer, yet.

[Wallington]

Ok because that's the bit I had no answer for, it didn't make sense.

The rest you said was there with old speedo and remains with new speedo. It may just be a 1970's design mechanical speedo. Changing gears and ratios will only affect if it reads fast or slow from a starting point of 0, not both. There's no fix for that. At what point is the speed accurate? have you run with a GPS app on phone or similar?

[Crisis]

The old speedo is just bouncing (between 0 - 15mph) but shows correct speed.
Yes, I use my GPS to know what speed I reach when driving.
The speedo is accurate at 44/45mph.