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[mantaray]

Question about speedo drive ratios.

Since doing all the mods to my manta A, I now have a speedo that is reading about 20% low, ie Speedo reads 50 mph when actual road speed 58 mph.

The gear box ( carlton R25) has a Yellow (19 teeth) drive fitted at the moment, but a Red (18 tooth) or Blue (20 tooth) drives are available.

Am I correct in thinking that by fitting a red drive, the drive will turn faster and therefore increase the speedo speed reading.

[cam.in.head]

. With the manta 'b' axles the yellow is for the 3.67:1 ratio and the red for the 3.44. With standard dia tyres. If the 'a' is similar ratios it should do what you want

[mantaray]

. With the manta 'b' axles the yellow is for the 3.67:1 ratio and the red for the 3.44. With standard dia tyres. If the 'a' is similar ratios it should do what you want

Thats the problem, I'm running Carlton R25 box, 3.307 diff with 205x50x16 wheels

But from what you are saying the red drive should bring it a bit better anyway.

[Andy H]

By my calculation you need 22 teeth

(58mph / 50 mph X 19 teeth = 22.04 teeth)

[Doctor Gollum]

Wheel diameter is largely irrelevant. The rolling radius is what you need to know... match the current tyres with the original sizes and you're sorted

ie 245/40/18 has an identical rolling radius to 225/55/16

[mantaray]

Tyre size is about the same.

Carlton had 195-65 x15, I'm running 205-55 x16, -0.4% differance.

[Andy H]

By my calculation you need 22 teeth

(58mph / 50 mph X 19 teeth = 22.04 teeth)

Scrub that - more teeth would give a lower indicated reading. I should have divided the current reading by the required reading:

50mph / 58mph X 19 teeth = 16.38

[mantaray]

By my calculation you need 22 teeth

(58mph / 50 mph X 19 teeth = 22.04 teeth)

Scrub that - more teeth would give a lower indicated reading. I should have divided the current reading by the required reading:

50mph / 58mph X 19 teeth = 16.38

You did confuse me there for a min.

[Andy H]

I imagine you will need something like this http://www.ebay.com/itm/Gear-Ratio-Adapter-to-speed-your-Speedometer-up-20-/132076508957

[cam.in.head]

I suppose it depends on the amount of absolute accuracy you are aiming for.when you replace a yellow drive with a red drive it will increase the speedo reading like you thought but will it be still not enough.are there any 16 tooth wheels. Maybee on a different gearbox that would fit the carlton box or even the actual nylon cog on the shaft ?

[mantaray]

I only found out it was reading wrong, after getting a speeding ticket, for doing 58 mph in a 50 zone. When i know I was below 50 on the speedo, as I am very carefull past that camera haveing been done by it a couple of years ago.

Since then i have been using a Satnav, and can see it is about 20% out. Not a big problem, but it would be nice if I can sort it.
Might take a speedo apart and see how esay they are to calibrat.

[cam.in.head]

Moving the needle to align with 50mph at 50mph would be easy enough but that doesnt mean other indicated speeds would be correct. I beleive there is a tension spring as well. Is this how they calibrate them ?

[ajsphead]

Easiest solution would be to get the speedo recalibrated, common enough in the classic car world. Try Richfield flexible drives in Nottingham

[Kevin Wood]

There was always a company called "speedy cables" who recalibrated speedos for the kit car community.

In fact, here they are: http://www.speedycables.com/

You used to be able to measure how many turns the speedo cable makes against how car the car has moved, send this info along with your speedo to them and they'd recalibrate it. Might be worth a call.

[Kevin Wood]

.. but a 3.307 diff is a very tall ratio for a 5 speed gearbox! Might be worth making sure you're happy with the performance with this ratio first. It would suit a diseasel engine nicely.