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speedo under reading !

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speedo under reading !

Postby mike En » Sun Jun 18, 2017 7:27 am

Since swapping over the speedo drive parts from my mk4 gearbox in to the replacement mk5 gearbox ive noticed the speedo is under reading my speed, ive only driven when using the satnav since the speedo was working. At 30mph on the satnav the van is saying I'm only doing 25-26mph. If I do 30mph on the clocks the sat nav says 35mph.
When pushing it up to 60mph on the satnav the clocks only say 50-51mph !

Ive got some borrowed steel wheels on it atm which are fitted with 195/14's and my alloys where only fitted with 185/14's so obviously this will make a small difference to the speedo reading as they'll be very slightly taller. But I don't think this will cause that much of a difference will they ?

I went from standard 27" tall tyres to 30" tall off road tyres on a Vitara and it didn't make anywhere near that much difference to speedo reading.

Any ideas what the problem is ? Are there different drive gears for the speedo drive ?
Ive fitted my mk4 one which fitted perfectly, and have also retained the mk5 speedo sensor aswell.
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Re: speedo under reading !

Postby loot » Sun Jun 18, 2017 7:48 am

Edited due to reading wrong

If you want it right then either play with tyre sizes (price different ones as some sizes are dirt cheap compared to others but consider load index) or reverse the speedo drive work you did.

Also i know that the milage difference is compounded but on old Ford speedo they were mechanical driven and based on friction, there was a metal slider that you could move to adjust the friction, perhaps you could adjust it with someone driving to a speed you are happy with?
It may well be electronic speedo i don't know, I'm thinking of similar age fords or use the same speedo from the vehicle the gearbox came from?.
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Re: speedo under reading !

Postby ceurieusneus » Sun Jun 18, 2017 8:22 am

As I see it, there is only a difference off 2.4% between the tires
Reading 60 miles on the clock with the old tires would give a reading of 58.56 miles now.
I believe the big difference has to come from the swap MK4 to Mk5 gearbox
Did both gearboxes had the same number of gears?
Perhaps a solution is the swap of the speedo clock.

Anyhow, I would be careful with the speed camera's :wink:
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Re: speedo under reading !

Postby mike En » Sun Jun 18, 2017 9:58 pm

Thanks for the help guys.
I cant reverse what ive swapped as ive fitted a mk5 gearbox that had a speedo sensor, popped the little core plug out and fitted a new seal and then fitted the drive gear from the mk4 gearbox, re fitted the mk5 speedo sensor as this works from the square hole on the opposite end of the drive gear ive fitted.

Yeah same boxes, both 5 speed mt75's. Only difference was the speedo drive being mechanical on the mk4 and electronic on the mk5.

Where there different ratio boxes available on the mk5 ?
What's involved with swapping to mk5 electronic clocks ?

I'm going to be fitting landie wheels with 205/55/16's so hopfully there a little smaller and will correct it slightly

Untill then I'll carry on using the satnav ! Tbh I cant really see the clocks past my small steering wheel ! Lmao.
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Re: speedo under reading !

Postby Mfr » Mon Jun 19, 2017 10:05 am

Instead of changing the tyres why not change the speedo gear? I fine tuned mine by using a yellow gear (I think 21 tooth) to bring it to spot on the sat nav, in theory its the wrong one for me ( 6 stud 225/70R15 with a 4.63 rear end) but they were always a little under to help with speeding. Just a matter of finding the correct gear, Takes 10 mins to swap over from the side of the gearbox. Sorry if this already known.
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Re: speedo under reading !

Postby ceurieusneus » Mon Jun 19, 2017 12:36 pm

Why not rework the scale on the speedo meter?
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Re: speedo under reading !

Postby sportscott » Sat Sep 23, 2017 8:35 pm

I made the mistake of letting somebody change my clutch for me and to cut a long story short they messed up everything, I mean everything even my speedo was reading way out, too fast. In fact when driving long journeys I disconnect the speedo from the sensor on the gear box as it cuts the motor out at 165kmh and am only doing 135kmh. I have just bought a set with tacho and speedo from a petrol transit but it's on a cable. The Tacho works but will need to fix it with a cable for the speedo, which I have.
My original speedo is electronic and now way out but not so far out as it was, I took off the plastic lens and when I was driving wound the needle back over the stop pin done it quite a few times while watching my speed on the satnav, now up to 80kmh it's well within tolerance and to 60 kmh spot on just when it gets to over 100kmh it's still about 10% out this was far less than it was.
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