Final drives again.

Post by **newkid** » Mon Sep 12, 2016 10:20 pm

Hoping someone can help. Ever since I had the final drive changed the speedo is way out. I know it's to do with the final drive increasing the rpms by 400 at any giving speed. Obviously the speedo gearing hasn't been changed so it's to do with this. Is there any type of speedo convertor I can get to rectify the 400rpm difference.

Example. Doing 3700rpm in 5th car shows you doing about 70mph. Your actually only doing about 70mph when your doing 4100rpm.

Cheers.

Post by **NafemanNathan** » Mon Sep 12, 2016 11:13 pm

Are you sure it's always 400 revs difference? It's a ratio after all and it'll depend what gear you're in also.

The right size resistor should do it in theory. The question is what size?...

EDIT: Sorry, you'll need an amplifier.

Post by **newkid** » Mon Sep 12, 2016 11:59 pm

NafemanNathan wrote:Are you sure it's always 400 revs difference? It's a ratio after all and it'll depend what gear you're in also.

The right size resistor should do it in theory. The question is what size?...

EDIT: Sorry, you'll need an amplifier.

No

Ok easier way is if the final drive works out 9.19% different then the clocks should also be 9.19% different to what there actually showing?

If so I'd need a resister that slows the clocks down by 9.19%?

Now im also presuming that if you have car tuned on say hondata, crome etc then whoever is doing the mapping can make that 9.19% change happen in the ecu instead of adding a resistor?

Post by **NafemanNathan** » Tue Sep 13, 2016 1:10 am

Not sure what part you're saying no to

But exactly, you're talking percentages or ratios (the same difference) 400 isn't the same percentage of 4000 as it is 5000.

The difference between how the car was at 0 revs before isn't now 400 revs, it's still 0. When a certain speed was 2000 revs in 1st and 5th, it isn't now 2400 for both at those same speeds. The difference will still increase as the revs increase and that will vary throughout the gears (The differences will be the same percentage though).

I'm pretty sure on the same page, but your 400 comment contradicts the rest of what you've said... Unless I've misinterpreted it, which is highly possible at this hour

But like you say, the difference you need to make to the reading is a percentage (or a ratio) of the original.

I don't think you can make a change to any EMS that is built around an OEM ECU. I think you'd need a standalone.

Post by **newkid** » Tue Sep 13, 2016 3:11 am

Yea ignore the 400rpms bit. Percentage will be 9.19% I'd need a resistor that knocks 9.19% of the actual speed clocks are showing.

Confused · Post by **Confused** » Tue Sep 13, 2016 12:11 pm

The speedo is driven by a pulse. So you need a box of tricks to be able to take the incoming pulse, and modify it by a specific percentage, and output another pulse - it's not as simple as a resistor!!

I need to do the same to feed into my engine ECU for the Anglia.

I did find this page where a guy has built a circuit based around an Arduino Nano - I've just not got round to ordering the parts to make it, yet...

Probably the biggest hurdle is to design the PCB and get it printed! Actually soldering the components together is a really quick job!

bennyboy · Post by **bennyboy** » Tue Sep 13, 2016 12:21 pm

Totally just thinking out loud, but technically could you use something like a Raspeberry Pi to do the work? especially if it's already in use as the head unit/media centre? I imagine anything is possible, in theory.

Confused · Post by **Confused** » Tue Sep 13, 2016 12:25 pm

bennyboy wrote:Totally just thinking out loud, but technically could you use something like a Raspeberry Pi to do the work? especially if it's already in use as the head unit/media centre? I imagine anything is possible, in theory.

Yes, probably - I believe the Raspberry Pi has some general purpose inputs/outputs. However, if it's running another piece of software, I wouldn't want to rely on that for something critical such as the speedo. A dedicated circuit would be best for that, in my opinion.

bennyboy · Post by **bennyboy** » Tue Sep 13, 2016 12:26 pm

True enough. Yeah, don't want to get an SP30 because it's too busy playing One Direction...

craig_jdm · Post by **craig_jdm** » Tue Sep 13, 2016 12:32 pm

http://www.jaycar.co.uk/corrector-speed ... e/p/AA0376