Changing final drive -- effect on speedo?

you need to change the plastic cog on the end of the speedo cable. I can't remember, but if yours is white, you need to get the red one or vice versa.

How did you sort the diff in the end Tub?

 

Sort the diff in which way mate?

 

I'm not bothering with the speedo on mine, it'll be way out anyway so I'm going to do the bike computer thing that Caterham people do. Gives me a completly accurate speedo (calibrated to tyre diameter) and some of the functions I'll have lost from the MFA, all for 20 quid at my local mountain bike place! I did it on my old mk2 and I was dead chuffed with it but it doesnt illumiate at night with the VDO one I bought, there are some that do. Plus I had trouble with the VDO and the small diameter wheel as it would only register so many 'clicks' per second and it maxed out at 47mph when calibrated to 15" wheels. OK for BMX or moutain bikes but crap for a GTI.... cant remember the one to go for

 

Get Gemini to swap to 02A for 020, no hassle etc

 

Quality idea!

 

It was the main shaft i was having the problem with.... the diff is a Quaife one that i had no probs with.

But, they did take the 02A off me and sent out an 020 - but that one was an 8v one!

Luckily Martin (Hotgolf) could easily sort it all for me

 

I took pics of the instalition and the bracket I made to hold the sensor over the inner CV and the magnet to the cv bolt but I lost the lot with my old digi camera.
Might do a FAQ as the golf ones are pitifully innacurate.

 

I was thinking about this - it's what Stack use for their several thousand pound dials!

But what about one thats permanantly back-lit when on - do they do them?

 

here's a link that explains it better than I could. just make it golf specific ad you away really, I did mine to the CV joint so it wouldnt be too remote and I could hang the sensor from the brace on the 'box.

 

They are driven from the pinion shaft, but the 3.94 R&P set and the common 3.667 R&P set both use 7 teeth on the pinion shaft, to spin the plastic speedo gear.

If his car was originally using a 3.667 R&P using the 15T red plastic speedo gear, and you installed a 3.89 or a 3.94 into his trans, the speedo will read fast....ie, he'll be indicating 70mph and actually be doing less. Not a HUGE difference, but enough that VW opted to change the driven gear one tooth larger to a 16T to match the lowered R&P sets.

The pinion shaft will still spin the same RPM as it did before, but now the ring gear it spins (and ultimately the tires) is now spinning slower than it was before, and thus, the car moves slower in a given RPM than with the taller R&P set.
The info being sent to the speedo is still coming from the old red gear, telling the cluster that the diff is spinning faster than it is.

There is also a green gear, and I am not sure on the tooth count for that, or if it even mates to a 7 tooth pinion shaft, but it is found in the trans with the 4.25 R&P, which, here in the States, was only offered on the 1991 Jetta EcoDiesel.
I want one of those for my own box....I am using a 3.941 in my 16V box (had to use 8V pinion and 4th driven gear to use the 3.941) now, and I love it compared to the 3.667 it did have...I can see I would really like a 4.25 set

Broke

 

My car was originally on standard 020 2Y ratios - so thats 3.6 i think.

I now have a VW Motorsport 3.94 in there - so does that mean i can use the red drive gear?

I was gonna go over 4 myself - but i can already hit 125+ on tracks i use... so needed to keep a top speed more than 120mph.

 

Just re-read your post. Using my red gear on the 3.94 would make it read fast. Or have i got it wrong again?

 

That is correct. You have a 3.941, like my own 2Y 16V box, and it requires the use of the White 16T speedo gear. The Red 15T gear will be spun faster than it should be according to the RPM of the ring gear, so it will be reading fast.
The higher tooth count of the White gear will slow it down a bit, lowering the indicated speed, to match the slower turning final drive set you have now.

Tire size can affect this reading as well, and VW speedometers aren't exactly right anyway, I think I read once that they are generally a few percent off actual speed, so check your indictaed speed against a friendly cop on the side of the road or using a GPS unit or something and then decide if the inaccuracy is bad enough to bother with swapping the gears out.

They can be swapped, a spring clip retains the gear onto the sqaured drive cable, and they are easily popped on and off....sometimes a bit too easily, as more than one person has asked about the plastic speedo gear dropping into the trans

On a side note, 3.941 is fun, init?

Broke
Edited by: Broke

 

Thinking about it your right, as the speedo is taken from the final drive which would give the true reading as it'll be the last drive before the wheels,
BUT, if you go putting a gear in of a different size its either gonna be too tight meshing against the opposite gear or not going to touch at all. So wouldn't you have to match the drive gear to the appropriate output shaft??
Just looked on Etka to double check, doesn't say sod all about what goes on what. Just that the white is 16 tooth and the red 15.
Looking gainst all the box ratiod though from 'Gearup' then it does look like that you'll need the white drive. Sorry mate (trust a bloody yank )

Edited by: Hotgolf

 

some great info there mr broke

 

Sort of on the same topic but do you know what my gears would be coming from a 4+e box? as it does about 125+ in 4th.

 

Normally, yeah, but for this application, precise mesh isn't required.
By VW only changing one tooth size, they can allow the gears to mesh, but yes, since the speedo gear is in a fixed position, and the tooth pitch cannot change (it has to mate to the pinion shaft), and we altered the overall diameter of the plastic gear, we have absolutely affected mesh.

Where it once engaged with a 50% contact area on each tooth, using the red gear, and we install a bigger around (ever so slightly bigger) 16T white gear, the mesh will get tighter than before, but since only half of each tooth was engaging the pinion teeth, we have room to play, and going one tooth up just results in the contact are increasing to say 80% or something.

The mesh will alter, but not enough to stop them from contacting each other, and not enough to have them forced together too tightly. If we changed like 3 teeth or something, then yeah, we'd have problems, but for a small change, it doesn't present a problem.

I thought I had a white gear handy out there, but I spent 5 minutes (barefoot on the cold garage floor!) looking, and couldn't find one....I have several red ones, but I wanted to measure the diameter of them, and see the % change in diameter....my 50% and 80% examples are exaggerated but you get the idea....the red gears have a "loose" mesh, white ones have a tighter mesh, but both are OK for the loads they see to spin the speedo cable.

Hey, you're a trans guy....over there out of the States ....have you ever seen a 16T white speedo gear that fits the electronic speedo sensors?
I've dug in etka, and I don't think it exists, but I keep hunting anyway...over here the electronic speedo came on the MK3 cars, and ALL of them use the 3.67 R&P, so they are all red 15T gears....the MK3 people love a 4K (MK1) trans installed into the MK3, but then nobody has a good speedo anymore.
If I could find 16T white gears to fit the electronic speedos (or even the entire sensor with the gear), there is a market here for those

I was hoping one of those 0.5L 3.5-cylinder odd-ball VWs they sell over there might have an electronic speedo, and be using a lower 3.89 or 3.941 R&P, so if you ever see one or know where to score some, let me know

Broke

 

If it is an 020, look on the bottom of the bellhousing to find the code, then the gear ratios and speedo gear and all that can be found out using that code. It'll also show you the exact date the trans was built, down to the day.

The code is found on a flat spot on the bottom of the bellhousing, shown here in the yellow sqaure in this pic...



Once you have the code, there are several sites that list the codes and ratios, I don't have any handy now, but if you can't find any, let me know. I might also have the code of your trans in some factory papers I have.

Broke

 

I'm tempted to buy a SPA Design Digital speedo from D Tweeks. 200 for loads of features!

 

http://www.scirocco.org/gears/

Code code site