1.6 Gearbox On A Mi, Will Speedo Be Right?

[floodys 4 1 Cars]

as per tthe title i have put a 1.6 box on the mi and was just wondering if the speedo will be right or do i need to put the speedo gubbins out of the the 1.9 box in or are they the same?? cheers

[Ryan 99]

The 1.6, 1.9, and Mi16 all have different speedo drive gears. You need the ones that match the tyre size you're using - so say you've got 1.9 wheels then you'd need 1.9 speedo gears.

[pug_ham 241 3 Cars]

It'll be closest with the 1.6 gearbox drives imo, its the fd ratio that changes the speedo reading not the size of the wheels as they are almost identical 1.6 or 1.9.

 

1.9 gears on a 1.6 box will slightly over read.

 

Graham.

[floodys 4 1 Cars]

if i put in the 1.9 drive, doi need to just put in the red drive bit that is under the 10mm bolt?

[Rich_p 23]

I run the normal speedo gears from the 1.6 box in mine and it's actually quite accurate when tested against a gps thingy.

[Tom Fenton 1,526]

The speedo drive picks up off the driveshaft, so match the speedo drive gears to the wheels and it should read right. The final drive is kind of irrelevant, because the speedo comes off the driveshaft, so the speed is simply a function of the speed at which the shaft rotates regardless of what engine rpm is, and the rolling circumference of the tyre.

[pug_ham 241 3 Cars]

The speedo drive picks up off the driveshaft, so match the speedo drive gears to the wheels and it should read right. The final drive is kind of irrelevant, because the speedo comes off the driveshaft, so the speed is simply a function of the speed at which the shaft rotates regardless of what engine rpm is, and the rolling circumference of the tyre.

Hmm, so what turns the driveshaft besides the diff which the crownwheel of FD is bolted to? The shaft turns faster as the diff turns faster therefore turning the wheel faster.

 

I agree engine rpm has nothing to do with it but the driveshaft only slides through the speedo drive that couples with the diff & it doesn't turn at a different rate to the diff so surely its the diff ratio matching the speedo drive gears that are important.

 

Tyre size is a side effect because as you change this the speedo will vary a small percentage but if both are wrong then your speedo can be miles out imo.

 

Graham.

[Ryan 99]

The gear reduction between the pinion and crownwheel takes place before the speedo drive, so it doesn't matter - one revolution of the diff is the same road speed no matter what FD ratio you have (assuming the tyre size is the same).

 

The FD ratio would only enter into it if the the speedo were driven from the pinion gear or the output shaft.

[jengis 0]

I don't think we got a definite answer on this one. What actually drives the speedo gear? I thought it was the crown wheel?

 

I have a 1.6 speedo cable on a 3.8 final drive and it DEFINATELY reads wrong (gps usually shows that a speedo is under-reading but mine is now slightly over-reading). So I believe that the speedo does not just read what the driveshafts are spinning at. Tyre circumfrence is a separate factor but largely irelevant compared to the differences in final drives.

[welshpug 1,626]

as mentioned several times, the final drive of the gearbox doesn't have any effect whatsoever on the speedo reading, the speedo drive is driven by a cog attached on the end of the diff, which rotates at the same sped as the driveshaft, which turns at the same speed as the wheels, so as long as the cogs in the speedo drive housing match the tyre size, you should be close.

Edited August 11, 2009 by welshpug

[jengis 0]

as mentioned several times, the final drive of the gearbox doesn't have any effect whatsoever on the speedo reading, the speedo drive is driven by a cog attached on the end of the diff, which rotates at the same sped as the driveshaft, which turns at the same speed as the wheels, so as long as the cogs in the speedo drive housing match the tyre size, you should be close.

 

Seems a very clear-cut answer. But different opinions have been given! So who is right?

"It'll be closest with the 1.6 gearbox drives imo, its the fd ratio that changes the speedo reading not the size of the wheels as they are almost identical 1.6 or 1.9.

 

1.9 gears on a 1.6 box will slightly over read.

 

Graham"

I don't want to dissagree with anybody, I'd just wanted to know if (as suggested by a gps test) my speedo is slightly over-reading with a 3.8 FD, or would it be the same with the standard 4.06 on the 1.6 box?

 

The joy of forums!

[Tom Fenton 1,526]

Road speed = rotational speed of driveshaft x rolling circumference of tyre.

 

Speedo reading is a function of the rotational speed of the driveshaft, as the speedo gear meshes into the end of the differential, which has a splined connection to the driveshaft, therefore the differential turns at the same speed as the driveshaft when going in a straight line.

 

Therefore,

 

Speedo cable speed = (rotational speed of shaft x rolling circum of tyre)/reduction of speedo drive gears

 

We then get to the speedo, which converts the rotational speed of the speedo cable, into a rate of speed displayed by the needle, using a magnetic slip joint of some sort.

 

However, in NONE of the above does the differential gears affect the calculation.

 

If you change the diff ratio, then more or less engine revs will be needed for a given driveshaft rotational speed. This is an expected result of the final drive change. However, this increase in engine revs and final drive will have no effect on the above formulae for speedo cable speed and hence speedo cable reading.

[jengis 0]

I get it now!

 

My gearbox is from a 306 so the speedo drive gear assumes I have 195/55 tyres, not the 195/50 on the 205. That is why my speedo now slighty over-reads..... nowt to do with the diff or ratios!

[floodys 4 1 Cars]

just to put my bit in here, i think my mi with the 1.6 is wrong, ( not checked against sat nav as silky has it!!) because i think i'm going faster than i am and also if the spedo was out my MPG wont be right? right!! cos i'll be doing more/less miles than i really am??

 

Also, my 306 HDI's speedo is out, at 80mph on the clock's im actually doing around 76ish? cos of my tyres being 195/50/15 and not what they should be 195/55/15.

 

so if i put in my 1.9 worm drive thingy that should help??

 

Anyone know the ammount of teef on a 1.6 and a 1.9??

 

just been on servicebox and they do a 17,20 and 21 teef pinion, and they do a 1 and 22teef tac screw. so what do you think would be right for me?

Edited August 22, 2009 by floodys

[dcc 855]

erm, you will never, ever have a correct reading from a speedo. it will always over or under read - as standard they over read - you do 30, the speedo shows 35 for example.

 

If you REALLY need to know exactly how fast you are going, get a satnav or gps, get a mate to jot down the speedo readings from 0-70, in intervals of say 5. and get him to take readings from the satnav display. dont do it yourself, taking you eyes off the road will make you more of a danger than you already are

Edited August 22, 2009 by dcc

[large 33]

my std 1.6 and 1.9 set up were both spot on.