Speedo drive cog.

[.]

Can someone please check my logic.

The background: I have just changed my diff from a 3.9 to a 4.6
My current speedo is in Kmph.
There is no speedo cog to suit the 4.6 ratio
When I am actually travelling 100Kmph my speedo tells me 140

So heres my plan (I'm broke so I can't afford a new speedo)
I figure by changing back to a Mph speedo but reading it as if it were Kmph would then read 87 when I was actually doing 100.
So then I need to adjust it a bit more to get it almost right.
Assuming I have the 19 tooth cog (I need to check this) I multiply 87 by 19 then divide by 17 to end up with 97.2

In other words if I change my speedo to Mph and get the black cog (17 tooth) my speedo should be usable again.

Thoughts anyone?

[garvice]

I think you have thought about this far too long.

Your logic appears correct.
The smaller cog (17 instead of 19) will be turning quicker then the larger cog.
So you should be pretty close to seeing the correct speed.

[stevo_gj]

Alternatively, you could just make up your own gauge face and overlay it on your current one, similar to the way the speedhut faces worked on mine. Since you have a GPS to measure speeds it shouldn't be too hard.

[garvice]

If you are already hitting the stopper, then with a smaller tooth drive, you are just going to be hitting the end of the speedo even quicker.

[stevo_gj]

I am trying to think through your original suggestion Greg, but I don't know the sequence of gears which are connecting the wheels to the speedo drive. If you could throw up a quick diagram then it would be a lot easier.

Edit: Can't seem to think straight in the mornings..

[sexual_sushi]

hey greg, I have made up designs to get some speedo faces made up on aluminium. It is quite expensive to print but I could use you as a guinea pig? I was gonna get tests done anyway

[Zedback]

Yep agree you are correct, I worked it out this way...
You're reading 140kmh when doing 100kmh which is about 87mph
You want to read 100mph (= 160kmh) when doing 100kmh (currently 140kmh)
Therefore you want to increase the speedo ratio by 160/140 = 1.143
Speedo cog is 19 tooth matching a 3.9 diff.  3.9/1.143 = 3.41  or a 19/1.143 = 16.6 tooth

So what diameter are your tyres compared to stock?  That should tell you whether to go 16 or 17 tooth!  Or better yet a GPS log telling you with greater accuracy what your true speed is.

What's a mph speedo read to, 160mph?  Do you care to read your speedo in the track?