Speedo Drive Gear calculations - LOCK PLEASE

[glytch]

Box is definately out of a GA70. I had to help the seller pull it out because he wasnt organised.

The one thing that your (glytch) and Norbies calculators have in common is that they dont take into account the inaccuracy of the OEM speedo in the car.
Ill be doing GPS calculations again in the near future (when i reinstall the exhaust)

Yeah true. My spreadsheet does indeed have a manual correction factor to allow for speedo variation - though you need to know what it is first :\

FWIW I ended up photoshopping a scanned image of the original speedo face to have the corrected speedo readings and sticking it on over the top. Though I don't recommend doing that to your car

[takai]

Hm, maybe it had a 4.3 diff then, i was just going under the asumption of a 4.11 due to SupraForums ramblings, although that can be notoriously dangerous.

The 4.3 would make much better sense though, as if it was 4.3 then the speedo would be out by more like the correct amount.

[PeteH]

Hi Pete - Sorry I am not saying that you are assuming anything other than that 33/10 is the correct existing speedo ratio. And yes everything is only based on the information Takai gave us.

The ratio will only be correct if it came from a car with a 4.11 diff on 225/50/16 factory tyres. Norbies axle database doesn't have a GA70 listed but a GA71 seems to have also come with a 3.9, 4.3 and 4.55 diff ratio, so if the gearbox had any of those diffs on it originally then the speedo ratio will not be 33/10

The formula I used to work out the required speedo ratio is this:

speedo ratio = (1000/rolling circ) * Diff ratio / speedorevs

Entering the known info
1000 is meters per KM
Where Rolling circ is in m (ie 1.924)
Speedorevs = 637
Diff ratio = 3.9
Gives a ratio of approx 3.18. This is the ratio required for, everything being as is, the speedo to work correctly.

All I am saying is based upon the calculated speedo ratio required and your calculation using the GPS /speedo reading data, Takai is probably wrong about it having a 4.11 diff originally (or 225/50/16 is not a factory tyre though it seems to be AFAIK)

Cheers

Luke

Hi Luke,

No reason to be sorry as there was no offence taken

Thanks for your explanation it makes sense to me.

While driving home I was thinking it is more likely that the donor vehicle originally had a 4.3 ratio diff (which Takai now thinks), and that your calc is correct as for the speedo ratio Takai should be going for.

I think my calc was right but I was trying to move 17% from the wrong ratio, i.e. 33/10 has been assumed and is not the correct ratio.

Takai, I'd go with Lukes suggestion.

Hey that was a good distraction for me today, thanks

Pete

[takai]

Ill have to pull the speedo drive gear out now and figure out which one it is.

[takai]

Ok, ive eventually been able to pull the speedo drive (after coping with it for a while).
Its a 33 driven gear, which i think i can correlate with a 10tooth drive gear.



Im thinking that a 31/10 ratio would be needed, and if so then its likely to be found on 3.9 diff equpped MA61.

Anyone want to confirm that there are no 33/11s and only 33/10s?

[wagonist]

You could also fit 5.4% bigger diameter tyres

[takai]

5.4%? its out by almost 10%. 3.9 to 4.3

[wagonist]

Out by 10%? Even better, then 3.2% bigger diameter tyres.

Waits for penny to drop....

[glytch]

Mmmm I like Pi.