Well, I've finally got there, 15 tooth sprocket fitted on the front.
This is what I've done :
The standard sprocket is 14 tooth and is 8.5 mm wide.
Unfortunately I couldn't find a diect replacement.
The DRZ400 sprocket is 10 mm wide and the TU250 sprocket is only 7 mm
Also, the teeth are central on the Inazuma sprocket whereas both the above are offset, with a flange.
After some careful measurement, I've fitted the 7 mm, TU 250 sprocket, with the flange at the back, this should mean the teeth are lined up as per the original.
Because the new sprocket is narrower than the original, I've made up a new washer (0.100" thick) to fit under the retaining nut, as the nut wouldn't tighten on the std washer.
As I thought, the speed sensor relies on the gearbox output shaft, so the speedo reading is unchanged despite the different gear.
To get some idea, I rigged up a Cateye cycle computer to compare against the Suzuki speedo.
(Obviously I can't be sure how accurate this is)
On the standard gearing, compared to the Cateye, the original speedo over reads by almost exactly 10%.
With the new sprocket fitted the Cateye and Suzuki speedos are within 1 mph of each other.
So, either the speedo is now more accurate than before - or - it now under reads by about 10%
Maybe at this point I should say that this is clearly not an approved modification. If anyone wants to folow what I've done, fine, but don't hold me liable
The sprocket I've used is from JT sprockets part no JTF434.15
I also pulled a favour, to get the new washer ground to the correct thickness.
Other than that it's an easy enough job for anyone with a decent toolkit, including torque wrenches and an impact driver, to get the sprocket nut undone.
All I have to do now, is go out and test it