thats the shortest ratio they do really
you should have either 13 or 14 teeth on the pinion gear and either 46 or 47 on the crownwheel (it's been a while since i did all this stuff)

you should also find that it's going to be the more common 7.5inch diff in the back of the twinky rather than the 7 inch diff you might have been expecting (for some reason the twinky cars are 90% fitted with the 7.5 inch diff)

I've got the other end of the scale in my twinky, as in i've got a 3.14:1, coupled with the 0.75:1 overdrive as fitted to the granny cossie auto box, i should see plenty of mpg crusing down the motorways