Wheel diameter

[Alan]

The problem you have is not with top gear, but the intermediate gears. 3.50x17 wheels with a 4.9 diff will give more or less the same gearing in top as 3.50x19s and a 5.25 diff. The problem is that second gear (3speed box) which was already too low, becomes even lower. The same is true for a four speed box. Making Austin 7s go fast is as much about gearing as it is engine tuning - you have to get it right. It matters not whether you use 17in wheels or a different axle ratio, the change to top gear speed will be the same,but it's the spacing of the intermediate gears you really need to sort. So, what I'd do - 5.25 or better, a 5.625 and stick with 19 in wheels (why? - they look nicer), but then swap the box for one with Ulster ratios. This will be ok for most places, but then if I was getting serious I'd have 17 and 16 inch wheels as well, because it really is all about gearing, and the more options you have, the better