However, a different approach is to use a lower-ratio (higher geared) back axle. The standard Javelin one, at 4.875:1, gives just 15.5 mph/1000 revs with 525x16" tyres. Some years ago, Mike Smailes and I found a manufacturer who could provide alternative crownwheel/pinion sets for Salisbury axles. We each ordered a 45 tooth crownwheel with 11 tooth pinion, giving overall gearing of 4.09:1. In practice, (using Kingpin 175x16" Taxi tyres) this equates to just under 20 mph/1,000 rpm.
I'd had my modified axle sitting around in the garage for some years, but decided to fit it to OS7943 Javelin last year pending the long trip from York to Goodwood for the 2013 taxi jaunt. Although OS has a pretty standard engine, the car coped very well with the higher gearing, lolloping along the motorway at a very comfortable 70mph, loaded with myself and 2 chunky adult sons, all our camping gear, lots of tools and spares, and a large amount of alcoholic beverage! Ok, let's be honest, acceleration is hardly neck-snapping. But the car gathers speed without complaint, and the (only mildly-tuned) engine copes remarkably well.
I remember the old magazine motor tests, where 'tractive-effort' and 'Maximum-gradient in top gear' figures were given. These things mattered back in the fifties, when even the A1 was like a modern secondary country road. Today however things are very different.
In conclusion, do I prefer an overdrive or a high back axle? Well, despite possible reliability issues, I'd go for the overdrive option, which gives the best of both worlds. But I do like my 4.1 axle. Perhaps, for the Javelin, a Jupiter axle (4.55:1) is a good compromise. But watch out for wheel clearance issues, because a Jup axle is slightly wider than the Javelin. Also the Panhard rod bracket is at a slightly different angle. Finally, I confess that I've plucked the above statistics from (fading) memory, so please forgive any unintended errors
Geoff McA
