Axle ratios, crown wheels and pinions
The standard Routemasters had
a rear axle ratio of 5.22, achieved by using a pinion with 9 teeth and a crown
wheel with 47. In the close up view of a standard crown wheel you will see the marking
9X47. The end-on view of the pinion shows the 9 teeth. Standard RMs had a top
speed of about 38 miles an hour which was perfectly suited to work in central
Country area RMs had a lower ratio of 4.77 (9X43). My RM has a country bus differential and has a top speed of 45 miles an hour with an ageing AEC AV590 engine (an accurate figure with a recently calibrated tachograph). Average fuel consumption is 13.5 miles per gallon. The downside is that it is not all that comfortable starting in second, especially with a load and it sometimes needs to crawl up hills in third gear.
The airport Routemasters (RMAs) had a larger AV690 engine and a rear axle ratio of 4.08 (12X49). It is said that they could achieve 60-70 miles an hour towing a baggage trailer!
Please email me with any corrections and additions to this article.