15-10-2018, 08:44 PM
Austin themselves recommend changing up at 8mph, 18mph and 25mph on '36 to '39 cars. However, maximum power is quoted as 17 bhp at 3800 rpm. Using the figure of 800 revs per mile for 4.00 x 17 tyres (17" rims) quoted in the back of Bill William's book, this equates to 1st 13mph, 2nd 21 mph, 3rd 33mph and 4th 56 mph for the 5.125 axle ratio. When driving my Ruby, I sometimes reach 30 in third (about 3400 rpm, apparently), but generally do not tend to exceed the Austin figures. (And there is no chance of attaining 56 mph in my car.) As already suggested, if the engine is not running smoothly at higher revs, it is best to look for the cause directly.
Colin
Colin