Hi Glenn
If your car has a normal 405 first gear it will give you about 5.3 mph per 1000 revs (may be some variance due to tyre profile) (this is the 15.24 ratio you refer to).
If it has an ex AC Ace first (or equivalent) it will give you about 6.7 mph per 1000 revs (again possibly some variance due to tyre profile) (a 12.26 ratio)
In a light car this will still seem very low (as you seem to be experiencing) because the diff ratio is low (4.22)
As you say, being a light car first is only marginally useful, and second at 7.7 or so is achieved very quickly.
These are NOT modified gears, they are stock ratios that were available in period and were delivered in the sports gearboxes at the time.
It is straight forward to check what ratios you actually have by driving the car at (say) 2000 rpm and seeing how fast you are going in first gear (see mph per 1000 rpm data above).
AC Aces delivered with Bristol engines all had Bristol gearboxes (usually BW CR 6/9/12) with "fixed" (non synchro, non freewheel) first gear with an internal ratio of 2.9:1 which would give 11.38 with a 3.9 diff (or as possibly in your case 12.26 with a 4.22 diff). This is all well documented.
AC Aces with AC engines did have Moss boxes - that's a completely different kettle of fish (and a completely different set of ratios).
It is conventional to talk about gear ratios as "higher" if they have a smaller number and "lower" if they have a higher number. This is because the gear ratio divides down (reduces) the revolutions per minute as you progress along the drive train. For example the diff in early (pre 405) Bristols is 3.9, this is higher than the diff on the 405 (and 406) which is 4.22.
To illustrate - if the engine was turning at 3000 rpm in top in a Bristol with a 3.9 ratio diff, the half shafts (the drive shafts out of the diff to the wheels enclosed in the rear axle) would be rotating at about 770 rpm and the car would be doing about 62.7 mph. If the Bristol had a rear axle with a 4.22 ratio (405) then the half shafts would be rotating at 714 rpm and the car would be doing 57.9 mph. "higher"= faster for given revs, "lower"= slower for given revs. This terminology convention applies for all gears in the drive train.
PM me if you want more detail
If your gearbox has ratios that are too low to be useful, when you have someone look at it, it may be useful to talk to Ian Nuttall who can do additional (higher) ratios for first gear (I presume you are in the UK, I'm in Australia so unlikely to be much practical use to you even though I can do a number of ratios and synchro for first). Again PM me if you want more details
Cheers
Julian
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