Higher? Sounds like a clash of nomenclature.
any higher gearing and i'll be doing 30 miles an hour in fifth
Gear ratios are referred to quite often as "high" or "low", and there is some ambiguity about it sometimes. First gear is low gear, and 5th gear (or whatever) is high gear, or top gear. It's a reference to the amount of speed generated in each one. So, if you go to higher gearing, you go faster in 5th, not slower.
What confuses the matter is that ratios are expressed in numerical terms. For instance, a 49/13 is a 3.77:1 ratio. A 53/13 is a 4.08:1 ratio. 4.08 is a higher number, but it's a lower gear. The 4.08 ratio is a greater differential ratio, but it's a reduction ratio, which lowers the speed of the driven shaft relative to the driving shaft. So, just like temperatures below zero, the bigger number is lower.