Standard is 4.7 as you have. 3.54 is fitted to Rangerover, Disco and 90/110 as well as Stage 1. Rangerover, disco, 110 front and 90 front and rear will fit the Series 3 except for the Salisbury diff fitted at the rear of the lwb. If you have a lwb, the Salisbury diff is not simple to change, but it can be done, and 3.54 gear sets are readily available. The Rover diffs especially from RRs are plentiful and cheap.
There are other ratios available, but they are rare and relatively expensive. 
Other alternatives include - 
1. High ratio transfer case gears - this has the advantage of only affecting high range.
2. Overdrive. In many ways the most satisfactory (you have the choice of gears), but against this is fact that the most common overdrive, Fairey, is not the strongest bit of gear out, and new or good ones are pretty expensive, as are parts. The Toro overdrive is better, but very rare.
3. Depending on what tyres you have at the moment, you may be able to fit larger diameter tyres to achieve a sufficient effective increase in ratio.
Depending on what engine you have, you may not find that a higher ratio is all that much advantage, and all the Rover engines are quite happy at the speed limit with standard gearing - although they may be pretty noisy (but most of the noise is not engine noise!). Rather than change the ratio, a campaign to quieten things may be something you could consider - replace body seals, fit insulation and sound deadening, possibly an electric fan, things such as that.
Hope this helps,
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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