Hi Paulcalf, theres several threads on this subject - but yes if your running a bigger engine anything from 1776 upwards its a good idea to fit a gearbox with a longer final drive.
This is because the extra power means you can run at higher speeds with lower engine rpm, which makes for quieter, more relaxed and, driven gently, more economical cruising.
I am currently running a 2 liter plus T1 engine with an 091 cw/pinion set fitted as used on the 2 liter T4 engined late bays and was pleasantly surprised to find that driving on a touring holiday the mpg actually improved from 25 to 28 average.
The various gearboxes all use much the same internals and its the cw/pinion set that changes the gearing. Fortunately the cw/pinions are interchangeable so if you have a good box from say a 1600 it can be fitted with a replacement longer final drive.
Despite running in southern europe and using only the stock oil cooler I have no problems with cooling.
Here is a summary of the different gearboxes out there but, as I said, its really only the final drive that changes the overall gearing.
1.6 liter and 1.7 liter
1st 3.80
2nd 2.06
3rd 1.26
4th 0.89
final drive 5.375
Overall in top 4.407
1.8 liter
1st 3.78
2nd 2.06
3rd 1.26
4th 0.89
Final drive 4.86
2 liter
1st 3.78
2nd 2.06
3rd 1.26
Top 0.88
final drive 4.57
Overall in top 4.02
Final drive ratios for all gearboxes; 95%of 1-4th gearsets are the same just the dif that changes much
1600 engine
CA - 5.375 68-71
CB - 5.857 68-71
CC - 5.375 68-71
CD - 5.857 68-71
CE - 5.428 72-75
CF - 5.857 72-75
CG - 5.375 72-75
CH - 5.857 72-75
1700 engine
CK - 5.375 72-73
CL - 5.375 72-73
1800 engine
CM - 4.857 74-75
CN - 4.857 74-75
2000 engine
CP - 4.571 76-79
CT - 4.571 76-79
1600 engine
CU - 5.428 76-79
CV - 5.428 76-79
CW - 5.857 76-79
CX - 5.857 76-79
Code Engine 1st 2nd 3rd 4th Rev. Final Years
CA 1600 3.80 2.06 1.26 0.82 3.61 5.375 68-71
CB 1600 3.80 2.06 1.26 0.82 3.61 5.857 68-71
CC 1600 3.80 2.06 1.26 0.82 3.61 5.375 68-71
CD 1600 3.80 2.06 1.26 0.82 3.61 5.857 68-71
CE 1600 3.80 2.06 1.26 0.82 3.80 5.428 72-75
CF 1600 3.80 2.06 1.26 0.82 3.80 5.857 72-75
CG 1600 3.80 2.06 1.26 0.82 3.80 5.375 72-75
CH 1600 3.80 2.06 1.26 0.82 3.80 5.857 72-75
CK 1700 3.80 2.06 1.26 0.82 3.80 5.375 72-73
CL 1700 3.80 2.06 1.26 0.82 3.80 5.375 72-75
CM 1800 3.80 2.06 1.26 0.89 3.80 4.857 74-75
CN 1800 3.80 2.06 1.26 0.89 3.80 4.857 72-75
CP 2000 3.78 2.06 1.26 0.88 3.28 4.571 76-79
CT 2000 3.78 2.06 1.26 0.88 3.28 4.571 76-79
CU 1600 3.78 2.06 1.26 0.82 3.28 5.428 76-79
CV 1600 3.78 2.06 1.26 0.82 3.28 5.428 76-79
CW 1600 3.78 2.06 1.26 0.82 3.28 5.857 76-79
CX 1600 3.78 2.06 1.26 0.82 3.28 5.857 76-79