Real world fuel economy improvement from a gearbox change

[drindiana]

Am seeking some advice hopefully from those who have gone before. I have a 71 bus with a 1915cc motor in it and a 1600 gearbox (with a 5.38 crown & pinion). Any motorway cruising >50mph has it revving much more than I find comfortable and have heard a 2L (091) box with its 4.57 crown wheel & pinion will help in the noise stakes by keeping the revs down.
That said there are other ways to help keep noise down in the bus generally adding weight with sound proofing etc.
What I was hoping that someone might be able to share if they had down the box upgrade and saw some fuel economy improvements and if they did was it significant?
I know there are lots of variables but sharing the experience would be helpful for me in making a decision.
cheers Shaun

 

[Trikky2]

A well built 1915 engine will easily produce enough extra power and torque to run longer gearing.

A 1600 bus returning say 23 - 25 mpg, when fitted with such an engine and gearbox (and driven in the same manner) will be perfectly capable of 27 - 29 mpg.

The only downside of an 091 2liter box and stock size wheels/tyres is the gearing is so long that 30mph often means keeping it in 3rd in town.

To avoid this its possible to use the box from the 1.8 which has 4.86 final drive.

The ratios in all three types of boxes are pretty much the same and its the final drive that changes. The different final drives can be fitted to any of the boxes eg. if your 1.6 box is in good condition then having that fitted with a longer final drive is an option.

 

[drindiana]

Thanks Trikky2, very helpful.
cheers Shaun

 

[Humphrey]

Yep I've got a 2332 mated to an 091 box and find the ratios perfect. Yes I'm mainly in 3rd around town but that's not a problem, revs are right and saves gear changes! Also motorway cruising is spot on, 78 mph (accurate) @ 3200. If you can find a good 091 5 or 6 rib box do it!...they are getting hard to find though.

 

[uber cool]

what sizes tyres do you have? that makes a big difference to gearing!

 

[drindiana]

Uber I have 215/50 series on 17x7. Did this to keep the speedo as close to stock as possible. So should be good there.