Gearbox Advice For Taller Gearing Please?

[triumph2.5pi]

Hi all. I’d really appreciate some gearbox advice please?

I have an ’88 Autosleeper hightop which was originally a 1.9 but has recently had a recon 2.1DJ from the VW Engine Centre. Even though it’s still on the 1.9’s Pierburg there’s been a noticeable increase in torque/power and now it feels like it could really benefit from taller gearing.

I have been offered an AAP 5 speed box which has just come out of a 2.1 and according to the seller is in good working order (only being taken out to install a more modern engine & box).

There’s nothing mechanically wrong with my current 4 speed box (as far as I can tell as a layman)  but it is of course it’s 35 years old and has done over 150k miles (presuming it’s the original ‘box which I believe it is).

(UPDATE) I'm assuming my current 4 speed final drive ratio is 4.83 (will check code asap) and therefore the AAP's 4.57 is about 10% difference.

So now I’m note sure whether to:

1) Have the 5 speed AAP box reconditioned and fitted?
2) Have the AAP box fitted as it is and hope it’s ok (and if it’s not, then have it reconditioned)? 
3) Have a 4 speed reconditioned box fitted, using the parts required from the AAP for taller gearing? 
4) Just stick with the current 4 speed box as maybe the slightly taller gearing from the AAP just isn’t worth the hassle / expense?
5) Something else I haven’t thought of?

Any thoughts would be much appreciated, TIA
 

[T3luestar]

Get the code of your 4 speed and compare the ratios with the 5 speed, you might not actually gain anything by swapping boxes and gear linkage etc.
https://rjes.com/html/gearbox_types.html

[Da dan]

As above. Not a lot of differences

[triumph2.5pi]

Thanks. I had just assumed my current 4 speed final drive was 4.83, but I don't know that for certain so will check the code on the box. 
If it is, I'm aware changing for a 4.57 isn't a huge difference, but a roughly 10% or 350rpm drop at 70mph to me would be "worth it" I think?

[Aidan]

roughly a 8.8% difference in top gear between ABD 4 speed and AAP 5 Speed, 3600rpm becomes 3280rpm

I have a refurbished rhd 5 speed gearstick and box under and front linkage in stock if you go that way; you can use the 4 speed stick and linkage set up but you'd drive it like a Syncro, under the gate and back to get 1st, some can cope with it, some don't like it, many just pull away in 2nd all the time and treat it like a 4 speed but the 2nd gear is too high really to use as a 1st and you will put extra wear on 2nd gear and the mainshaft or you'll be slipping the clutch a little to get moving

[triumph2.5pi]

roughly a 8.8% difference in top gear between ABD 4 speed and AAP 5 Speed, 3600rpm becomes 3280rpm

I have a refurbished rhd 5 speed gearstick and box under and front linkage in stock if you go that way; you can use the 4 speed stick and linkage set up but you'd drive it like a Syncro, under the gate and back to get 1st, some can cope with it, some don't like it, many just pull away in 2nd all the time and treat it like a 4 speed but the 2nd gear is too high really to use as a 1st and you will put extra wear on 2nd gear and the mainshaft or you'll be slipping the clutch a little to get moving

Thank you. I'll bear your gearstick & linkage in mind. Would the original 1.9 '88 Autosleeper box definitely be ABD do you know?
 

[Stesaw]

Crikey, 3280rpm sounds better than 4000+ for my 5 speed

[Aidan]

Most 4 speed 1.9 petrols had ABD in 1988, a few had ABB which is 4.57 final drive with the 0.85 4th gear, only way to tell is check the box itself
Gearbox Codes - VW-T25-T3-Vanagon-Bus-custom- Gearbox- Vantopia

[Smiffo]

Not wishing to hi-jack, but just my two pence worth ( as someone who hasn't had a 4sp ) is that my ABN or ABM ( can't remember which ) is great between the gears when moving, but not for first gear pulling away.

I find 1st too low and almost pointless, awkward to get into, but 2nd is too high to pull away in without slipping the clutch - which I don't like doing.

I think what I am trying to say is I sometimes wonder whether I'd sacrifice higher revs at motorway speed for better drivability around town at low speed. Given the option for a 4sp with good final drive, I may do that if I ever need to swap my gearbox out.

 

[triumph2.5pi]

roughly a 8.8% difference in top gear between ABD 4 speed and AAP 5 Speed, 3600rpm becomes 3280rpm

I have a refurbished rhd 5 speed gearstick and box under and front linkage in stock if you go that way; you can use the 4 speed stick and linkage set up but you'd drive it like a Syncro, under the gate and back to get 1st, some can cope with it, some don't like it, many just pull away in 2nd all the time and treat it like a 4 speed but the 2nd gear is too high really to use as a 1st and you will put extra wear on 2nd gear and the mainshaft or you'll be slipping the clutch a little to get moving

I've sent you a pm Aidan

[Rosie n' Jim]

An easily reversable option would be larger wheels. Either tyres, or rims and tyres. My speedo over reads by around 5%, so an increase tyre circumference of 5% will both correct the speedo and drop the revs 5%. 

[mrhutch]

An easily reversable option would be larger wheels. Either tyres, or rims and tyres. My speedo over reads by around 5%, so an increase tyre circumference of 5% will both correct the speedo and drop the revs 5%. 

I'd agree with this..

I spent a few months with a bus playing with gear ratios etc..  my advice would be to make a choice that gives you acceptable revs for the work being done...

At the time (a good few years ago) I was regularly driving to the south coast or north yorks for work  - each trip was at least 200 miles in one sitting..
so I got a setup that suited, IIRC it was 16" wheels with tall tyres and an Aircooled (DK) box  -  4 speed with high fourth - running off a 2.0 gti engne

Motorways were lovely, cruise at 65 - 70 at about 3500 rpm (maybe a bit more, this was back in the times where everyone was going TDi)

country/a road driving was awful...