Hi, I’m new to owning a T25 having previously owned a couple of T1s and a T2 bay many years ago, however I just was given a t25 on condition I got it home, well after being stood for a few years and a few swear words she started and is now home, it’s running rich and drank fuel like a teenager at the weekend but that aside, the one thing that I noticed that even with a 1900 water cooled DG code 76 BHP it’s not fun cruising on the M1 at 55-60, so the question is how straightforward is a gearbox swap from a VAG group 6 speed manual ?? Just to give it chance of 75 so I can safely overtake this century. I am aware the rough running doesn’t help. Yet the question remains is it simple and cost effective to drop a modern 6 speed in or some 5 speed with modified 5th gear cogs ??
There's no point putting a taller gearbox on a DG - VW chose the gearbox ratios for a good reason! This final drive ratio was the best compromise. The engine just doesn't have the power to push the van at the combination of higher speeds and lower RPM you're hoping for.
That said, a healthy DG will do 75mph on a stock gearbox... it'll be revvy and you won't want to do it all day, but it'll do it.
You need to service the engine and sort it first
You need to check if the gearbox fitted is the correct one for the vehicle, if a previous owner has stuck in a cheap used box out of a n/a diesel then the gearing will be wrong and it will be more snail like than it should be.
These engines make torque at 2800rpm and power at 3500rpm so that's where the engine is happy, over gearing it will mean an awful driving experience, they rev, it's a pushrod engine running a carb not a modern ohc fuel injected engine with multiple sensors and a digital brain, and it's a bit heavier and displaces more air than a T1 or T2
There are conversion parts available for fitting a five speed early Passat inline type box in a flipped installation but finding the right variant now won't be easy and it's very much a compromise install, mostly done by the TDi boys in an effort not to spend money having the original 5 speed built to suit the engine (Which is easy if not cheap). None of the transverse engine fwd transmissions will fit, the only other rear engined, rear wheel drive vehicles produced are Porsches, so anything from a 914 or 911 will be a lot of money and not at all straight forward.
Check the white sticker if still on the vehicle (usually knee high under the steering wheel) and the M plate under the left seat to find out what should be in the vehicle and then check the code stamped on the gearbox matches.
As campervans T3s are not really 75mph vehicles, suspension and brakes mitigate against that sort of driving - if that's what you want I'd advise you sell it on and buy a T6, otherwise chill out, 60-65mph is the happy place for a T3 and with the engine giving its best that's a reasonable overall compromise and relaxing holiday vibe. Every 5mph over 60 will cost you an extra 10 %, with everything sorted and driven with mechanical sympathy you can get 25mpg on a run.
Tyre pressures are quite high by modern standards so check they are correct and the tyres are still flexible, old tyres are often awful to drive
Enjoy
Hi, I’m new to owning a T25 having previously owned a couple of T1s and a T2 bay many years ago, however I just was given a t25 on condition I got it home, well after being stood for a few years and a few swear words she started and is now home, it’s running rich and drank fuel like a teenager at the weekend but that aside, the one thing that I noticed that even with a 1900 water cooled DG code 76 BHP it’s not fun cruising on the M1 at 55-60, so the question is how straightforward is a gearbox swap from a VAG group 6 speed manual ?? Just to give it chance of 75 so I can safely overtake this century. I am aware the rough running doesn’t help. Yet the question remains is it simple and cost effective to drop a modern 6 speed in or some 5 speed with modified 5th gear cogs ??