Custom gearbox or bigger wheels/tyres?

[Allanw]

Tyres and final drive together looks like 20% drop in revs, but I bet the cost of both is high? If I could get a better percentage drop just via gearbox final drive changes and perhaps custom gear sets it might just be worth it?

Be really careful with expensive gearing changes! If you gear too tall, you'll have a slow van that uses HEAPS of fuel and is a pain in the a*** to drive! Tyres you can always sell on, but if you change gear sets and hate it, that's a LOT of money to waste!

I assume a diesel box was doing very high revs to start though.

I assume Aidan can give pointers there.

I live on mostly winding hilly road types (nearest motorway is 1.5 hours away, we just have state highways, mostly 1 lane each way). My 2.5 Subaru powered van has a 4 speed AOJ box. With the taller tyres, it's running about 3000 at 60 mph. While it would be nice to have a 5th for less noise, a taller 4th would be a pain in the a***, and that's with 165 HP, a lot of torque and 6800 rpm max. Because of the time I'd spend in 3rd gear, the van would be noiser and hungrier with a taller 4th. I also have the luxury of top gear being useable from 25MPH upwards (never been past 90). 3rd also works from 15MPH to a bit over 80 mph.

When I convert to SUbaru 5 speed, I'll have a 5th that is about 2700-2800 at 60 in 5th and my new 4th will give me about 3100 at 60, which should suit my engine well.

I don't know what would suit your though

[Aidan]

i can add a longer 5th to a 4.14 but the scooby is just a wbx and happy to rev and you can't gear it to the same as the car it came out of cos the van is heavier and displaces twice as much air
low revs does not always equate to lower fuel usage, and the scooby lumps aren't economical in a car, they always run rich to protect the cat and don't use lambda till fully hot which is why they are rubbish on fuel on short runs
14% is a big difference
205r14c is nearer 10% on the gearing which means on a standard dg engine it's over geared and fuel economy isn't improved - I have done it on a 2wd and have the fuel receipts to prove - nor is driveability
a quieter exhaust will make a subaru installation easier to live with for a lot of people
most scooby installs are running the original wbx gearing
once i get my scooby project running I'll be playing around with gearing it a lot, 4 and 5 speed for different applications but I have enough boxes out there, standard, slightly modded, and heavily modded to have a fair idea of what works without the benefit of VW r and d

[busbuddy]

you need to spend time with pen and paper working all the scenarios out including the cost, is a couple of hundred quid worth it to you for fitting new tyres to see if it will feel better? every person has their own comfort revs and I've been in vans that sound like the engine is going to explode and the owner thinks its fine and has been that way for years



when I fitted my tdi on a 3H 4 years ago i did the number crunching but as my gearbox was an 'unknown' because the van is over 20 years old and the box is probably original I decided to try tyres first and let the gearbox decide if it could cope with the tdi before I spent any money on it, I'm running 255/55x18s because I can and the set up is fine but now I must admit that I'm getting curious to see just how much more mpg I could get with a sorted box and will probably spend the cash at the end of this year

[Allanw]

For some reason... I thought the OP was talking about a later VW diesel on a early diesel box ... so some of my last post doesn't make sense! Maybe I should have re-read the earlier posts

Luckily, Aidan knows what he's talking about!

The economy of my 2.5 Quad cam is 12L/100kms on my commute, which is about 23.5 MPG - I give it hell though, and I live on the outskirts of town, and travel to the other side for work - it's mostly town driving. On a really cruisey run, I once got 34 over 3 tanks.

Like Aidan says - you can't run the same gearing as the car, when it's in a brick.