Gearbox question diesel / petrol swap !

[937carrera]

Last weekend gave me the opportunity to see how my cheap Chinese tacho worked, so I took note of revs and road speed from the speedometer. Interestingly my speedo was accurate to GPS speed, which really surprised me as I am on standard 185 tyres.

What I saw was 55 mph = 3000 rpm, so 4000 rpm would be 73 mph, that was a bit higher revs than I was expecting so I checked with my T25 gearbox spreadsheet and I saw that with an ASM gearbox (late WB) 70 mph would be 3581 rpm, that set me wondering what box was fitted to the van.

I got under today and cleaned the box to remove the corrosion and I found that I have a 1987 3H box - my van is a 1990 van so the box has been swapped.

According to http://oldbluesblog.com/files/VanagonTransSpecs.pd the 3H box is from a 1.6TD.

Well that was a surprise, diesel box in a petrol van. Looking at the ratios top doesn't seem much different, but through the gears it is about 5% shorter so a bit nippier.

My maths says 70mph is 3619 revs, so no noticeable difference, but I also need to check the tacho accuracy.

Is this a common swap, any ideas why the 3H could have been selected or are there any internal differences than would mean a 3H was a better, or worse, choice ?

First and reverse are difficult to get but I understand the 5 speed boxes do have a dodgy linkage anyway

[Aidan]

3H is exactly the same internally as the ABN which is the 1.9 petrol standard 5 speed box, the only difference between them is the input shaft and bellhousing which have been swapped so essentially the 3H is a perfectly acceptable swap for a 1.9 van
The ASM is a weird thing, I really don't understand what VW was doing with the S boxes, the higher 2nd gear may be just to stop people pulling away in 2nd all the time (which wears the gear - and since the fixed gear is part of the mainshaft equates to noise and expensive rebuild in future) but it means if you pull away in 1st you have to rev slightly longer before going to 2nd then you are only in 2nd for a moment before going to 3rd as the gap there is less - the 1.9 can't pull the 0.82 fifth gear with the later cleaner carb jetting so they fitted the 0.85 instead so the gap 4th to 5th is small and you always feel like you want another gear
ASM is generally good for rebuilding with a longer 5th for a better engine mind as they don't tend to be as trashed as the AAPs and since they are always late will have the late syncrohub and often a better case than the majority of used boxes
just mho of course

[937carrera]

just mho of course

More than good enough for me, thanks Aidan. I have seen that sometimes boxes on other cars have hardened teeth in particular configurations, or other non obvious changes. I'll live with my 3H

Still need to figure out the tacho /speedo error. I've double checked my maths against other calculators and I got the correct answers, unusual for a tacho to be 10% out