It is a fallacy that regearing the Vanagon tranny for lower rpms at the
same road speed will make much difference in MPG of gasoline. The reason
is simple. Modern gasoline engines burn nearly the same fuel/hp across a
fairly wide range of rpm. This data is available for many engines in
published SAE papers. At the speed where the stock .85 4th gives 4000
rpm the .77 (from the 5 speed 5th) gives 3600. It takes the same HP to
go 70mph whether you are doing 4000 rpm or 3600 so gearing for the lower
rpm can't have much direct effect on MPG.
In practice, making the top gear taller may even reduce MPG for indirect
reasons.

Over the past many years I have often driven a certain route of about
300 hundred miles in my Syncro. On occasion I make the trip without
using 4th gear and I cruise at the upper rpms of 3rd gear instead. I
find I get better MPG when I do this even though I spend hours at a much
higher rpm than if I was in 4th. That is because I drive a little slower
when I leave it in 3rd.

Earlier this year I regeared my tranny by changing the 2,3,4 ratios
along with the r+p. I intentionally lowered 4th to INCREASE rpms on the
the highway. My overall highway MPG improved slightly after this though
that was not why I did it.

Driving speed is far and away the largest factor that influences highway
fuel economy in Vanagons. Making 4th taller encourages you to drive
faster, at least it does me. Then MPG goes down not up.

Mark

Daryl Christensen wrote:
>
> I put a taller 4th in during rebuilds quite frequently. The .77 4th drops
> about 500 RPM at cruising speeds compared to the .85 stock ratio. So far
> everyone loves em but I can't swear it gives you anymore mileage. Haven't
> actually heard of any definitive results.
> Daryl of AA Transaxle
>