The speedos are the same so I assume you mean the tachs. The gas tach
gets pulses from the ignition coil and diesels don't have ignition
coils. The diesel tachs from the Vanagon era use a much different signal
from the alternator "W" lead.

Mark

Ralph Meyermann wrote:
> I think the diesel one hooks up different or the pod is somehow
> different. I went from a 185/75/14 to a 215/75/15, just by
> guessing/eyeballing I'm about 5 faster than Speedo shows. I would like
> to keep up on my mpg while tuning my fuel/boost for happy oomph, temp
> and fuel economy without over taxing my Power Plant. My phone doesn't
> have good enough signal to use its gps
>
> Velma 82 1.9L AAZ TD Westy
>
> On Jul 30, 2014 7:33 AM, "mark drillock" <mdrillock@cox.net
> <mailto:mdrillock@cox.net>> wrote:
>
>     The "W" factor has to do with the tachometer, not the speedo. The speedo
>     and odometer are driven by a cable that does not know or care what
>     engine the vehicle has or even that it has an engine.
>
>     Mark
>
>