I used a diesel speedometer for a while in my '81 gas fueled Vanagon.
It worked fine until it started screeching somewhere in Oregon.

Having taken apart and repaired two different Vanagon speedometers, I
can say with some certainty that depending on how one treats the uber
delicate spring (or magnetic wheel-cup relationship. No. Don't ask.
;)    ), the speed displayed can be affected to a greater or lesser
degree depending on the actual speed. Some pics I took if one is super
bored:

1985 Speedometer repair:

https://picasaweb.google.com/musomuso/1985VanagonTransporterSpeedometerGuts

198x diesel speedometer repair:

https://picasaweb.google.com/musomuso/1982Or83DieselSpeedometerClipOnCableType

In general, with the 1985 speedometer I'm running (the one I dug
deepest into), speed displayed gets more inaccurate as speed
increases.

Well intended owners like myself can tamper with things like a
speedometer which can cause issues for the next owner. However. A GPS
unit rocks.

Neil.

On 7/29/14, Al Knoll <anasasi@gmail.com> wrote:

> Other than the "W" gears that drive the odometer, what is different about
> the "Diesel" speedometer?  The rotation of the input shaft is magnetically
> coupled via eddy currents to the spring-biased device to which the needle
> is attached.  The faster the shaft turns the more torque produced to
> counteract the spring force.  The angular displacement thus created is
> resolved by the scale on the face of the speedo and the needle position to
> indicate speed.

> On Mon, Jul 28, 2014 at 9:58 AM, mark drillock <mdrillock@cox.net> wrote:
>
>> The GW diesel verbiage is BS. The 2wd speedo they are selling will work
>> fine with a diesel.

--
Neil n

Blog: tubaneil.blogspot.ca

'88 Westy http://tinyurl.com/c8rlw6p

'81 VanaJetta 2.0 "Jaco"  http://tubaneil.googlepages.com/

Vanagon VAG *Gas* inline-VR Engine Swap Group:

http://tinyurl.com/d7gd5ej