I just bought a speedometer off Ebay and installed it yesterday. It has a MPG indicator on it. I was watching it today. When I take off from a light the MPG go down which seems correct as I sit in neutral they are high also seems correct. When I got on I95 and was in fifth going at a steading speed it was all over the place. Does anyone else have this indicator on their speedo and does it do the same thing?

Here is a crappy cell phone picture.

 

I thought there was a shift light there for the n/a's but it doesn't light up for 5th gear

the light is a recommended point to upshift for better fuel economy

 

Yea I have the light up in the tach. I was talking about the MPG indicator under the tach in the picture.

Does anyone else have this MPG feature on their speedo?

 

I used to in my auto.
The thing really sucks. It barely gives a rough idea of your real MPG.
In top gear it seems to be really touchy to how far down you're holding the pedal which may describe why yours was all over the place.

 

i have it in my early. the needle wiggles at the slightest change in throttle (even if you are at a steady speed you might have to vary the throttle to maintain it)

usually is pretty accurate for me

 

It's reasonably accurate on my '86. You might have a bad ground

 

Mine is around 40 at idle, and very close to zero even when cruising at constant speed, no matter the gear!

 

Mine works pretty accurately at corresponding to my amount of throttle. That said it is kind of a useless read out unless you can't work out that more throttle means less fuel economy.

 

does that mpg gauge run off vacume or the o2? I tried disconnecting the o2 and the needle still moved so I 'assumed' it was based on vacume but i don't know what else it could be except if the computer was doing a calculation.

tom

 

Its based off your speed vs. injector duty I'm assuming. On my 924S it runs off the speedo and some other signal I think.

 

Per the FSM.........

 

But my 84 has a cable driven speedo. Maybe there is a hal sensor in the trans just for the gauge ?

Tom

 

If you look at the top of the back of the speedo, is there a small circuit board? That is to prep the signal from a Hall sensor inside the speedo. I haven't opened one up, but it appears they put the sensor on the drum that moves the needle.