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Real-time Mileage Display?

M

matinau

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Is there an option for a real-time mileage display?

I found one model online, the ScanGuage II - but a local 4x4 shop says it won't work on my 91 Blazer.

I'm happy to wire something in (if it comes with directions).

Any ideas?

Thanks,
Mat
 
So if there's not an OBD-I tool like the ScanGuage, is there a way to wire-in a mileage readout out of another vehicle? Or does the computer in the 91 simply not know about mileage?
 
Only way I would know of is if you were swapping in say a 6.0 out of a newer truck and grabbed EVERYTHING.......then it would be feasible.
 
Grab a Sharpie marker and write "10 mpg" somewhere in your sight line from the driver's seat. You'll always be close...:D

Rene
 
It can be done, cheaply, but not necessarily easily unless you have a good background in electronics.

Doing so on a carb'd motor would be more or less impossible without some fairly sophisticated equipment to measure the fuel flow going into the carb, as well as any that got returned to the tank.

Basically, the DIY versions I've seen used a small computer to sample data from the EFI setup...specifically the pulse length of the injectors. Using known data, like flow rates for the injectors will let you make a fairly accurate guess as to how much fuel is being consumed at any given time based on how long the injector stays open under "X" amount of fuel pressure.

Combine that data with an electronic speedo setup to let the computer crunch the two numbers together (fuel consumed and distance traveled) and you'll get a number expressed in MPG.

I don't know how accurate this is, but I'd assume no less accurate than the stuff on OBDII cars. Most OBDII cars I've seen don't show you instant MPG. Not because they're not capable, just because the designers decided the car owner didn't need to know. My girlfriend's 2006 Tahoe did not have the option to display instant MPG, but her 2004 Yukon XL does.

The Scangauge II just pulls the info from the OBDII computer. It's not really working very hard on its own AFAIK.

You *may* be able to pull the pulse length data from an OBD-I ECU, but I wouldn't have a clue how to make it happen. Probably pretty easy to do on a vehicle equipped with aftermarket or DIY EFI setups, like Megasquirt.
 
Can you get the injector pulses right off the data port? I just had a pretty cool idea.. there are scanners you can get that hook up to a laptop. Could use a GPS to determine speed... wouldn't be that hard of a program to write. I'd suggest it to my bro, but I don't have anyway of hooking up a laptop to my truck(yet).

ETA: Doesn't the computer get pulses from the VSS as well? Wonder if that data can be read right off the computer..
 
jonathon said:
Can you get the injector pulses right off the data port? I just had a pretty cool idea.. there are scanners you can get that hook up to a laptop. Could use a GPS to determine speed... wouldn't be that hard of a program to write. I'd suggest it to my bro, but I don't have anyway of hooking up a laptop to my truck(yet).

ETA: Doesn't the computer get pulses from the VSS as well? Wonder if that data can be read right off the computer..
Click to expand...

I'd imagine most of that information is available in the computer, although whether it's accessible or readable through the port is unknown to me. At least with an OBD-I ECU. OBDII, yes...all of that should be in the ECU.

I have yet to see a GPS that reacts to changes in speed as fast as something measuring it directly off the moving parts of the car.

A simply VSS or something cobbled together with a magnet and pickup would probably be much easier to make work...for instance a drive shaft sensor. Each time the magnet passed the pickup, it would pulse. For vehicles already equipped with VSS, half the work is done.

3.73 gears and 31" tires at 60mph would be 2491rpm in "D" or any gear that has a 1:1 drive ratio. That'd equate to the drive shaft spinning at 2491rpm as well. All of that is irrelevant except to point out that under those conditions, 2491 pulses per second would equate to 60mph. 41.51rpm at the drive shaft would be 1mph, 415.1rpm at the drive shaft is 10mph, etc...

Take that info, then calculate how much fuel is dumped into the engine for a given dwell time on the injectors. Obviously, if you're sitting in traffic at a stop, the computer would see that you're still burning fuel, yet not moving. 0mpg.

I don't think it would be all that hard though.
 
If you are concerned with getting better mileage, just hook up an intake manifold pressure gauge. Higher vacuum = better mileage. Lower vacuum = worse mileage. Many German cars had analog mileage gauges like this in the dash 30 years ago, the dial had a Miles per Gallon scale but the gauge was nothing more than a vacuum gauge. You will be surprised at how much intake vacuum varies with throttle position and engine load.
 
jonathon said:
The anal retentive part of me would really love to see fuel consumption under various conditions :doah: :haha::haha:
Click to expand...

fill the tank, drive, then fill back up and divide total milage traveled by total gallons used:D:haha:

of course if its lifted with different tires the speedo is gona be off so the milage will be off anyways.

does it really matter? you cant really change it, it is what it is unfortunately:doah:
 
Spend some time on thirdgen.org's forums.

I *believe* some of the later OBD1 setups had the MPG setup, either Camaro or Corvette, if not more, so it's pretty likely that someone has already figured out how to do it using a stock ECM with TBI.
 
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