tripminder accuracy?

[gadget73]

Just a question for you folks who have them, both factory install and self-installed. How accurate is your average fuel mileage and fuel used vs the miles / gallons at fillup? I'm curious because consistantly the tripminder tells me better things than my calculator does. Its also off by about 10-15% on the gallons used. Tonight for example I put 15.7 gallons in the tank, while the tripminder said 14.2 gallons used. This showed 16.8 MPG by the tripminder's average economy, and 15.3 by my calculator. I checked for fuel leaks with the engine running. Not a wet spot anywhere on the frame that I could see, no leaks at the injectors, or really anything else that seems wrong. The injectors are not old, and before I installed them, I wasn't really doing any different for fuel economy. Last I checked the plugs, none of them were excessivly wet though it has been a while since I last pulled and inspected them. Is it maybe just my tripminder install because it wasn't factory, or is something odd going on here? I was considering maybe it has a bad ground or something, but I don't know. Mileage figures are dead on, usually in 250 miles it will show about 1/4 mile difference between the tripminder and the mechanical trip odometer.

[PICKUP6772]

It depends for some strange reason. When I wrecked my 89 CV, I took out the tripminder, and put it in my 89 GM. In the CV , the mileage was off, but the gallons where on par, but in the GM the mileage is dead on, but the gallons are off, thankfully in my favor. When I say mileage BTW, I mean what the tripminder reads out compared to the trip odometer, on the dash, not MPG.
Mike

[turbo2256b]

Things noticed with my trip minder install.

Total Miles driven is accurate. The service manual states something like 2 or 3 miles differance allowed dont remember over what distance.

Ricent short trip at 70 / 80 showed instant economy readings at 14.6 up and down hills more or less. Calculated showed 17.01.

Long trip with ported lopo upper and HO throttle body showed about the same. Calculated 18.45 in windy conditions includes 2 traffic jams creeped along for about 1/2 hour each or better.

Gallons used dosent work well at all. Possibly recieving intermitit signal.

Average gallons way off. Calculated off gallons used so expected that.

Clock works good.

Plugged in another tripminder it showed clock and reset push any button nothing couldnt even reset. Popped it open to see whats inside.

In the car the tripminder came from the factory instant economy always showed low to calculated. According to service manual the ECM is programed to the tripminder or viseversa. Big ssection on diagnostics for it.

[gadget73]

Hm so the tripminders are specific to the ECM then? That could be the problem. I'm using a Crown Vic tripminder in a Town Car with a Mark VII ECM. I know there is an adjustment screw on the side but I don't know what its all about. Which manual has the info, the EVTM or the regular service manual?

[turbo2256b]

Some other thoughts I had were how does it figure eeconomy. Were is the flow meter and how does it figure return flow. In a marine flow meterr showing gallons usedd with a return system you have to haave a special system compaired to a non return system.
Figure it nust calculate it from gallons used and a something charted in the tripminder or ECM. Started me thinking if one increased the efficency of the engine enabling it to burn less fuel would you fall off there charted values and make the acccuracy fall off.

What do your tripminders read steady state at 30, 40 , 50 , 60, etc? Mine run from like 5 to 14.7

[turbo2256b]

Hm so the tripminders are specific to the ECM then? That could be the problem. I'm using a Crown Vic tripminder in a Town Car with a Mark VII ECM. I know there is an adjustment screw on the side but I don't know what its all about. Which manual has the info, the EVTM or the regular service manual?

Manual A Body chassis electrical.

So far though mine is more accurate on creamy than the doner car.
Dont figure.

[gadget73]

I think the flow data is based on injector pulse width on these, or at least I'd assume that since the flow data comes from the ECM, not from any in-line sensor. There isn't anything in the fuel line to directly calculate flow. It would probably assume a certain fuel pressure though, so I'd imagine changes to fuel pressure would affect the accuracy. If not for the fact I distinctly remember setting my fuel pressure to stock, I'd think that might be related. I'll have to double check the pressure though sometime.

Mine reads about 16-17 at steady cruise ~70 mph. It shows what seems to be reasonable figures, and the average shown seem to jive with the instant economy, but overall its just off. If I calculate the tripminder's total gallons used over the distance travelled, the average number is dead on, but since its not showing the right gallons used, its sort of irrelevant.


I'll go have a look at my BCE manual. Was hoping it was that book and not the EVTM, since I dont have a Crown Vic EVTM. Vics are covered in my BCE though.

[gadget73]

Hm, further investigation does show a flow sensor for non-CFI models (the 86 book is screwed up, thinks its CFI or nothing) so that lends support to the idea of a flow sensor on carb cars. Must find one of these mystery beasts and see if that flow sensor is useable on non-original carbs.

[Tiggie]

Mine's fairly accurate.

Gallons used is always right, when I can get a gas pump that allows me to fill it up without it kicking back.

MPG usually matches my odometer readings, though the 6mph speedometer error throws things off.

Distance traveled always matches my odometer readings to the wire.

Heh, mine is so accurate, but we have yet to figure out how it works...