Just curious if anyone cross-checks the economy that the computer tells you that you're getting?

Accuracy?

Thanks!

Regards - Randy




If there's one thing I've learned in online forums its this:
Never argue with an idiot, they'll drag you down to their level and beat you with experience!

Yeah there has been a lot of discussion on that, check the threads.

Basically your speedo is off 2 to 3 mph at 70 mph. However that does not affect the FE readings because it seems that the odometer is also off by 1.5% and not in the same direction as the speedo (go figure?)

I track the FE by indication, by manual calculation and by a corrected odometer reading.

The Computer indicator is actually more accurate (or less error) than a manual calculation.

Currently after 13,700 miles I have these approximate calculations:

FE by computer 39.4
FE by manual calculation 38.6
FE by corrected odometer readings 39.2

The odometer has been checked by several people (myself included) using GPS and is off by 1.5%.

How are you able to determine that? I'm assuming that the fuel pump is pretty accurate. The calculated mileage is based on a number of small instantaneous fuel measurements. Since each measurement is going to have some degree of error it's more likely that these will add up to a greater error than what you'll get from a fuel pump dispensing 14 gallons at a time.

I also find that my measured mileage is about 1 mpg less than the TCH calculates, but I believe the car's calculation to be less accurate.

I have a quick question: in those states with strict emission controls, is it possible for gas pump's evaporative system to suck the fuel back up, if you overfill your tank? Just about anybody I see filling up the tank, they will keep on overfilling the tank long after the pump kicks out. I always wonder if all that extra fuel really goes inside tank, or is it slowly being sucked back into the pump, avoiding spill out? And if that's the case, wouldn't some people be off, just for that reason?

I saw the same thing. Originally, I thought the computer was high by about 1/2 MPG. But after I correct for an odometer off by 1.4%(my best estimate over a 10 mile measured distance), the adjusted manual calculation is just about even with the number on the nav screen at fillup time.

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Thanks for all the replies..

I'm curious about the speedometer and odometer error. I know that in my 3/4 ton Avalanche, my indicated speed was almost 3mph higher than the GPS Nav speed until I bumped the tire pressure from 32# to 45# - After the pressure adjustment, the MPH was less than .5mph off.. Now these are some pretty huge tires and certainly not low rolling resistance.
I'm wondering how these LRR tires "measure up" with different pressures?

Regards - Randy




If there's one thing I've learned in online forums its this:
Never argue with an idiot, they'll drag you down to their level and beat you with experience!

The "speedometer error" is not actually an error, it is off by design. I also checked it against my GPS and it seems to be right on the money up to 10 MPH and then it is somewhere between 2 and 3 miles too high, pretty much regardless of speed (up to about 95 MPH, that's the max I drove it so far). This seems to me that the "error" is artificial, otherwise if you had 2 mph error at 25 miles an hour, then it should increase to 4 mph when going 50, shouldn't it? I think this is per some agreement. Yes, higher tire pressure will lower the discrepancy, on the other hand since I know the speedometer is shy by about 2-3 miles per hour, I always take it into account when driving and problem is solved. I wonder if odometer is not calibrated in such a way to account for tire thread wear? Maybe it shows less miles now, but by the time the tire is worn out, it will be showing more than actual and at the end averages out? Of course the speed "error" will only get bigger as the tire wears out.