The Stovebolt Site Forums The Shop Area General Truck Talk Speaking of fuel mileage...........

Juat haven't stopped long enough to think about it and work
out the math, which by the way, I'm reallllll bad at.
But...I've kept a running record of every fill up
since I brought me truck home.
With different sized tires, different rear end gear ratios
it's bound to throw the speedo/odometer off by some factor.
It's easy enough to check how much with a GPS and in my
case it comes out to be right around 5 mph slower than
he actual speed.
In my mileage calculation I've always just used the odometer
reading without correction, so....it's really not that
accurate. Mostly I keep the record just so I can tell if
a mechanical problem seems to be headed my way. I've
never been all that concerned with the actual fuel economy.
I never would expect to get good mileage with a car/truck
that was dang near as old as I am, I mean my mileage is
down now a days also.
So the question that has been sitting on the shelf for
a while is, how would you make the correction for a
mpg calculation if your speedo read 5 mph slow??

DG

I have the same question, mine reads about 8 mph slow.

I have thought about moving the needle on the speedometer dial.

So your actually traveling 5mph faster than/at what indicated speed?
Calculate that % and apply it to increase your odometer reading.

I'm afraid the percentage method won't work.
5 mph error at 60 mph is 8% 5 mph at 25 mph is 20%

You just need to drive a measured mile and record the odometer error. If there aren't any "measured Mile" sign markers left around you, use mile markers on the main roads. I see that they are marked in tenths now so I would assume they are very accurate.
If you changed tire sizes over the years, you can get the mfg data for rolling diameter of each size you had and correct for the percentage difference to what you have now.

Still pretty sure the percentage will work, that's why I asked at what (exact) indicated speed is he 5mph off.

If he is also showing 5pmh at rest the speedo/odometer is suspect.
If it is working correctly except being driven at the wrong ratio given tire size and axle gears the percent error should work.

Measured mile is the better way to figure because the odometer is geared to the transmission output, no slip. The mph indicator is just an indicator, not as accurate as the miles and the thenths ticking a away.
But Denny didn't offer this info, or hasn't yet.

No, no, no! the speedo is just fine and dandy. Just as I
posted, different tires, different gear ratio. The 5 mph
error is pretty consistent over all of my average driving
speeds. It works at 65, works at 50, works at 35 and even
down to 15 or 20. All run pretty close to 5 mph off according
to the GPS. When I'm in a speed zone all I do is keep it about
5 under and it's close enough. Had the local copper shoot me a
few times with his speed trap in a 20 mph school zone and he
said I was within a mile per hour every time. I was holding
it at 15 indicated at the time.
The reading on the head should be fairly consistent just as it
would be if you changed the speedo gear in the transmission or
used an ratio adapter off the transmission for a correction.

Percentage wise....yep. the difference percentage wise of
5/15=.33% vs 5/50=.10%, etc. So....as Jim posted, I don't
think you can use the percentage as a constant for the distance.
If I'm indicating 55 mph and travel for one hour my odometer
will read 55 miles traveled. In reality I've traveled 60 miles.
So at this speed, I've traveled 5 miles further on my tank of
fuel than my calculations would indicate using a direct reading.
So using the additional miles traveled with a correction means
that the mileage that I got on what I thought was a 154 mile
trip in reality was closer to 170 miles which means my
mileage for the day was more like 14.5 mpg.

And this would account for the drop in mileage around the time
I rebuilt the engine. Just prior to that I had change tire
sizes and installed the 4.10 gear. Then just last summer I
changed tire sizes again, each time calculating my mileage
without a correction. And thinking my mileage had gotten
worse then it was when when I first got the truck.

Thanks for figuring that out for me guys.
I'm happy with 14.5 given the other problems that winter
driving can impose on fuel mileage.
dg

I'll bet this is more than anyone wants to read but...I checked things 3 different ways. Note the tires and rear end below.

First, I went past a neighborhood radar sign.
Speedo=15. Radar=18. Actual speed is 20% higher than speedo says.
Speedo=30. Radar=35. Actual speed is 17% higher.
Speedo=39. Radar=45. Actual speed is 15% higher.

Second, I timed how fast I went through a highway mile check.
Speedo = 40 MPH. 74.63 sec for the mile = 48 MPH. That's 20% higher than the speedo's 40 MPH.

Last, I calibrated the odometer at a highway mile check. 1 mile on the highway changed the odo 1.19 miles.

So, I'm saying I'm off by about 20%. I multiply the odo reading by 1.2 for figuring MPG. I keep in mind the discrepancies in speed, especially at the school zones.

I've done the GPS check a hand full of times. A few times with the GPS on my wife's cell phone and a few times with a handheld Garmin. They both told me the truck was travelling 10 mph faster than what the speedometer was reading i.e. a speedo reading of 50 had the GPS telling me I was running 60 mph. Of course this is sort of subjective with a speedo needle that is wavering ever so slightly.

Now, the Garmin also has a distance readout. So twice I compared my work commute distance (43 miles) with the GPS and the trucks odometer. To my surprise the odometer reading over the 43 miles agreed to with the GPS within a tenth of a mile each time.

I think of the speedometer error in terms of an offset, since the odometer seems to be pretty accurate. And I don't think I'd need to adjust my MPG numbers because the trucks odometer is accurate.

Denny, it would be interesting to know how your odometer readings compare to GPS readings.

I would not depend on the speedometer at all for judging whether or not the odometer is calibrated. Doing so is relying on, either a 60 year old hair spring, or a potentially inaccurate hair spring in a re-manufactured unit.
The odometer is what matters. It is driven directly by the cable.
Carl

The odometer is direct gear driven, the only thing that can
effect it's accuracy is a change in the rear gear ratio or
the tire diameter or a change in the driven gear at the transmission.

The needle on the other hand, is driven by a magnetic field
and can easily loose calibration from a dry bearing, a bad
bearing or damage to the air gap clearance if someone messed
with the head, the air gap sets the calibration.
A jerky needle is almost always caused by a worn or
sticking cable or cable housing.

dg

I usually use the chart at TCI or The Transmission Center(preferred). You plug in your tire size and rear end ratio and the chart gives which drive and driven gear to use.

In your first post you posed this question, "how would you make the correction for a mpg calculation if your speedo read 5 mph slow??"
Based on your most recent post, is there still a valid reason for determining a correction based on 5 MPH speedometer reading (irrespective of the odometer)?
If so, then I would suggest that, while the math equation can be formulated, the premise of the request may be invalid. Is it off by 5 MPH at all speeds? If that is the case, your speedometer is defective. If the inaccuracy were due to gears/tire size, then the error would be a precise percentage of error at any given speed.
Carl

WAAAAY over my head. I just divide miles driven by gallons used equals MPG.





Weeds

I agree with Carl, or he may agree with my earlier posts, I just couldn't say it as clearly as he just did.


Grigg

Here is how I interpret Denny's words:

"check how much with a GPS and in my case it [the GPS ?] comes out to be right around 5 mph slower than he [the] actual speed [speedometer readout]."

For example: the GPS reads 50mph and the speedometer (Denny's "actual speed") reads 55mph.

Excuse me here: the GPS is really the actual speed.

The GPS is the real/actual speed and the speedometer is the actual read out, which has to be re-calibrated.

If I am correct (and I probably am not), take off the needle and put it back on, pointing to -5mph.