How accurate/inaccurate is your mpg display??

sturge said:

One other thing to look at - is your temperature guage going reasonably quickly up to 3 bars and staying there regardless of speed? If it takes ages and/or drops down at speed, then your thermostat is likely stuck open, which will affect mpg. Fixing that gave me around an extra 8mpg (going from mid-30s up to mid-40s on reasonably sporty riding, more on easy highway cruising).

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Quote from post #1:

"............The coolant temp gauge reads a steady three bars after it gets warmed up (I read the threads on the failed stat causing poor mileage) and the brake rotors aren't hot."

So, yes, it seems to hold steady three bars (whatever temp that is) when it finally warms up. Ambient temps have been rather low when I've ridden it and it took several miles to get up there.​

Sorry, missed that. On the bright side, you don't have to pull it all apart to change it

The results can be skewed if the speedometer is not calibrated accurately. After I installed my Speedo-Healer, my MPG actually wen't up by 7.5%. The miles traveled were now accurate with my GPS. The difference between actual and indicated also got closer than before being calibrated.

wdupmults said:

The results can be skewed if the speedometer is not calibrated accurately. After I installed my Speedo-Healer, my MPG actually wen't up by 7.5%. The miles traveled were now accurate with my GPS. The difference between actual and indicated also got closer than before being calibrated.

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Did you correct the odometer and speedometer with one calibration change? On US market ST1300s the error is not the same for both so usually only one can be corrected to no error and the other is still off.

wdupmults said:

The results can be skewed if the speedometer is not calibrated accurately. After I installed my Speedo-Healer, my MPG actually wen't up by 7.5%. The miles traveled were now accurate with my GPS. The difference between actual and indicated also got closer than before being calibrated.

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Generally US bikes have reasonably accurate odometers, because the allowable error is governed by a DoT standard, whereas the speedo is not. If your speedo was off, I presume it was reading too fast, so if anything by recalibrating the speedo to read slower, your distance traveled would have decreased by that same percentage. Not seeing how that would increase your MPG, seems like it would decrease it. Perhaps you are running non-standard tires (or darkside) and the speedo/odo were both reading too low before recalibrating?

A bit of an update with a fair degree of "wait and see" until more fuel ups and further averages.
After the first fueling after a prior full tank the display read 37.7 and the "actual" miles/gal worked out to 40.48 mpg. About 3 mpg better than the display if the fill up was accurate either time, although I filled to the ring on the centerstand both times. Lots of short trips and cool temps, which really seem to make a big difference on the ST1300. Its a bit of a fuel hog during warmup and in the sub 50* temps it takes about 7-8 miles of riding to reach "normal" 3 bars coolant temp. Warmed up and longer trips would see the tank average go over 40 indicated, only to plummet the next short cold trip.
So, it is what it is, until the season warms up and I quit riding it to the current job. Warmer temps and longer rides will get the average up a bit.
Question is, will the spread between display and actual remain constant, get closer or worse? I'll find out soon enough!

On my latest trip the instrument said 46.2 mpg over the entire distance. Arithmetic said 48.x. Side stand fill to the ring each time.

The numbers included a fair amount of interstate at (generally) 75+/-, a large amount of rural back roads at 50-something, and a bit of twisty at varying speeds, keeping rpm in the 3000-3250 range in anything between second and fourth gears. Lots of up and down in those sections both in terms of gear and also elevation change.

It does seem to prefer warmer temperatures.

jettawreck said:

Its a bit of a fuel hog during warmup and in the sub 50* temps it takes about 7-8 miles of riding to reach "normal" 3 bars coolant temp.

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That still has thermostat written all over it. There is no direct connection between the coolant temperature sensor and the gauge, and my theory is that the ECM firmware has a lot of hysteresis built in because the engine temperature has to rise or drop considerably to make it budge from three bars. Mine used to get there with an open 'stat, and you'd see it fluctuate down to two at freeway speeds with temperatures just above freezing. Behind the scenes, the ECM will run a rich mixture and fuel economy will drop, even in warmer weather.

If you're not sure whether your bike has the original thermostat, the only way to know for sure what state it's in is to remove and inspect it. This was a very common condition on early bikes, and that's where my money is.

--Mark

Blrfl said:

That still has thermostat written all over it. There is no direct connection between the coolant temperature sensor and the gauge, and my theory is that the ECM firmware has a lot of hysteresis built in because the engine temperature has to rise or drop considerably to make it budge from three bars. Mine used to get there with an open 'stat, and you'd see it fluctuate down to two at freeway speeds with temperatures just above freezing. Behind the scenes, the ECM will run a rich mixture and fuel economy will drop, even in warmer weather.

If you're not sure whether your bike has the original thermostat, the only way to know for sure what state it's in is to remove and inspect it. This was a very common condition on early bikes, and that's where my money is.

--Mark

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I tend to agree with you given it takes as long to reach three indicated temp bars as it does.
I need to research the stat change procedure. I'm sure it's probably more plastic removal than mechanical.
I will no doubt realistically not get around to changing the stat until the riding season up here (or at least my riding) is over.
I have more pressing projects for summer and it's running fine, even though using more fuel than it needs to.
On my VWs I run a digital gauge system (Scan Gauge ll by Linear Logic) thru the OBDll port that besides being a great trip computer system also displays dozens of data points. I always monitor coolant temp and at least three other items per screen. Wish there was something easy to do likewise with the bike, or maybe there is?

Part way thru an engine swap (VW TDI into a gas engine chassis) that has turned into a wiring nightmare and it would be nice to get that done and out of the garage.
Then, if/when we close on selling a rental property, a complete long distance house/garage clean out.
Boat to get ready for the water.
Then, hopefully, there is also golf to play. Maybe lots of golf if the elbow ever gets better.

I'm with Mark on this one. It usually only takes my 2004 STeed about 2 miles to reach three bars in some pretty cold weather (under 40 deg F). Obviously some of it depends on how long you let your bike warm up. I usually take off after the idle drops down and I'm on the first bar. 7-8 miles is just too long. I suspect a partially stuck open stat.

Really not too difficult of a job if you're used to removing the plastic. All you have to do then is just remove the radiator to access the stat housing. If you haven't changed out your coolant in a couple of years it would be a good time to kill two birds with one stone...

Not that it matters much, but here is what I have observed on my 2004 over the course of a few months.
The display mpg reports about 3 mpg less than I usually actually get as documented by miles driven and gallons used. Fills are as consistent as I can get it. On the centerstand up to the ring below the cap. Average over the last 7 tanks is 48.52 mpg. The display usually reads 45-46 mpg at fill up time. 280-300 miles to the "miles remaining" flashing one bar on the gauge.

the last tank was my longest-347 miles and it took 7.358 gal to fill. Not my best mpg tank, but close to an average one. Also learned that the "miles remaining" display once it drops below 20 instantly goes to 0. The best mileage tank was 51.13 mpg, so far, although with the weather cooling the mileage goes down. The ST likes it hot. After a good road trip the slow warm ups ceased and for whatever reason(s) it now warms up to three bars within a few minutes, even without a pre-warmup.

jettawreck said:

...the last tank was my longest-347 miles and it took 7.358 gal to fill....

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You were probably minutes from walking ... There are numerous reports from others of being able to put in only about 7.3 gallons after they've run it dry.

Is your mileage measured with the odometer,,, or via gps ?? Reason I ask,,, is that few honda odometers are accurate,,, same as the speedo is usually out by 3-5 percent. If you are calculating your consumption using your odometer,, the result will be affected. Try using your gps and the refill method you described is fine providing you refuel after using a decent portion of your tank. Cat'

BakerBoy said:

You were probably minutes from walking ... There are numerous reports from others of being able to put in only about 7.3 gallons after they've run it dry.

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Yep, I was sort of pushing it a bit, perhaps out of neccesity. Knew it was going to be close and don't/won't make a habit of it but like to know the range of most of my vehicles. The VW (2) TDIs I have are capable of 900 miles tanks if things go good. Never ran one of them out yet either, but been very close. It's a bit of a "procedure" to get them re-primed and started and hard on the injection pump.

Catmandu2 said:

Is your mileage measured with the odometer,,, or via gps ?? Reason I ask,,, is that few honda odometers are accurate,,, same as the speedo is usually out by 3-5 percent. If you are calculating your consumption using your odometer,, the result will be affected. Try using your gps and the refill method you described is fine providing you refuel after using a decent portion of your tank. Cat'

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speedo is off the typical 3-4 mph at 60 mph, but the odometer racks up the miles pretty close to actual per the old Garmin and known mile markers over a distance. That can/will probably change depending on tire wear and selection.

jettawreck said:

speedo is off the typical 3-4 mph at 60 mph, but the odometer racks up the miles pretty close to actual per the old Garmin and known mile markers over a distance. That can/will probably change depending on tire wear and selection.

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Yes,, it varies from bike to bike. I just got back from an 850km trip,,, and the two (odo vs gps) disagreed by about 25km. Cat'

Last fill up the gauge showed 39.7 average. Actual was 43.49. It will start going down as the weather gets cooler.

Funny story. I was filling up at Costco when a fellow drove up and started filling his blue beast, the same as mine. I asked him just out of curiosity what his range was. His reply...I get 508 KMS out or her...I needed 512. We both laughed. True story

My speedo and odo are both off by approx 4%,, and I expect that the mileage computer is affected the same way. If it was just the speedo that was out,, I would not be to concerned. But now I am considering a Speedohealer to correct all three read outs. It is kind of like my headlight aiming motor,,, which gave up recently. I hate having things on the bike that don't work,,, Cat'