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Old 11-18-2012, 03:07 AM   #1 (permalink)
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Default trip meter/fuel mileage discrepancy?

Have been doing a massive 20 day assignment in every direction of the island province I live in, and of course am keeping tabs on how much I spend on fuel. I have trip meter no1 set up to run over the whole 20 days, and trip meter two for each time I refuel to check on the mileage I get. I also once in a while check and see what my average mileage is (actually in kilometers per liter) on the dash read out.

With the Zard exhaust installed and a Bazzaz (commander type thingey) installed and tuned I was hoping to get a decent 20km/l (with stock exhaust I was getting up to 21). But it looks like I now average something like 18 km to a liter. Yet the fuel mileage indicator is telling me I am getting 20.5 . I was discussing this with my daughter's boyfriend who studied industrial design, and he reckons that because I have put on larger than stock tyres at the back that the trip meter is underestimating the distance as the tyre circumference is slightly larger than stock, and therefore I am doing slightly more kilometers than the trip meter reckons. But if that is so why would the readout for my average mileage per liter be so different? After all it's the same mechanism feeding the trip meter isn't it?
Anyone out there who knows about this?
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Old 11-18-2012, 03:20 AM   #2 (permalink)
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Old 11-18-2012, 07:46 AM   #3 (permalink)
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That would be a given. But tyre size messing with the trip meter's (relative) accuracy?
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Old 11-18-2012, 09:58 AM   #4 (permalink)
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Your speedo likely reads 3-5% fast. So ar any rpm the Cpu thinks you're going further than you actually are. If you want accuracy use a gps to determine distance travelled and always fill the tank full. Your numbers and the bikes numbers are both estimates.
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Old 11-18-2012, 10:46 AM   #5 (permalink)
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get a speedoDRD and set it based on gps and you'll never have a question on whether or not your speedo is accurate again.
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Old 11-19-2012, 07:13 AM   #6 (permalink)
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Speedo reads about 7% fast on the stock bike, but the bigger tyre (which alters the gearing) will reduce that degree of error. But...

The speedo error is deliberate and built into the display. The odo and trip are as accurate as the manufacturer can make them, so if anything they're probably underestimating the distance covered because of your bigger tyre.

That said, I doubt if your tyre has a very big effect, perhaps about 3%, and I did say 'as accurate as the manufacturer can make them'. That isn't as accurate as you might like to think lol.

The average mpg from the trip computer is never accurate. The bike just isn't very good at measuring the amount of fuel used. I suspect that there is also a deliberate error built in. Every vehicle I've ever had has had a trip computer that's read about 10% better on fuel consumption than the real fuel consumption.

As posted, for an accurate mpg figure, use a GPS to record your mileage and be aware that that's probably only accurate to around 2 or 3%, but as it can both under read and over read it tends to balance out over a distance. Gps or trip, record the amount of fuel you actually put in to the tank and calculate your mpg yourself.

Why does the tyre make a difference? Because the sped sensor measures the rpm of the front sprocket. Tfe speedo calculates speed by knowing how far the bike moves for each revolution of the sprocket. If you change the gearing or the tyre size it moves a different distance from that programmed in to the ecu.

One other thing.

Because the speedo reads high and the odo doesn't with the bike in stock trim, if you use a speedo healer to correct the speedo you'll throw the odo and trip out.

Rob

Last edited by williamr; 11-19-2012 at 07:22 AM.
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Old 11-19-2012, 07:45 AM   #7 (permalink)
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I always notice a difference in speedometer error as my tire wears. By the time its worn out it is 2-5 mph off when compared to a new (and slightly taller) tire.

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