Overstated MPG?

[Upright]

Hi All

If we agree that the speedo is overstating the speed then do we think that the odo is overstating the distance???????????

The reason I ask is I have just completed (I'll do this in km's and miles - I'm determined to convert yiz all to the better system) 525kms (325miles) from brimfull, dripping out of the overflow pipe onto the ground. I then put did the distance and put 21litres to full again.

I did my own calculations and then re did them using internet calculators. Both methods tell me this is 70.28 imperial MPG.

I chuffed as sh1t about this coz the longest trip is 33kms (20 miles)with engine cold at start all the time.
Now the details of this. Firstly this was not "spirited" riding. Great weather here, Never really over 90kms per hour (say 55mph), never over 4500rpm.Engine up to temp very quickly Lots of looking ahead and coasting to a stop. I do live in a rural hilly area so I did reach 4500rpm a lot and then parking and traffic at work so lots of clutch slipping so revs a bits.

Front oem trailwing 36psi and rear oem trailwing 41psi. Very square rear so don't know if maunfactures take into account acutaul circumference when it comes to gear. Stock chain and sprockets.  13oookms (7800miles) on every - bike, tyres, drive train, suspension. 2500kms (1500mles) on halfords 15W40 fully synth. Stock screen on madstad at fully down fully back. No engine bars. Givi 55 maxia and givi e21's on sw motech evo racks. forks drop 7mm,  barkbuster hand guards preload to the max front and back. I'm 13.5stone and 5 foot 9a dn a half.

So am I deluding myself with 70.28 MPG? Has anyone a gps ( I ain't got one) way if doing this? Is my life in need of something else?

[Juvecu]

The speedo is out, but the odo is accurate by all accounts. On my Wee the speedo is out about 8% and this seems to be the norm for the older Wees. I'm not sure if the Glee is roughly the same out on the speedo, but pretty sure the odo will be accurate.

Tyre pressure on the front is quite high, particularly for the hot weather. I'd recommend dropping it to 32psi and having the rear on 37 psi. You are only a stone heavier than myself and these pressures work for me pretty much all year round (might lift them by 1 psi front and rear in the dead of winter when the roads are very cold.)

[greywolf]

The speedo is about 8% happy and the odo about 2% on Wees. My Glee in stock condition has a 7.2% high speedo and a 2.7% high odometer.

[Juvecu]

How are you measuring the odo accuracy?

[greywolf]

How are you measuring the odo accuracy?

I compared it to mile markers posted on an Interstate highway. IIRC, the check run was for a distance of 37 miles.

[Andy M]

Same sort of numbers here measured against GPS and the local measured mile (Bingley).

About ten years ago I had access to one of the UK official measured miles (MIRA). GPS with five satellites showing was accurate to under 1%.

The speedo vs odo is a tolerance issue. The EC standard is -0 +10% for speed readings to make you "safer" in a speed limit while the ODO standard is +/- (can't remember the number) as that is there for service needs and to help determine sale value. Set both to middle limit and they differ.

There are multiple variations in MPG measurement. The petrol pump is accurate to about 0.5% but there is an unknown density variance with temperature (more bang to your gallon from cold petrol as there are more molecule chains for you to react with the air). Filling to full by eye will be the biggest variance. As a saddo currently working on such things I have done Statistical Process Control on my MPG measurement. Cpk, a measure of the measuring system is about 0.8, well under any acceptable engineering standard. If a micrometer got Cpk 0.8 I'd send it for scrap or kick the inspector who did the R&R.

Your 70 mpg is between 60 and 80. Be happy, don't worry, calculate range via the lower figure.  

Andy

[Jacko]

70mpg is perfectly achievable. As said, odo has to be accurate and isn't optimistic like the speedo. Trust your odo and litres used. Use the readout as a guide, in my experience its very accurate, usually with 1mpg to measured.

[Fatbelly]

Surely when talking about such extreme accuracy the amount of wear on the tyres makes a difference?

[Andy M]

Surely when talking about such extreme accuracy the amount of wear on the tyres makes a difference?

Fuel economy will fall as the tyre radius falls and the engine revs faster. 2 lots of 5 mm tread on a 17-inch tyre make a whopping 30mm or so difference on the circumference or 2-3%. Tyre pressure (low pressure = smaller circumference = higher revs).

Won't change the accurancy of the measurement though, an accurate plot will just slope and show you the tyre wear.

Andy

[Fatbelly]

But doesn't the speedo count how fast the wheel turns?

[greywolf]

More rpms do not necessarily translate to poorer fuel economy. The gain in mechanical advantage of the smaller wheel diameter plus the higher horsepower at the higher rpm can negate the effect of more power strokes per mile. I once tried a 16T front sprocket on my old Wee in place of the 15T stock sprocket.  I was seeing a drop of about 500rpm at comparable 6th gear speeds but noticed no change in fuel economy. Others who have used sprockets from 14T to 17T have also reported no change.

[Keith Cross]

Why are we talking about the rear tyre, thought the 650 speedo was driven from the front tyre.  Well my 2010 model is.

Keith C

[Oop North John]

Why are we talking about the rear tyre, thought the 650 speedo was driven from the front tyre.  Well my 2010 model is.

New wee speedo is driven a reading off the front sprocket.

[greywolf]

Wee - front wheel. Vee and Glee - front sprocket shaft.