After last weeks debate on fuel consumption I have done an accurate test this week end.
I was on a run with 6 or 7 other guys. The riding was through holiday traffic at some points and country lanes lots of the time. The speeds where around the 30-40 mark with a few thrashes at 50-55. I filled my tank before leaving home to the filler neck and re-filled it exactly the same on my return.
The distance covered (and my speedo is accurate)= 93 miles. The fuel used 5.25 litres (1.155 gallons) I make that 80mpg. Which is about the same as my calculation on last weeks 112 mile thrash.
Someone tell me if I've cocked up. Ron

email (option): ronpier@talk21.com

Ron, your calcs are spot on 1 gall =4.546 litres.
I have logged every tankful i have ever used and the running ave is 65.00 mpg..i got 80mpg once ..but the jet was so small it was spitting back all the time...70mpg is achievable on my G3L if i dont thrash it to much.

email (option): edd@abbottracing.com

Ron, you've got to come and tune my M20. I've never seen anything remotely like that kind of mpg.

I checked an old Motor Cycling road test, and they report 82mpg at 30mph, and 56mpg at 50mph. But Motor Cycling was notorious for flattering manufacturers with all kinds of nonsense numbers (and no one rides around at 30mph; not even way back when).

Sidevalves are famous for being inefficient. Thermal losses are huge. Gas flow is about as bad as it can get. And low compression engines are the complete opposite of what's needed if you want to maximise mpg.

Even a modern lean burn Royal Enfield with (technically superior) OHV returns only 70-72mpg (under steady riding conditions). But if you say your M20 is getting 80mpg, who can argue with that? Mine sees around 40mpg riding at around 40-45mph. Maybe a little more on a good day. Say, 45mpg. But NEVER more than 50mpg.

Not that I don't trust you. Far from it. I think you're as reliable as a hammer. But those numbers make no sense to me at all.

By the way, doesn't fuel expand in a hot petrol tank?

email (option): dannydefazio@sumpmagazine.com

Well I don't understand it either? I never ride my bike with fuel economy in mind. I know my speedo is accurate as far as speed camera's are concerned. I'm also quite sure that the mileometer is correct, based on the fact that I know from driving a truck or car, that Sammy Millers place is 15 miles from me and that's what my speedo reads when I go there.?? Ron

email (option): ronpier@talk21.com

'This is a two pipe problem, Watson'.. ...very strange...I've never got that out of my B33, never mind an M21...Ian

email (option): ian@wright52.plus.com

The only thing I can do now is a serious check on my mileometer. Although I'm convinced it's correct. I'll own up if I'm wrong. Ron

email (option): ronpier@talk21.com

Hello Ron, if you find that your speedometer/mileometer is incorrect (which I'm sure it isn't), can I buy it?

It would at least let me feel better about my mpg.



But seriously, if you work out why your bike is so economical, pass the word. As I said, I don't for one second doubt your integrity, but I've seen you ride your bike, and you ride it as hard as I ride mine - and yours carries a little more weight with luggage and Vokes and so on.

email (option): dannydefazio@sumpmagazine.com

Even if Rons milometer is a bit out, it should not affect the result by more than a few %, and I am sure his calculations are accurate.
So such a good result should be attributed to a well set up engine, spot on ignition and carburation, low friction losses through the transmision, and a light low drag rider

email (option): gasboy@btinternet .com

Hi Peter...you haven't met Ron then?... .If all of the above was correct then what would a well set up B31 be doing to the gallon? I'm hesitant to assume Ron is wrong but those figures are remarkable and not typical of the figures quoted for an M21...or any other sidevalve come to that.....Ian

email (option): ian@wright52.plus.com

Even with double-checking the odometer, etc., generally a one-fillup test will tend to be inaccurate, even if it is only by a few percent. Add a few percent on the odometer variation and it starts adding up. Generally you would want a test involving several fillups, say a few hundred miles, so that any errors at the pump cancel each other. Is it also possible that some are measuring in imperial gallons and others in US gallons? I've never seen more than 50 mpg on any M20 that I've ridden with any regularity and even that is very rare. In my experience 45 is more the norm - but then our gas is 10% ethanol or whatever they call the additive-du-jour.

email (option): jonny.rudge@verizon.net

Ron,

Are you using LPA?

Regards,
Leon

email (option): leonhop3@planet.nl

john i hope the error at the pumps you mention is in my favour i dont know what my M20 does to the gallon i just fill it up when needed and on all my vehicles for year i only buy my fuel by the pound ie £10/£20 worth that way you dont notice the prices going up so much now being old i forgot went the last time i filled up

Buying "by the pound" doesn't work so well for me. It used to be that I always filled up with dollars in the single digits. In the recent past it became always in the double digits. Now it's going up and down, depending on where I fill up etc. so it's noticeable. Thankfully, none of my bikes require the equivalent of twenty quid for a fillup, that's restricted to my wife's big car, as even my econobox doesn't have that much capacity at the current prices.

As for the error being in your favor, that's the problem, you just can't tell. Bike leaning slightly left, right, front, back, the pump popping a little prematurely or foam in the tank from the filling, means a little less or a little more gas getting into the tank. That's why the averaging method is necessary - the errors cancel out.

email (option): jonny.rudge@verizon.net

Well now Ron
I get 100 MPG in my WM20,everywhere I go is downhill,so I free-wheel a lot,when I want to get back home again,its only half the distance back,so if thats not nonsense I dont know what is!

email (option): glesgaman@tiscali.co.uk

What about different 'aspect ratios' of tyres?..ie the rolling diameter/circumference. A change here from original would affect the drive ratio of the speedo and the recorded miles... and therefore the consumption calculation. In the past the Police also accepted a 10% potential error in speedo readings, inferring a degree of innaccuracy there as well. Finally a 'visual' as opposed to a measured recording of fuel used may not be entirely accurate..Ian

email (option): ian@wright52.plus.com

I agree that Rons MPG figure is amazingly good, and there are variables that affect that figure. But say all those variables are 20% out on the happy side, that is still 60MPG.
Ron, let them scoff, I have faith in you and you have set the bar for us to all acheive

email (option): gasboy@btinternet .com

I have an open mind on the fuel consumption of Rons M21...The figure is certainly not what would be expected and therefore all possible variables that may affect the result need to be considered and quantified to establish an accurate, actual figure.
Personally I would rather see 80mph on the speedo, my M20s 80th. birthday and the 80th anniversary of D Day more than 80 mpg ...Ian

email (option): ian@wright52.plus.com

Ian--- I agree with what you say-- so long as mine starts and runs well I am more than happy with it. MPG is an accademic number and fuel consumption is just a very small part of the overall running cost-- but still an interesting discussion.

email (option): gasboy@btinternet .com

Well, if one of you is game enough to weld up their heads to increase compression then you certainly will gain some mph and mpg!

I totally agree with most of the comments made. For me the fuel costs of my M20 (600cc) are completely academic. I'm just intrigued at the figures I have come up with and am determined to get to the bottom of it. I have only done this on the last two runs and both times I came up with around 80 mpg. Taking into account the 3.50 tyres instead of 3.25 and speedo error, I'd still say 70 mpg. My next mission before another test is to follow my wife or friend in a modern car to get speedo comparisons. Ron

email (option): ronpier@talk21.com

Ron,,if you can borrow a hand held Sat Nav...the type walkers use..my wife gave me a Yellow Garmin one years ago,,,you can tape it to the fuel tank and check Speedo accuracy and Odometer accuracy....and lots of other useful things like altitude..only joking..

email (option): edd@abbottracing.com

hi,this is what you need to do like pratap henri from my earlier post( m20s abroad) a bit heath robinson but accurate, watch hi little video 1945 BSA M20 (Trial Run after 40+ years) just copy and paste, cheers rick

email (option): richardholt@rocketmail.com