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Thoughts on fuel economy calculations

Discussion in 'The Pub' started by gsxxer, Sep 20, 2010.

  1. So Ive been thinking about how much petrol my station wagon drinks. About 13.5L/100km or down in the 12's with ultra careful straight driving.

    It seems like a lot to me, but then again its speedo is very accurate against a GPS, so am I right in thinking that the distance meter also works off the same signal?

    On the other hand my blackbirds speedo is optimistic, by about 7%, so going off its distance meter, its fuel econ is probably worse - around 7L/100km rather than the 6-6.5 Im currently calculating.

    So I know my old tank of a car is not efficient, but just how bad is it compared to modern cars when you factor in the speedo?

    Ive driven foresters and outlanders and they return 9-9.5L/100km but I am thinking its closer to 10-10.5

    Any input? Any owners of newish commodores or falcons?
  2. what's the car? 13.5 seems a bit high for a newish wagon - I'd expect maybe 10 or so.

    the speedo on most modern cars is electronic. I can only speak definitely about Ford, but others should be similar. The speed signal is generated by counting pulses either from the ABS sensor or from a sensor on the gearbox output shaft. Knowing tyre sizes etc allows an accurate calculation of speed. Hence the PCM knows accurately how fas the car is going. this then sends a signal so the speed on on vehicle can bus. What is read out is dependant on how the instrument panel does any calculations, but i should be pretty close. It will certainly be a much more accurate reading than the older mechanical systems. I don't know what's fitted to the Blackbird.

    I believe that the often misquoted speedo tolerances are that legally a speedo is NOT allowed to underread, but may overread by up to 10%. In almost all cases it is more accurate than this (except on my Vstrom where it seems to exactly overread by 10%)

    the biggest variable in fuel economy calculations is the amount of fuel you add. The only reliable way to calculate is to brim the tank, drive, and reill at the same bowser to the same level.

    don't trust any in-car trip meter

    hope this help
  3. Oh my car isnt new, its a nissan stagea. Turbo, 1.7 tons, 4WD....so I can see what contributes to the fuel consumption.

    I was just curious as to how far fuel efficiency has really come with comparable vehicles in 10 years. An optimal running stagea should get down around 11L/100km

    I am thinking safety is the only thing to encourage me to upgrade my old car.
  4. +1000 And I agree, 13.5 litres per 100 is shit. My 4 litre dedicated gas AU series 3 Falcon will do about 9.6 litres per 100 ON GAS on a trip. And gas is supposed to be between 15 & 20% less efficient than petrol. Depending on what you read. The 9.6/100 was calculated by filling with gas, driving 700+ kms and then refilling by the way. Used to drive back and forward from Melbourne from Geelong every day for work and got consistent 10 litres per 100 also.
    Mind you, she drops to 14-15 litres per 100 just hacking around town for a week or so. So conditions make a big difference.
  5. Dont know what your all complaining about, my ute sucks 15.5/100 around town but on the hwy she gets a very smiley average of 9.5/100
    As for the VTR if I nanny the throttle 8.5-9/100 but if I run her hard that drops down to around 6.5/100 [ she runs better and leaner with the 48mm carbs sucking air properly ]
  6. It's worth remembering that tyre wear will make a small but measurable difference to your odo reading. Of the order of a couple of percent from new tyres to bald at least. Maybe a bit more if you own a 4WD with chunky tread.

    Coincidentally (and at risk of a threadjack), I've been conducting some experiments with the DR. My employer relocated to an office 15km further from my home. The previous commute allowed me to get exactly two days commuting, full to reserve. Not wishing to visit the forecourt more than once every two days, I've been seeing how far I can stretch a tankful. Given a slightly more open commuting route and a conscious effort to shortshift and use steady throttle openings in high gears, I've seen the full to reserve range go from ~190km to ~260km, and I'm not getting anywhere any slower. Something to think about next time fuel prices go through the roof.

    Needless to say, I'm impressed by both the bike and by my own self-control :D.
  7. I am on an international Blackbird specific forum and world wide the Blackbirds speedo is exactly 8 % over, GPS corrected. With new and correct size tyres.
    100 klms per hour on the speedo is 92 klms true speed,
    A copper who clocked me with a stationary hair dryer said I was doing 91 Klms, I was sitting on 100 kays as I knew he was waiting some where up ahead as he passed us on Mt Hotham.
    He actually complimented me on my speed. as I was well under the limit.
  8. While I can't comment specifically on the Blackbird, for the Triumph Daytona 675 the speedo is generally about 7% optimistic, but the odometer is pretty much bang on the mark. This was true of my old R1 too (but there the speedo was about 9% optimistic).

    I think what you'll find is that most manufacturers design in optimism for the speedo for purposes of giving riders/drivers a safe-ish buffer for the typical ~10% speed creep variations that are present in every day driving (ie. when watching the road conditions your speed can vary up and down by around 10%). The odometer however is generally very close to accurate for newish standard profile tyres for the vehicle in question.
  9. Owned a BA, currently own a forester and in and out of pool and hire cars a fair bit.

    13.5 does seem a bit high, though giving a new commodore a bit in traffic could well return that.

    Forrester would be closer to 10-10.5 as it weighs so much. Like the new Commodores, on the hwy at constant speed they return OK figures, but as soon as you try to make these relatively small engines accelerate these heavy cars, they start to return some ugly figures.

    In theory the Falc should be much worse, but because the engine is so relaxed it returns good and constant figures.

    I did a decent stint in a diesel Prado once and was shocked at how bad the figures were for a diesel. about 11.5 (from memory) on the hwy.

    The Aurion on the other hand is really quite good. (9.5 over mixed city and hwy and not shy on the right pedal)
  10. [QUOTE='[FLUX]I think what you'll find is that most manufacturers design in optimism for the speedo for purposes of giving riders/drivers a safe-ish buffer for the typical ~10% speed creep variations that are present in every day driving (ie. when watching the road conditions your speed can vary up and down by around 10%). The odometer however is generally very close to accurate for newish standard profile tyres for the vehicle in question.[/QUOTE]
    I don't know that it is so much a deliberate ploy, but rather a result of the hardware used. the sensor has a production tolerance, the wiring has resistance characteristics that will vary with time (age) and the speedo itself has a tolerance on the needle position. Since legislation requires the speedo never underreads, the system will be designed such that on worst case tolerances, and with crappy 10 year old cables it still works correctly. this means that with nominal performance parts it will overread.
  11. My 96 EL Fairmont with modifications managed between 7-8L/100km on the highway (on the way to Sydney). City driving is about 11L/100km...giving it a bit of stick might get me to 13L/100km.

    Motorcycle city riding only gave me about 8L/100km or so.

    So 13.5L/100km is a fair bit if you're driving it around normally, however age of car will play a role in it's economy too...may be getting tired.
  12. Really? That's v.poor. I'm disappointed 'cos I drove a Prado fairly extensively when I was working up north and found it so pleasant that I'd consider buying one myself.

    Oh well, back to HiLuxes.
  13. My odometer reads 250km less than my GPS after 34,000km: quite good enough to rely on the fuel calculations from the bike's computer (y)
  14. i just did a 600km run in a turbo diesel auto prado, 4 adults with overnight bags and sitting 'around' the posted limits. it has less than 1500km on the odo and i got 8.8l/100km according to the trip computer.

    wasnt too bad.
  15. i get around 16l/100 in my car (v6 commodore) that's all city and i never go easy on her. only drops down to around 10/100 on long trips. perils of owning an old car i guess...

    bike gets around 5/100 if i calculated right (math isn't my strong suit, 217km from 11l for any nerds out there). again, all city and i wrong its neck as much as you can wring a 250.....
  16. Thats a burn. I am trying to think of simple things to do to increase the efficiency without spending too much on a service, otherwise the cost/benefit doesnt make any sense if your spending $100's to save a few dollars every 100km.

    I think Ill ultrasonic injector cleaning a go if its not too expensive.
  17. So, how does any of that explain the digital speedometer (no cable/wiring resistance errors), and that the speedo is driven by a pulse feed coming from the front-sprocket (no resistances, it's a periodical pulse - and the speed is determined by counting the number of pulses in a certain time).

    When talking about the old analogue systems, I agree with your post. When talking about the more modern digital speedometer systems, there is no comparable age degradation error aspects.
  18. Doesn't mean there isn't an error to the display or a hysteresis in the system.
  19. As I implied earlier, a little moderation of the right foot combined with reading the road ahead will net pretty much anyone at least 10%, with no measurable increase in journey times. Costs nothing and is quite satisfying to do, smoothness being about the only vehicle control skill it's possible to meaningfully practice in urban traffic. Brake pads and tyres will see decent savings too.
  20. if you're really keen on saving fuel it can be done, on a 200km highway trip in a ford territory awd sitting on 100 ish and through some towns i got 7.1l/100km. not bad for a big ship.

    as PatB said, you really have to read the traffic ahead. very far ahead. and read the speed signs so you arent using the brakes. each time you brake it means you've wasted fuel.