dojj

iTrader: (3)

seems like the maths peeps are on form with their calculators in their noggins today so here's my one

drove the car ther other day and, on the trip to the place i was going to, the trip computer read:
108 miles distance covered
33 mpg
average speed 60 mph

i drove back along the same roads and when i got back home it said:
216 miles distance covered
30.9 mpg
average speed 38 mph

so the question is this:
what were the return trip's figures had i remembers to reset the trip computer then?

and wh does it use more fuel driving slower?

AlexF

slower average speed is more stop start driving - hence less mpg.

rog

iTrader: (2)

I woud say your difference in speed on the return journey means you have been accelerating and decelerating, i would assume there was more traffic??? Explains difference in MPG.
MPG is just an average so it ould be 28.8 MPG on the way back if i am correct.

33 - 30.9 = 2.1

30.9 is the average of the two journeys, so you subtract the 2.1 from the 30.9 also, which gives 28.8 MPG

rog

iTrader: (2)

33 + 28.8 / 2 = 30.9

dojj

iTrader: (3)

ok then, how does that work with the speed then?

it took roughly 2 hors to get there and nearly 3 hours to get back, using your mpg logic, i can't have been doing 16 mph could i as that wold have taken me ages to get back

rog

iTrader: (2)

The simple answer is, it doesn't.....
Speed is not part of the eqauation for MPG is calculated by how much fuel you have used over a distance covered, but you engine is more efficient at the higher speed on the outward journey so it uses less fuel to get there.
As for average speed, you HAVE to have been going slower than 38 mph for enough time/distance for it to register 38 as the AVERAGE, it wouldn't drop from 60 to 38 if you were just doing 38 all the time without zeroing it? Fuck knows....

alistairolsen

it took you an hour and 48 minutes to get there (108 miles at an average of 60mpg = 1.8 hours)

It took you 5 hours and 39 minutes for the complete journey (216 miles at an average of 38mph = 5.68hours)

therefore the return journey took you 3 hour 51 minuted for the return journey!

And yes, your average speed for the way back was 16mph which must have been painful!

Your fuel consumption is not a function of average speed, its a function of speed an acceleration. Your car WILL use more fuel at 90 than it will at 40 if youre at a steady speed, but it will use a lot more fuel accelerating from 0-60 and then braking than it would cruising at constant 30.

dojj

iTrader: (3)

i left at just after 9am from my house, went along the M4, round the M25, along the M26 and to the end of the M20
i got there at roughly 11am
i spent roughly another hour there doing what i was doing
then, at just after 12 midday, i returned, along the M20, along the M26, round the M25 and into london on the M4 to get home for just before 3pm

so how is my car telling me it's taken me an hour longer to come back than it actualy took?

or should i just regard the trip comp as being broken

fuzzy

all downhill on the way and uphill on the way back? or you stopped at a cafe for a greasy fry up and a read of the paper and forgot to turn off your engine ?

Chip

iTrader: (3)

108 miles distance covered
33 mpg
average speed 60 mph

time taken : 108mins
fuel used : 3.27 gallons




216 miles distance covered
30.9 mpg
average speed 38 mph
time take : 341 minutes
fuel used : 6.99 gallons


So for 2nd trip:
Distance: 216 - 108 = 108
Time: 341 - 108 = 223
Fuel: 6.99 - 3.27 = 3.72

therefore mpg = 108/3.72 = 29mpg
average speed = 108 * 60 / 223 = 29 mph

dojj

iTrader: (3)

your maths is better that the computers chip

(it's also better than mine as i'm a little confused as to what it all actually means but i'll wait till later for you to reply )

jammerrs

maybe...you ate burgers for that hour, therefore on the return trip the weight in the car was alot higher and so mpg drops,

i got an A btw in my maths

dojj

iTrader: (3)

i was chopping up an exhaust for that hour so deffo no burgers

there was lots and lots of traffic on the way back though, i got caught up for absolutly ages on the 25 through clackets and then from around junction 8 onwards to 15

jammerrs

maybe you put all the exhaust in the car

dojj

iTrader: (3)

i spent ages chopping it up to fit in the car

st3v3

fuel consuption is gonna be RPM related and gearing related, an engine will work better at 2500rpm in 5th gear compared to 1200 in forth gear, thus become more efficient at 2500rpm in 5th because of the gearing with that making more speed
That why manufactures say xx mpg at xxxx in x gear, or they used to back in my day.

jammerrs

well it started as a big exhaust from a...hmmm cant think of a big car, but it did

dojj

iTrader: (3)

it's an auto mate and i set the cruise to sit at 70, same as i did on the way there, obviousluy there is osemthing wrong with either the figures or the space/time continum

the tank holds 13 odd gallons of fuel when full and there was more than half a tank left when i got home, so if i've done 30 odd mpg then i would have used the thick end of 7 gallons and more than half a tank of fuel

it's exciting this figuring out malarkey isn't it, i might apply to join the sherlock holmes fan club