Page 14 of 15

Re: My new project

Posted: Fri Aug 05, 2022 7:27 am
by maxferreira
I’ve even calculated it using maths! Same results

I’m quite unsure how it’s managing it, but I can’t complain, rather impresssed I must admit

Re: My new project

Posted: Sat Aug 06, 2022 8:11 am
by Dieselman
On a run the auto can theoretically give greater mpg than a manual due to having full torque converter lockup in 3rd and 4th gears and the final ratio being slightly taller than the manual gearbox.
Manual gearbox gives 27.2mph/1000 revs
Auto gearbox gives 28 mph/1000 revs.

Given the Xm has a low drag coefficient (0.28), then that small increase of final drive is apparent when the gearbox is fully locked up.

During regular driving the auto gearbox suffers greater losses due to pumping the Atf taking energy and the torque converter slipping.

Re: My new project

Posted: Sat Aug 06, 2022 4:37 pm
by Dieselman
Your mpg result has prompted me to look up some official figures.

2.1 td.
Urban
Manual = 35.8
Auto = 29.1
23% difference manual to auto.

Constant 56 mph
Manual = 57.6
Auto = 52.3
10% difference manual to auto.

Constant 75 mph
Manual = 43.5
Auto = 39.8
11% difference manual to auto.

Drag coefficient for the 2.1Td is 0.29Cd and the frontal area is 22.7 Sq-Ft.

The 2.0i has the drag coefficient of 0.28Cd. The only change I can see is the tyre aspect ratio is slightly lower profile.

The longer final drive ratio never fully compensates for the additional energy losses of the auto gearbox.

Re: My new project

Posted: Sat Aug 06, 2022 7:16 pm
by maxferreira
interesting.....

it's got me rather interested in how mine is being so staggeringly good on fuel

i wasn't hanging around either, but yet it's been calculate with filling at half a tank and working out fuel consumed and miles covered

i'm not going to complain, i seem to get around 500 miles to half a tank, plus the guage is quite accurate

Re: My new project

Posted: Sat Aug 06, 2022 8:51 pm
by xantia_v6
Is it accurately measuring distance traveled?

Re: My new project

Posted: Sun Aug 07, 2022 8:30 am
by Dieselman
xantia_v6 wrote:
Sat Aug 06, 2022 8:51 pm
Is it accurately measuring distance traveled?
The only way I can see the mileometer over-reading is if the drive gear at the gearbox has been swapped for a different one.
iirc, the 2.1td uses a green drive gear.

Re: My new project

Posted: Sun Aug 07, 2022 1:40 pm
by Julianw
maxferreira wrote:
Sat Aug 06, 2022 7:16 pm

... i seem to get around 500 miles to half a tank...
:o

Re: My new project

Posted: Sun Aug 07, 2022 8:18 pm
by maxferreira
Surely it has to be?

But admittedly it does seem to last a very suspicious amount of time, certainly I’ve apparently done 200 or so miles and it’s just above 3/4 tank…..I’d say I’ve done around that too

Re: My new project

Posted: Mon Aug 08, 2022 7:00 am
by Dieselman
The first quarter of a tank takes longer to register due to the filler pipe adding to the volume.

Just keep logging your brim to brim mileage and fuel quantity and any inaccuracy will balance out.

Re: My new project

Posted: Mon Aug 08, 2022 10:38 pm
by Julianw
maxferreira wrote:
Sun Aug 07, 2022 8:18 pm
Surely it has to be?

But admittedly it does seem to last a very suspicious amount of time, certainly I’ve apparently done 200 or so miles and it’s just above 3/4 tank…..I’d say I’ve done around that too
I may have used the wrong emoji. I was trying to convey shock, but not disbelief. My relatively short period of XM use led me to believe there were lots of 'free' miles to be eaked out of a tank, and i can't find it now, but iirc an owner (dollywobbler?) on another forum got near to 1000 miles from a tank on a 2.1td XM... Whatever, i hope it can be true, but I'll accept it may be unfounded