MPG v Fuel weight

[Si Vxr]

I was on my way to work one morning stuck in traffic and was a little bored so I started to think about stupid things like why is the world round? did that fly see me comeing before it died any way I got to thinking how does the amount of fuel in the tank i.e its weight affect the AVG MPG. As im sure everyone notices a difference in performance when they have a full tank oppose to say an empty tank diesel it's heavy stuff. So as one of the houses I now manage means I have to visit it a few times a week comeing from home I thought I could do a test so here goes.

The Route:- A mixture of A and B roads over 12 miles one way 30-40-50mph speed limits same time of day roughly so same traffic levels.

Fuel:- Total Excellium Diesel

Perfomance mods:- Celtic Tuneing Remap (182bhp-302ft-lb), PD160 intake, Pipercross panel filter, Custom FMIC (only mods that should affect it)

Car:- Skoda Fabia vRS 1.9TDI PD130 53 reg 164k miles covered (due a oil service soon).

All results are taken from the on-board computer so should give a controlled result and results are averages over total distance for that trip.

Results:-

Full Tank:- 41.6 MPG, 24 mph, 31 mins, 12 miles

3/4 Tank:- (to be updated later)

1/2 Tank:- 49.0 MPG, 26 mph, 30 mins, 12 miles

1/4 Tank:- 55.0 MPG, 23 mph, 32 mins, 12 miles

Empty Tank:- 58.4 MPG, 23 mph, 32 mins, 12 miles

To try get fair readings I didnt drive overly economically this was "normal" driving with out the traffic light GP's ect so changeing gear at no higher than 2k and trying to sit at 1.5k ish or so the engine was happy not struggling.To be honist im already surprised at the results and thats just showing half to empty results oh and empty was just before light came on. Ive just brimmed it so I will add some more data for the full tank results and report back in a few hours.

[adm]

you must have been bored!

interesting to see the difference

[Si Vxr]

The day I thought this up I had been stuck in traffic for an hour but to be honest ive enjoyed doing this its been IMO interesting findings

[wilsy7]

massive effect, i didnt think weight would really hinder economy with the amount of torque these cars have, didnt expect lol gonna run on fumes from now on lol

[Si Vxr]

first post updated

[wilsy7]

thats shocking lol

[GrahamVRS]

Interesting results. I too find how much diesel you're carrying makes quite a big difference and stopped brimming the tank. On my 45 mile commute to work (majority motorway cruising) I averge 55mpg on a full tank, but creeps up to around 63-64mpg on 1/4. That's 16%, so given I spend £200 a month on diesel, that's a potential saving of £384 annually!

[JLneonhug]

Hmm..

Weight of diesel is less than water (slightly) so even assumed it was 1:1 (1ltr for 1kg) we're still only talking about 40ltrs or 40kg's maximum difference.. We're talking less than a weight of an additional person, which affects the mpg by 16%?

Does sprung weight really have that much difference in mpg..? :| Im quite sceptical about this, more research needs to be done :p

If this deemed plausible though I may trade my sub for one of those sw-anky compact ones.. get my money back in fuel savings

[Mike B]

Diesel weighs between 820g-900g/litre.

I'd suggest ripping out 35-40kilos of stuff like rear seat, Spare and tools carpets, And eat fewer pies.

No point compromising on range!

[GrahamVRS]

Hmm..

Weight of diesel is less than water (slightly) so even assumed it was 1:1 (1ltr for 1kg) we're still only talking about 40ltrs or 40kg's maximum difference.. We're talking less than a weight of an additional person, which affects the mpg by 16%?

Does sprung weight really have that much difference in mpg..? :| Im quite sceptical about this, more research needs to be done :p

If this deemed plausible though I may trade my sub for one of those sw-anky compact ones.. get my money back in fuel savings

Please don't take my figures too literally, as we're going by the onboard computer, but I can assure you I personally get much better mpg. It could be an unconscious effort to be lighter on the throttle as the tank gets closer to empty, the wind might have always been behind me close to empty, or any other factor... who knows? Even a more realistic true life 2% saving would be £50 annually, might not sound much to some, though I'd bite your hand off for £50!

[JLneonhug]

Please don't take my figures too literally, as we're going by the onboard computer, but I can assure you I personally get much better mpg. It could be an unconscious effort to be lighter on the throttle as the tank gets closer to empty, the wind might have always been behind me close to empty, or any other factor... who knows? Even a more realistic true life 2% saving would be £50 annually, might not sound much to some, though I'd bite your hand off for £50!

Nah it was good for you to share! I just never realised there was any significant effect. Im currently looking to swap out my fat sub/amp combo for a smaller one now

[MoggyTech]

It takes energy to move weight, so a 16% difference sounds about right. Imagine the savings if you stripped out interior and spare wheel, and then ran on 1/4 tank of fuel.

[sydoggkdy]

It would make sense running on sub 1/4 tank if you live next door to a petrol station or pass one most days not out your way, but it would be completely counter productive constantly making trips to the petrol station just to increase your mpg a little.

I wonder if the people with no interior see an improvement. That person with the ibiza FR with no dashboard must be getting a fair old MPG

si

[andypandypoos]

It would make sense running on sub 1/4 tank if you live next door to a petrol station or pass one most days not out your way, but it would be completely counter productive constantly making trips to the petrol station just to increase your mpg a little.

I wonder if the people with no interior see an improvement. That person with the ibiza FR with no dashboard must be getting a fair old MPG

si

you're always going to travel further going into the garage more often, plus you've got the extra stopping/starting hurting your mpgs and you'll be hit by the rising cost of fuel more often by running 1/4 or 1/2 a tank. does it work out a saving in the long term?

[sydoggkdy]

you're always going to travel further going into the garage more often, plus you've got the extra stopping/starting hurting your mpgs and you'll be hit by the rising cost of fuel more often by running 1/4 or 1/2 a tank. does it work out a saving in the long term?

never thought of the stopping in the garage lol I'm lazy and fill up once every couple of weeks with one big heavy tank full.

It would probably make more of a difference if everyone went outside to their cars and took in all that sh*te they don't need. my boots full of things I can't be bothered taking inside because I might need them again, sometime.

[bryand]

I don't think the weight of fuel makes much difference - about as much as having an 8-year old in the back. One point to consider is that PD engines pump an excess of fuel from the tank to the engine, heat it up and return it to the tank. I've never known the fuel cooler actually do anything, so the returning fuel will have more heating effect on the fuel in an empty tank than a full one. In other words, a full tank has cooler fuel in it than an empty one. And cool fuel = dense fuel = more energy per injected quantity. So MPG should be better on a full tank than an empty one.

Also, how warmed up was the engine? It can take up to 20 minutes for the oil to get up to working temperature, and that has quite an effect on MPG.

[C.D-vRS]

Bangor north Wales to the New Forest 300 miles

Average over 40 miles:

Average over the total 301 miles (was on 75mpg until the last 30 miles when I put my foot down a bit)

Filled the tank up before I left.

Was doing ~50mph on A/B roads, 68mph on cruise control on motorways.

Oil & air filter changed less than 5k miles ago.

Excess weight was just a backpack weighing roughly 5-6kg

Remap to 170bhp (RR tested)

Toyo Proxes T1R tyres, all freshly inflated to 32psi

A couple of pics of the car in the mountains.

[Si Vxr]

There has to be something in my readings even if there out slightly due to the on board comp in accuracy but you can still see it improve when the fuel goes down I'm doing the run back again on 3/4 tank on Thursday so I will post an update then

[xman]

You must have pretty flat back tyres. Weight doesnt make that much difference unless a lot of accelerating/braking is involved. Pump em up hard and repeat the test....

Edited 15 May, 2011 by xman

[KR MAN]

Strangely the first half of my tank gets my about 250 miles, the second half only gets me 150 or so miles :'( (til its well into the red and showing about 30 miles left in the tank on the computer) which kind of suggests that the less fuel I'm carrying the worse the economy gets.

Unless the air in the tank is heavier than the fuel

[Si Vxr]

That's the fuel gauge most cars do that when I brim mine it doesn't move off full for 80 ish miles around town or over 100 if on a run but once it gets to half it drops very quick

[bryand]

...Unless the air in the tank is heavier than the fuel ...

You mean topping off the tank with helium doesn't work? Scheisse!

[prolfe]

I've been half filling the CR and my previous A3 PD for years cos of the weight of 27 litres.

[veryrapidskoda]

Been thinking about this for a while (what, with a new high-milage daily commute and rising fuel costs), and couldn't resist piping up. Couple of points I've noticed;

Don't assume the fuel gauge actually shows what's in a tank - tanks are irregular shaped and gauges don't compensate, plus they are not callibrated, therefore the dial may show 'half-full', but in reality it could be anywhere from 1/3rd to 2/3rds full.

And the only way to properly check Consumption is by proper measurement, so you really need to log millage per tankfull. If you start half filling the tank you wont be able to do that.

Personally I'm thinking about carefully covering and relocating the rear seats and parcel shelf to my loft and swapping the spare wheel for a can of quick-fix tyre repair. As previously mentioned, tyre pressures are crucial as well (at least check 'em once a week), and economy will improve with higher a PSI but stability will suffer dangerously if you over-pump!!

[marcusthehat]

Might I add that way back in the 60's (possibly?) when Mercedes were setting all sorts of records for diesel engined cars, max hours at max mph etc etc round a circuit.

The Mercedes engineers were perplexed to find that the got the best/highest mpg/mph? figures on a FULL tank of fuel.

This they had not predicted/anticipated as common sense would indicate that the less fuel the lighter the car the higher the mpg's/mph's.

Except the full tank of fuel was COLD fuel, therefore denser therefore the injection pump was able to squirt more into the cylinders.

I liked that.