Bigger Wheels - Better MPG

[ringo]

Just to be controversial, and not using excell.

700 miles @ 25 mpg on unleaded = 126 litres @ 1.16p = £146.16
700 miles @ 32 mpg on diesel = 98.4 litres @1.31p = £128.95

a saving of £17.21

Isn't that taking the two extremes?

When does a petrol get 25mpg?

My diesel gets 32 when i drive like i stole it - which is all the time

Ringo

[syncrosimon]

[quote="ringo
When does a petrol get 25mpg?
Ringo[/quote]

Thats what I got on a 2500 trip to Corsica and back, 4 persons, tent, 70 litres water, no prob.

[toomanytoys]

My syncro 1.9 DG does 23-24mpg sitting at 4000 rpm on the autoroutes for hours........ the Caravelle did about the same but always sitting at 65-75mph....

[in_the_garage]

I thought 25mpg was terrible - I honestly thought my engine was on its last legs...which it was, to be honest, as it blew both head gaskets and emptied itself of coolant and oil in one, big, noisy death-rattle...hence my AAZ conversion.

I've been looking around for the CD figures for a T25, but with no luck - I did find this though, for anybody interested in a full on, graphs and all explanation of the implications of putting a 1.9TDi engine in a T25 (I know it's not the same as a 1.9TD before anybody says anything...)

[HarryMann]

Reading that confirms my view above, 45 mpg is ridiculous for a TD at any sensible cruising speed and 32 ish about what one should expect for a Tdi and what Martin actually got in practice too..

Cd (Drag coefficient) assume 1.0 (flat plate)

[in_the_garage]

Ridiculous

...well, that's me told!

I did find somewhere that a smooth brick has a CD of 2.0, so your estimate of 1.0 isn't too unkind

When I've used my current tank of fuel, I'll tell you what I managed to get MPG-wise...

[HarryMann]

OOps!

Didn't mean to be rude, but ...

When I've used my current tank of fuel, I'll tell you what I managed to get MPG-wise...

When you've used several tanks of fuel to remove any filling errors and unusual conditions, then drop the fillups litres and mileages in here and we will all chew the fat over them, that'd be great

Expect your hot summer day consumption to be better, whatever the Cd is, air is a deal thinner, as anyone with a marginal car or van will know.. despite reduced power with hot air inlet, they do go faster and easier in summer than winter... providing we have a summer that is.

NB. Is that brick Cd based on plan area or frontal area? Road vehicle Cd is traditionally quoted on frontal area, aircraft wings on planform area, unless it's just the skin-friction component being quoted which is often normalised using total wetted surface area.. as usual, its that chalk and cheese problem

Yes a flat plate normal to the flow will have a Cd based on frontal area well above unity, and it varies a great deal depending on Reynolds Number (the golf-ball dimple thing)... so 2 for a brick is probably about right at some speed or other (i.e. at some Re). Try using a Cd of 0.5 for a T25, but will see if I can guesstimate or find a figure that I can believe in... its been suggested that it drops around 75 mph (from someone with a fast van), which is quite possible...

But think my twin-cab has a Cd around 2, a real brick

[in_the_garage]

Well OK...so 45MPG was optimistic.

I've still not got through my first tank, though...which is better than my old 1.9DG at any rate

I reckon (and this is calculated using GPS and my fuel gauge, so is subject to change) I'm on target for between 30 to 35MPG and that's all B roads and backlanes, no motorway driving to speak of...

I searched and searched for a set of bigger wheels, but in the end I couldn't find anything more than about 3% bigger, which hardly seemed worth the hassle...it would have meant my 60mph rpm dropping by about 70rpm at best...

Oh well

[HarryMann]

Ah well! The laws of nature don't change much

and this is calculated using GPS and my fuel gauge

So when you finally re-brim it should eliminate fuel-gauge from the equation, 'tis the best way unless you invest in £1000's worth of fuel-flow instrumentation...