R

ROY_H

New Member
OK, just one for a little discussion. I had a 300TDi, now regarded as a low-tech mechanically injected 2.5L diesel. Regularly got 30mpg from it no problem. I've moved up to a TD5. 2.5L engine but now with super-high-pressure injection, electronic control, offering greater efficiency etc. etc. but I can only get 26mpg from it.
Where's all the extra fuel going? Where's the efficiency improvements from all that high-tech? Why have I gone backwards?

I think I know some of the answers but can anyone else shed some learned enlightenment on this.
 
Correct me if i'm wrong here, but doesn't the td5 have an extra cylinder ? 5 opposed to 4 in the tdi . If so, thats where the extra fuel is going.
 
ROY_H said:
OK, just one for a little discussion. I had a 300TDi, now regarded as a low-tech mechanically injected 2.5L diesel. Regularly got 30mpg from it no problem. I've moved up to a TD5. 2.5L engine but now with super-high-pressure injection, electronic control, offering greater efficiency etc. etc. but I can only get 26mpg from it.
Where's all the extra fuel going? Where's the efficiency improvements from all that high-tech? Why have I gone backwards?

I think I know some of the answers but can anyone else shed some learned enlightenment on this.
Click to expand...

26mpg sounds about right, they're good on a run up the motorway but real bad round town, the reason for all the electronic control is for emmision regs. manufactures are constantly pushed to produce cleaner engines and there are certain criteria that they have to meet before an engine goes into production, as legislation changes, the engine gets modified to the point where they can do no more to improve it and so bring out a new engine, hence the new TdV6 in disco 3.

All 4x4's suffer with crap fuel consumption because you're driving 4 wheels instead of 2, they are heavy and have the aerodynamics of a house
 
diamond dave said:
Correct me if i'm wrong here, but doesn't the td5 have an extra cylinder ? 5 opposed to 4 in the tdi . If so, thats where the extra fuel is going.
Click to expand...

Ah, but the total cubic capacity is the same, thus the fuel/air ratios you would presume stay the same. My point was that with the improved atomisation of the fuel using the extreme high-pressure pumping it should burn more efficiently and use less for a given power output.
Given my TD5 has air-con and ACE, is it likely this is where the extra fuel is being burnt. The ACE I would imagine is quite a high-pressure hydraulic pump. Does anyone have a non-ACE TD5 Disco and does it use less fuel than mine.
 
A 5 cylinder motor has more internal friction that a 4, although the volumetric efficiency is better on the 2.5 5cyl. Fuel atomisation is better on the Td5, so the fuel efficiency will be better. But we're not comparing the same cars. The TDi Defender scored a composite 27.5 mpg, the Td5 score 27.9 mpg. So the root cause is down to the vehicle.

Weight and drag - the car is bigger and heavier than the Disco 1. Add 0.1-0.2 mpg for every 10kg of weight. These are probably the most significant factors.

The additional power draw and a/c (which uses between 5-7% extra fuel on average) will have an effect. Any component that draws electrical power will derive it from the engine.

Tyres: got knobblier ones on? I reckon my BFG Muddies soak 10% compared to the AT's.

I may have missed something else - anyone?
 
I agree with you it does seem nuts, for example the 2.5N/A in my 90 can and has done over 30MPG.

My engine will regularly beat the MPG of my friends TD5, but as said its all in the emissions, it hardly ever black smokes, never, not even on start up does it get that black puff asociated with diesels.

Give me a 200 or 300TDI engine any day.
 
I'll add my two bobs worth. I currently drive a td5 ES auto,
when new gave about 20mpg, which improved slighty after about 20k on the clock.
Then a piece of luck, a leaky rocker cover gasket caused oil to leak into the engine management system - or so my local LR dealer informed me.
This was replaced, under warranty, and hey presto better throttle response and 29mpg, has now got 110000miles and still 29mpg.
 
I have a 4.5 year old TD5 Disco (owned from new) with 89k miles, and my average MPG is 30.39 over a lot of tankfuls. It runs on good synthetic oil, and BFG AT's, I don't usually thrash it, but it does some trailer-towing occasionally. Hardly any in-town use though, mostly A-road work on the A9 and M8. I also get better MPG in summer than winter (don't use aircon much as I live in Scotland! - but the aircon does seem to affect MPG a bit) by about 1 or 2mpg. Worst ever is 26mpg with a big trailer, but never normally below 29mpg, and often 32 or 33 mpg...
 
There you go, a prime example of ideal driving conditions, M8, A9 will be giving this TD5 a good run, engine is getting up to temp, not to many start stops unless its 5:15ish.

Poor consumption can be linked to all sorts, I have mud terrain tyres on, so my MPG is somewhat reduced in that I have the rolling resistance of these tyres.

I also have some standard kit that I carry in the load space that will add weight, hence more fuel... So thinking about it, I could get even better consumption with my 15J if I had AT or plain road tyres, and didn't carry recovery gear etc.

Air-con, I know people who use it ALL the time, unless its actully freezing, their consumption must be much higher than someone who doesn't.
 
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