Petrol VS Diesel i'm confused!

pault123

Wig wrote: »

Well you really shouldn't be comparing the diesel mpg to your old car (the one you are about to sell) You should be comparing it to the 'petrol equivalent car' of the 'diesel car' you are thinking of buying.

Fiat Punto 1.2 16v petrol can easily return 43 mpg all day long. There are probably other equally frugal petrol cars available.

Hi Wig,

Looking through all the petrol models the best mpg I can find is 34. I could save myself £200-£400 initially if I went for a Petrol model.

So the diesel I think is still looking preferable. :money:

pault123

Altarf wrote: »

The 'urban' part of the test is 2.5 miles, with an average speed of 12 mph and at no time does the speed exceed 31 mph. The 'extra-urban' test immediately follows on and is only an extra 4.3 miles, with an average speed of 39 mph. The 'combined' is just an average of the 'urban' and 'extra-urban'. So the whole test is less than 7 miles.

The stated extra urban figure is 64 mpg, if I can get that after 2.5 miles @ an average speed of 39mph, motorway cruising mpg of 55mpg hopefully should be achievable :beer:

(its a VW engine based on the 1.9 TDI 90bhp year 2000 i think)

Altarf

I think that you may be disappointed because you are not comparing like for like.

You are comparing the actual mpg you are achieving in your current car, with the theoretical mpg that is possible to achieve under lab conditions.

You say that your current 1.4l car achieves 30 mpg and you are planning on buying a 1.9l VW based diesel that achieves 64 mpg. To take an example, if you compare a 1.4l petrol VW Golf with a 1.9l diesel Golf you will see that the extra-urban figure for the petrol is 51.4 against 65.7 for the diesel, so only a 15 mpg difference. The same difference applies to the urban or average mileage. Now neither car is actually going to achieve these figures, but you could expect the difference between them to be a reasonable indicator.

So at the current 7p per litre price difference between petrol and diesel, driving 12000 miles you would save perhaps £200 per year. However given that diesel cars are more expensive to buy than petrol cars, you may no actually see this saving.

You say that you are doing a straight swap with a dealer, petrol for diesel. Unless you have found a very generous dealer, I assume that the diesel is an older or inferior car to what you are swapping, so there is going to be a cost to change that you should factor in to your calculations, and then see whether the £200 fuel saving is worth it.

mitchaa

Petrol vs Diesel

MK4 1.8T 150bhp Golf Gti (Petrol)
Mk4 1.9T 150bhp Golf TDI (Diesel)

Both cars near enough identical, however the TD car costing £2000 more at purchase price than its petrol equivelant.

The petrol should average 35-40mpg (Lets take 37 mpg)
The diesel should average 50-55mpg (Lets take 52 mpg)

The average annual mileage = 12000 miles,

TDI you would have to buy (12000/52= 230.77 gallons of diesel)
Petrol you would have to buy (12000/37 = 324.32 gallons of petrol)

1 gallon of fuel = 4.54litres

TDI you would buy (230.77 x 4.54) = 1047.7 litres of diesel
Petrol you would buy (324.32 x 4.54) = 1472.4 litres of petrol

According to this site .... http://www.petrolprices.com/

The average cost of a litre of petrol in the uk at the moment is 106.9p
The average cost of a litre of diesel in the uk at the moment is 114.8p

The cost of fuel per year for the diesel = (1047.7 x 1.148) = £1202.76
The cost of fuel per year for the petrol = (1472.4 x 1.069) = £1573.99

A difference of £371.23 per year

Now considering the TDI car originally cost £2000 more to purchase it would take at todays fuel rates about 5 1/2 yrs to break even.

So yes the AA do have a valid point, diesel isn't neccessarly the best option

Additionaly, you can easily opt for a small engined petrol car (Clio 1.2) that will return 55mpg and the fuel costs would actually work in favour for the petrol

pault123

Thanks Altarf and Mitchaa, certainly valid points.The official "theoretical mpg that is possible to achieve under lab conditions." combined figure for my current car is 40mpg, in actuality I get 30mpg.

I think i'm probably putting too much faith on the 55mpg figure quoted for the diesel as being what I will get (althought Conors " I'm getting an average of 57MPG from a 2L Mondeo TDCi." has spurred me to research some real figures being achieved by 1.9 Tdi VW drivers of the same car/model)

Paul - also off to look at some Petrol alternatives with the possibility of :money:

smoky_joe

pault123 wrote: »

Thanks Altarf and Mitchaa, certainly valid points.The official "theoretical mpg that is possible to achieve under lab conditions." combined figure for my current car is 40mpg, in actuality I get 30mpg.

I think i'm probably putting too much faith on the 55mpg figure quoted for the diesel as being what I will get (althought Conors " I'm getting an average of 57MPG from a 2L Mondeo TDCi." has spurred me to research some real figures being achieved by 1.9 Tdi VW drivers of the same car/model)

Paul - also off to look at some Petrol alternatives with the possibility of :money:

well i have had a 2001 passat for 3 years which the computer regularly tells me i get 53mpg around the doors and a 3 hour motorway drive 64mpg at 55 to 80 mph now i regularlly check the computer figures between fill ups and they do overestimate a tad but it is still allways above 50mpg which is excellent being its a big heavy car oh and its only £120 yearly tax

cyclonebri1

smoky_joe wrote: »

well i have had a 2001 passat for 3 years which the computer regularly tells me i get 53mpg around the doors and a 3 hour motorway drive 64mpg at 55 to 80 mph now i regularlly check the computer figures between fill ups and they do overestimate a tad but it is still allways above 50mpg which is excellent being its a big heavy car oh and its only £120 yearly tax

I can second the figures for the passat 1.9tdi I was loaned for a week to cover commuting to a training course, A38/Walsall/Wolverhampton so fairly mixed conditions given rush hour and Jan weather. Over 400+ miles it averaged 52mpg and I wasnt exactly hanging about.

greenman7

Diesel engines get better fuel economy simply because they do not need to burn as much fuel as a petrol engine to get the same power. Diesel engines are also built heavier than a petrol engine to sustain the added stress of the higher compression ratio. Diesel engines do not have an ignition system so you will never have to tune them up. Exhaust systems last longer because diesel fuel exhaust is not as corrosive a petrol engine exhaust.

With lower depreciation, lower maintenance costs, higher mpg, less CO2 emmissions, same insurance costs diesel must be the way to go.

pault123

cyclonebri1 wrote: »

I can second the figures for the passat 1.9tdi I was loaned for a week to cover commuting to a training course, A38/Walsall/Wolverhampton so fairly mixed conditions given rush hour and Jan weather. Over 400+ miles it averaged 52mpg and I wasnt exactly hanging about.

Its actually a Passat i'm looking at :money:

thescouselander

greenman7 wrote: »

Diesel engines get better fuel economy simply because they do not need to burn as much fuel as a petrol engine to get the same power. Diesel engines are also built heavier than a petrol engine to sustain the added stress of the higher compression ratio. Diesel engines do not have an ignition system so you will never have to tune them up. Exhaust systems last longer because diesel fuel exhaust is not as corrosive a petrol engine exhaust.

With lower depreciation, lower maintenance costs, higher mpg, less CO2 emmissions, same insurance costs diesel must be the way to go.

I'm not sure the maintenance costs are actually lower. Maybe in the old days but diesel engines are now more complicated. On the BMW forum I frequent it is the diesel drivers who are spending a fortune on replacing turbo chargers and leaking high pressure injectors. Be warned, these are expensive parts and the same problems are found on other makes too.

Also, with regard to emissions, although diesel has lower CO2 emissions it is far worse than petrol in respect of the other emissions put out - especially particulates that have been linked to a number of serious illnesses.