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Petrol versus sludger mathmatician needed...

2K views 26 replies 15 participants last post by  dailydoggy 
#1 ·
filled up today and noticed the derv is 10p a litre dearer than petrol...

a 330d does roughly 10 miles to the gallon more than my 330i

derv is roughly 46-47p more expensive a gallon than petrol.


so whats the real MPG justification figure for a diesel over a petrol these days?


also a 330i has practically zero uber expensive bits under the bonnet that ever go wrong, unlike the diesel which has the usual suspect expensive common failures...
 
#5 ·
so whats the real MPG justification figure for a diesel over a petrol these days?
I worked out the MPG both petrol and diesel have to do to match my price per mile on LPG and due to the price difference the MPG figures for both were way less than 1 MPG apart
 
#14 ·
Anyone who really knows cars knows that new diesels are a waste of time for saving money if you do short miles. It is a false economy. DPF's clog up just like a customer who came in to our workshop the other day with hers knackered. She and her husband were not best impressed and were going to trade the vehicle in against something new. 58 plate, low miles their car.
 
#15 ·
so in 12000 miles yad save roughly £250 in fuel costs against the petrol, so yad have to do at least 48000 miles just to save enough to pay for 1 blown turbo...



hmmmmm il stick wiv me petrol i think...
 
#16 ·
Hi, I'm new (whatup!), but I kind of did this calculation before buying my BMW (was choosing 330i vs 330d - went 330i...)

You need to compare the difference in fuel costs per mile (multiplied up by some mileage), to the difference of the non-fuel running costs.

To get the difference in fuel cost per mile:

Take the difference in MPG figures, divide by 4.54 to get difference of mileage per litre.
Take the difference of fuel cost per litre, in pence, divide that by the answer above.
This gives difference in cost per mile, in pence.

Example:
From Autotrader listings, the e46 330i manual gets 31mpg combined, The 330d manual gets 42mpg combined.
So; (42 - 31) / 4.54 = 2.423 (i.e. 330d does 2.4 miles more per litre of fuel)

From my local petrol station; unleaded 130.9, diesel 138.9 (so not yet 10p more here, also premium petrol will massively change the comparison!)
So; (138.9 - 130.9) / 2.423 = 3.302 (i.e. 330d is 3.3 pence per mile cheaper for fuel)

So if you take your 'additional non-fuel cost' for the diesel (I'm going to take £500 for an example), turn that cost into pence, and divide by the saving per mile on fuel.
So; (500 * 100) / 3.302 = 15,412 miles to break even, more miles makes the diesel the more cost effective choice.

I guess you could take a 'nothing major goes wrong' cost, and figure the mileage for that, then whack on a 'turbo blows' addition, and see how many miles extra means even that cost makes the diesel cheaper?

You have to remember to figure in all the differences in running cost when making that other figure though; road tax and insurance should be cheaper, but then you have the issue of the initial cost of the diesel being higher, or the costs being the same but the diesel therefore likely having many more miles on it...

Like I said; I did this calculation when buying my 330, and stopped roughly at the point where I realised the additional cost of the actual purchase of a similar diesel was likely to take me 4+ years to recoup on fuel savings - given that I average ~2 years with each car, and was buying a 2000 era car anyway...forget that!

Since this is my first post; I'd like to say a big thanks to the people on this forum; I read a LOT while researching the purchase, and you were all a really great source of info and advice (not that I asked any direct questions, but I read a lot of replies to questions I also had!) - especially Daily Doggy, actually, so its cool that my first post is a reply to thread of yours...
 
#18 ·
I only buy diesels but thats because i do the mileage and i do all my own repairs.
When friends ask me what to buy for themselves, i ask them what they want the car for - i.e. do they just want to do the shopping and get A to B reliably which the majority do, i tell them to buy a Japanese petrol car as they cannot go wrong with that (Which more than likely they will ask me to service for them on a diy basis).

If you do 12,000 miles or less a year, then it has to be petrol unless you are buying new with a 3 or 5 year warranty.

Modern diesels are too expensive when something goes wrong - and it will. Turbo's, fuel pumps (high and low pressure), injectors, egr you are looking at a few hundred pounds if not 4 figures.
 
#21 ·
Same old sh1t different day, these threads are getting boring now. Bring on 2012 where I hope the forums topic of choice will become interesting again...
 
#23 ·
Anyone show me how to paste an excel doc into a post?
When I try to add an attachment - excel docs don't seem to be shown in the folder that I want to upload from.

I've got a rough work out of cost per week etc based on price per gallon/mileage covered.

Thanks.
 
#24 ·
This work?

Type what you want in colomn A - car1 etc.
Green boxes contain displayed MPG/worked out MPG fill to fill, price per litre and journey distance.
Blue boxes show cost per week month etc.

This does not include purchase price/servicing etc. If you want to try different MPG per car - enter pence per litre in uppermost line - ones below will reflect this.

HTH.
 

Attachments

#25 ·
With all due respect DD you mpg in your 330i is very impressive and above average ;)

Mate of mine has a 330ci manual same age as mine he gets 300miles a tank compared to my 500, and we both do the same sort of driving, using them figures I'll stick with my derv :thumbsup
 
#26 ·
Hi, I'm new (whatup!), but I kind of did this calculation before buying my BMW (was choosing 330i vs 330d - went 330i...)

You need to compare the difference in fuel costs per mile (multiplied up by some mileage), to the difference of the non-fuel running costs.

To get the difference in fuel cost per mile:

Take the difference in MPG figures, divide by 4.54 to get difference of mileage per litre.
Take the difference of fuel cost per litre, in pence, divide that by the answer above.
This gives difference in cost per mile, in pence.

Example:
From Autotrader listings, the e46 330i manual gets 31mpg combined, The 330d manual gets 42mpg combined.
So; (42 - 31) / 4.54 = 2.423 (i.e. 330d does 2.4 miles more per litre of fuel)

From my local petrol station; unleaded 130.9, diesel 138.9 (so not yet 10p more here, also premium petrol will massively change the comparison!)
So; (138.9 - 130.9) / 2.423 = 3.302 (i.e. 330d is 3.3 pence per mile cheaper for fuel)

So if you take your 'additional non-fuel cost' for the diesel (I'm going to take £500 for an example), turn that cost into pence, and divide by the saving per mile on fuel.
So; (500 * 100) / 3.302 = 15,412 miles to break even, more miles makes the diesel the more cost effective choice.

I guess you could take a 'nothing major goes wrong' cost, and figure the mileage for that, then whack on a 'turbo blows' addition, and see how many miles extra means even that cost makes the diesel cheaper?

You have to remember to figure in all the differences in running cost when making that other figure though; road tax and insurance should be cheaper, but then you have the issue of the initial cost of the diesel being higher, or the costs being the same but the diesel therefore likely having many more miles on it...

Like I said; I did this calculation when buying my 330, and stopped roughly at the point where I realised the additional cost of the actual purchase of a similar diesel was likely to take me 4+ years to recoup on fuel savings - given that I average ~2 years with each car, and was buying a 2000 era car anyway...forget that!

Since this is my first post; I'd like to say a big thanks to the people on this forum; I read a LOT while researching the purchase, and you were all a really great source of info and advice (not that I asked any direct questions, but I read a lot of replies to questions I also had!) - especially Daily Doggy, actually, so its cool that my first post is a reply to thread of yours...
A most interesting first post - I think you will have made a friend for life in DD:thumbsup:hihi
 
#27 · (Edited)
official BMW figures a 330d saloon will do a max of 10 miles a gallon mixed driving more than a 330i petrol saloon mixed driving so the equation still roughly works out the same..

yad save roughly £250 in 12000 miles (19 full tanks) with the premium cost of diesel factored in compared to the petrol if ya drove it on a motorway day in day out, but going on the premium and additional cost of buying a diesel to start with and the expensive twitchy components under the bonnet, yad have to do behemoth sort of mileages and be very lucky with any potential failures to get any financial benefit from it at all long term...


ppl will say yeah but i can get way better mpg on a motorway in my diesel and i say well so can i in my petrol it goes both ways, its not a valid argument the official figures are the only thing that is credible and has bin worked out scientifically under strict testing conditions,a 330d owner will say i can get 45mpg on the motorway well i can get 35mpg on a motorway all day long in mine...

and its still 10 miles a gallon difference between the 2 wotever cobblers anybody comes out with in defence or argument..

so do the maths yourselves!!

330d 490 miles to a tank average mixed
330i 350 miles to a tank average mixed
diesel is 10p a litre more than petrol
45.5p a gallon dearer than petrol
£6.37 more to fill the tank on the diesel..
 
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