Fuel economy

Submitted: Wednesday, Mar 22, 2006 at 22:53
ThreadID: 32059 Views:2614 Replies:4 FollowUps:4
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When I puchased the 2.8 TD GU a little over 12 months ago within the first 4 weeks I had fitted 3" lift, bar, winch, battery, alloys and 265/75 cooper st's.

Over the ensuing months I was frequently reading on here how others were getting 8 km/lt or 12.5lt/100 from their 2.8 TD GU and was rather dissapointed when the best I could get was 6.7km/lt or 14.9lt/100.and that was after I'd had the injectors done and a tune. Every so often somebody else would pop up getting no better than me, so at least I wasn't alone, but it didn't solve my problem.

I know how you drive a vehicle can make a big difference, so keep in mind I drive for a living and have done both local and interstate in everything up to road trains, so have a fair idea on how to get the best economy out of a diesel.

We recently purchased a 4.2ltr GU and therefore transfered all the gear over to it.
Now my father has a 4.2 dsl GQ with safari turbo, and running 31 10R15 tyres and most times returns about 8.5km/lt or 11.76lt/100 average economy.
I also owned a GQ shorty with a 4.2 n/a dsl that would give me the same, well blow me down if the 4.2 GU only gives me a best of 6.7km/lt or 14.9lt/100 !
I know there is a weight difference of about 300-500kg, but even if Dad puts 500 kilo in the back of his there is no way his economy would drop to 6.7km/lt.

Now here comes the interesting bit, the 2.8 TD GU is now back to stock standard with all the stuff from the 4.2 ltr on it, right down to factory supplied 265/65 bridgestones, the only thing still on it is the motson's power-up chip. As our turbo VL crapped itself, we are using the 2.8 as a second car, well bugger me if we're not getting 8-8.5km/lt out of the thing around town!
The missus drives about 30km each way to work every second day with not many lights and about 15ks of this is 100kph on the motorway.

I did a test with the shorty while I had it, with the 31 10R15's I could get normal 8.5km/lt and even 9km/lt when I was real conservative, I then fitted some 33 12.5R15's maxxiss buckshot's (muddies) and my economy took a dive to around 6 to the best of around 7km/lt or 14.2lt/100. It was hard to get an accurate figure as the car spent quite a bit of it's time getting hammered off road.

Now, back to the 2.8, the biggest difference from where it was to now is the tyres, the cooper st's are 265/75 ie 704mm dia. compared to the standard bridgies 265/65 ie 677mm dia. a 27mm or 1 inch difference, interestingly I have run the buckshots on the 4.2 GU and they felt no different to the coopers, so after some measuring I discovered they were only 5mm taller in dia. than the coopers.

So if I haven't confused you enough already, the final conclusion I have come to is that an increase of 1 inch in tyre size along with a slightly more aggressive tread is more than enough to knock your fuel economy around by anything up to 20%, ie. from 8km/lt, 12.5lt/100 down to 6.7km/lt, 14.9lt/100.
After all the reading I have done on this subject, I would never have expected such a big difference in economy by only a 4% increase in tyre size.

So for those of you contemplating fitting bigger tyres to your diesel, keep this in mind, and for those of you who are trying to figure out why your economy has gone down the drain, might pay to check your tyre size.
Obviously the extra weight of the bar/winch combination (which in my case was 95kg ) and extra battery had a little to do with it aswell, but I still think the tyres were the main culprit.

In trying to get the economy of the 4.2 back to around the 7.5-8 mark, which I think it should be capable of, seeing as Dad gets 8.5-9 out of his, and I got the same out of the shorty, I have one more test to try, I am going to fit the standard 265/65 bridgies back onto the 4.2 and see what that does to my economy.
If it goes back up to where it should be around 7.5-8km/ltr then I think I will contemplate sacrificing 1 inch in trye dia. and going back to 265/65's for an extra 120km from every tank of fuel.
I won't be doing this test though untill I get the MTQ/Northside 4x4 power upgrade preferably with intercooler done, and see if the extra power pulls the bigger tyres without affecting economy so much.
Then again, I might just do it before the upgrade to get a more accurate difference in figures for future reference.
I will certanly post the results when I get them.


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Reply By: Notso - Wednesday, Mar 22, 2006 at 23:06

Wednesday, Mar 22, 2006 at 23:06
Don't forget that unless you are using a GPS or something to measure your kilometres the readings will be out as well. You will seem to be doing more Ks with the smaller tyres, eg more revolutions. so less distance travelled.

The gearing in the GU is pretty accurate with 265/75s on and about 8 to 10% out with the smaller tyres 265/70s on. If you go bigger still the difference will be greater still. So that could get rid of 10% of your difference straight away
AnswerID: 162382

Follow Up By: Member - Pezza (QLD) - Thursday, Mar 23, 2006 at 10:01

Thursday, Mar 23, 2006 at 10:01
G'day Notso,
I agree that the km will be different with larger tyres on, but, the tyre increase was 4%, therefore my speedo would be out by 4%, in the 4.2lt case the speedo is actualy accurate to the metre when compared to the GPS with the 265/75 coopers on, and 4% out with the standard 265/65's on.
The 2.8 was the same, though either way it doesn,t matter, lets say we did 500 trip metre kms with the smaller tyres and used 62.50 lts this equates to 8km/lt, now do the same 500 trip metre kms with the larger tyres, this would actually be 500 plus 4% (remember a 4%increase in size ) therefore 520 GPS km, this time using 74.62 lts,
500 divided by 74.62 equals 6.70 km/ltr (this is what I used)
520 divided by 74.62 equals 6.96 km/ltr (this is what I should have used)
So the difference in figures is still only 4%, now keeping in mind these figures are not in a test envirionment in a lab, they are in real life conditions that change daily, which is the way we all do our figures, so having a difference of half a km/lt, ie just over 4%, wouldn't bother me, but a 20% (ok, it should have been 16%) difference I think is significant, considering they,ve started a whole business and will probably sell millions with the 'filth coke can' by promising a 10% increase.
Now that I've confused myself even more, either way, this post was to point out that an increase in tyre size will create more fuel usage not less, due to the motor having to work harder to pull the larger tyres.


FollowupID: 417140

Follow Up By: Notso - Thursday, Mar 23, 2006 at 10:23

Thursday, Mar 23, 2006 at 10:23
Yep, I was really making the point that the difference in fuel economy is affected by the size of the tyres in other ways not only the extra effort required from the motor.
FollowupID: 417144

Reply By: Member - Doug T (QLD) - Thursday, Mar 23, 2006 at 07:32

Thursday, Mar 23, 2006 at 07:32
Read your post with great interest, Like you a Truckie ...or was i gave it up but yeh i think your on the right track ,I had 15" wheels on my troopie for a while and on a trip from brisbane to Mt Isa by my self i just poked along at that speed where you know your car is doing it easy ie 80kph and i topped up the tanks at Rusty's roadhouse between Ipswich and Toowoomba and run on the rear tank and did measure distance with a GPS and decided to try and make it to Blackall or get the 900klm from a 90lt tank, i did achieve my aim and even went up the street past the BP to make it drop over to 901 and turned over to the front tank, OK now when i decided to return to 16" wheels the dealer said ....well now with the extra circumferance you should get a better economy,Well he was wrong, I dont think i could get same now though i have not tested it i reackon i would i would be struggling to get 800klm .Reason being i have worked out is that with the extra size the gearing is just a tad gigher thus making the engine work just a little harder , with the 15s on it run free-er over undulating country,
I also feel that at times i get better economy than other with different fuels,Woolworth fuel seems to give a lower economy, might be my imagination but when BP or Shell is used it seems to be better and yet i have asked tanker drivers and they say it all comes from the same refinery ??? What do you reackon Pezza.
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AnswerID: 162408

Follow Up By: Member - Pezza (QLD) - Thursday, Mar 23, 2006 at 10:22

Thursday, Mar 23, 2006 at 10:22
G'day Doug,

Yes, you are correct, I forgot to mention that in my original post, the extra usage is due the fact that the motor has to work harder to turn the bigger wheels, we can't forget, the factories aren't stupid, far from it, they have spent millions in testing and coming up with the perfect gearing to suit different hp outputs, we see this with the 4.2 GQ n/a having 4.11 dif gears, they add a turbo to it in the GU and go to the taller 3.9 gears, also the 2.8ltr, having less hp runs on 4.3 gears.
That's why I was originally not going to change tyres till I got the power upgrade done, to see if the extra power will turn the larger tyres easier and give me my economy back, I also have the option of staying with the larger tyres and going back to 4.11 gears, giving an overall gearing near the same as 265/65's with 3.9's, ie standard.
Though I would have to do some costings first to se if it would be viable, no good spending 2 grand on a gearing change for a 16% better fuel economy, would take too long to break even.

80km/hr all the way to Blackall ! Geez mate, I'd go loopey doing that, I'm flat out staying below 110, and even then I get bored silly. LOL.

FollowupID: 417143

Follow Up By: Footloose - Thursday, Mar 23, 2006 at 12:32

Thursday, Mar 23, 2006 at 12:32
Doug, Blackall is nothing. I came back from Alice Springs to the Gold Coast at 80/h in an 80 series with crook suspension. Some people passed me so many times that they said on the UHF that there must have been multiple copies ..the only thing I passed the entire trip was a grader and I rekkon even that was a race .
FollowupID: 417168

Reply By: rolande- Thursday, Mar 23, 2006 at 14:57

Thursday, Mar 23, 2006 at 14:57
G'Day Pezza,

I found similar to you that when changing from 265/70 road tyres to 265/75 LT's that fuel economy dropped off. Getting around the same as you at 6.5 - 7.0 km/litre.

I have also noticed a huge drop off in economy between driving at 95km/h compared to 110 km/h. Seems 95 is where it likes to sit economy wise, any faster and you pay through the wallet.

AnswerID: 162485

Reply By: rolande- Thursday, Mar 23, 2006 at 14:57

Thursday, Mar 23, 2006 at 14:57
G'Day Pezza,

I found similar to you that when changing from 265/70 road tyres to 265/75 LT's that fuel economy dropped off. Getting around the same as you at 6.5 - 7.0 km/litre.

I have also noticed a huge drop off in economy between driving at 95km/h compared to 110 km/h. Seems 95 is where it likes to sit economy wise, any faster and you pay through the wallet.

AnswerID: 162486

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