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.
Avagoodn
Pezza