Do the extras and tyres make that difference to fuel?

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martys0uth

martys0uth

Active Member
Posts
170
Location
The Wirral, Merseyside
Firstly, I understand the laws of drag and air resistance etc associated with my kit, maybe i'm over estimating what is to be expected?.

But on my Disco TD5 manual (2000) I have a winch on a bumper, Lift kit, snorkel and light bar (one of those LED ones I picked up for 84 quid from germany) and a set of Hankook Dynopro MT's 265/75/16's, granted they are far from road tyres, I'm getting 21 mpg on average, I'm not flooring it at lights etc and tend to sit at 65 on the motorway, sometimes (occasional) tipping 70 on a 25 mile route to work in the morning and back.

Hasn't got the EGR valve installed, luckily as I was about to order the kit for it this weekend just gone!

I know I'm not looking to expect 40+ etc from it, but I was hoping to get a little closer to 30 rather than 20?
 
Your tyre size is wrong, raising the gear ratios. With those tyres fitted the speedo will read 8.9% less than true speed. At an indicated 60 you will actually be doing 65.4 MPH. At 70 indicated a true speed of 76.23 MPH. Extra weight and air resistance from light boards Etc plus the higher gear ratios created by the over diameter tyres will load the engine more and cause your poor consumption figures. Plus of course the trip computer will be seriously effected.
 
wammers said:
Your tyre size is wrong, raising the gear ratios. With those tyres fitted the speedo will read 8.9% less than true speed. At an indicated 60 you will actually be doing 65.4 MPH. At 70 indicated a true speed of 76.23 MPH. Extra weight and air resistance from light boards Etc plus the higher gear ratios created by the over diameter tyres will load the engine more and cause your poor consumption figures. Plus of course the trip computer will be seriously effected.
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Ahh, that may explain a little bit, do you have any website you got this handy info from? been looking for somewhere to find this
 
wammers said:
No web site, just used a tyre size calculator compared what you use to standard, the rest is obvious. Tyre size calc is http://www.kouki.co.uk/utilities/visual-tyre-size-calculator
Click to expand...

Thanks for that :D

Think I'm going to look at replacing the intercooler on it, not only for power increase, but I understand it adds a little to the efficiency side alongside a remap, or atleast this is what I understand reading previous threads,
 
martys0uth said:
Thanks for that :D

Think I'm going to look at replacing the intercooler on it, not only for power increase, but I understand it adds a little to the efficiency side alongside a remap, or atleast this is what I understand reading previous threads,
Click to expand...

Cooler more dense air is good, but what you will get for the expenditure is minimal. Only way to increase power on a diesel is to give it more fuel. Use the power use the fuel simple as that. A more linear fuel map may give you very slightly better economy on a long constant speed run. But not in general use. If your mods are so you can enjoy off road activities you have to live with the consequences of that need. You cannot have the best of both worlds doing one will impact on the other. If you are using the trip computer as a reference to MPG, you may be actually getting more than it shows because the tyre size is cocking it's mileage travelled to fuel used calculations up.
 
Put a sail on it. It'll help one way. :p:D

Picked the wrong car for economy. Like trying to get a springer to walk at heal. Wrong dog.
 
Bobsticle said:
Put a sail on it. It'll help one way. :p:D

Picked the wrong car for economy. Like trying to get a springer to walk at heal. Wrong dog.
Click to expand...

I've considered the sail mod already! :p

I'm not too bothered about the economy, anything above 20 is fine with me, I was just hoping someone might have said there's an issue that causes overfueling or something like that :(

Thinking of what Wammers has said, seems to thing I've been doing a 'little' more speed then I thought, I'll have to do a tank-tank doing 60-65 rather than 65-70 and inspect the differences. I'm getting 400 miles out of a tank at the moment though :D
 
martys0uth said:
I've considered the sail mod already! :p

I'm not too bothered about the economy, anything above 20 is fine with me, I was just hoping someone might have said there's an issue that causes overfueling or something like that :(

Thinking of what Wammers has said, seems to thing I've been doing a 'little' more speed then I thought, I'll have to do a tank-tank doing 60-65 rather than 65-70 and inspect the differences. I'm getting 400 miles out of a tank at the moment though :D
Click to expand...

Tank is 95 litres or 20.8971 gallons. So 400 divided by 20.8971 = 19.2 MPG in round numbers. So your 20 MPG wasn't that far off. Should be getting about 24 around town with a manual and 34 on a steady run and 30 combined.. Factory fuel figures are always a bit hit and miss though. If you live in an hilly area up hill and down dale all the time, specially with the big tyres giving higher ratios, meaning the engine has to work harder to turn them it will drink fuel.
 
Further to what Wammers has said,you have got a lift kit fitted,so it sits higher,therefore more wind resistance,the lightbar and snorkel will add to that as well,all that will increase fuel usage.
 
brian47 said:
If you're looking to improve the fuel efficiency then as they say "I wouldn't start from here".
You're driving around in a one and a threequarter tonne metal box with all the aerodynamics of a finely moulded house brick ....... 'nuff said.
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I'm not complaining in the slightest, I'm more than happy with whatever I get out of it... I love the bugger even though I slap it with a hammer on the weekends.

goonarmy said:
That's v8 economy. So by my logic you might as well get a v8. Any excuse.
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Don't tempt me, with all my goodies I would probably get about 12 to the gallon!
 
I've a td5 90 and I get 28 to 30 with big tyres etc. but I do have disco gearing.

I have to sit at 60, if I go to the speeds you are doing I'm in the low 20's/high teens

I used my bicycle gps and my phone gps to work out my speeds as I'm over tyred so the speedo reads low. About 54-55 on my speedo is 60
 
wammers said:
Your tyre size is wrong, raising the gear ratios. With those tyres fitted the speedo will read 8.9% less than true speed. At an indicated 60 you will actually be doing 65.4 MPH. At 70 indicated a true speed of 76.23 MPH. Extra weight and air resistance from light boards Etc plus the higher gear ratios created by the over diameter tyres will load the engine more and cause your poor consumption figures. Plus of course the trip computer will be seriously effected.
Click to expand...

Just had my brain thinking the other day (which tends to be a bad thing most the time), if my tyres are 8.9% bigger, my odometer is 8.9% under? meaning I would need to times my mpg by 8.9% to get the 'true' mpg? at that I would be averaging about 23.3MPG? :eek::p
 
brian47 said:
If you're looking to improve the fuel efficiency then as they say "I wouldn't start from here".
You're driving around in a one and a threequarter tonne metal box with all the aerodynamics of a finely moulded house brick ....... 'nuff said.
Click to expand...
Just because it's not a featherweight hybrid city car, is no reason to ignore the fact it should operate within an mpg range. If you are at the lower extreme or below it, then it is well worth doing some investigation.

I'd say 21mpg is fairly poor for the vehicle and given mods. A stock Discovery 2 Td5 when touring should be able to top 600 miles to a tank.

If it was fuelled up and being driven hard, then lower mpg should be expected. But driving it fairly sensibly with just some bolt on goodies shouldn't really being having such an impact.

Of course, it may be handy to know how the op is calculating mpg... as that can often be error prone.
 
martys0uth said:
Just had my brain thinking the other day (which tends to be a bad thing most the time), if my tyres are 8.9% bigger, my odometer is 8.9% under? meaning I would need to times my mpg by 8.9% to get the 'true' mpg? at that I would be averaging about 23.3MPG? :eek::p
Click to expand...

Yes because the wheels go round slower the distance travelled compared to fuel used is out on the fuel computer.
 
300bhp/ton said:
Just because it's not a featherweight hybrid city car, is no reason to ignore the fact it should operate within an mpg range. If you are at the lower extreme or below it, then it is well worth doing some investigation.

I'd say 21mpg is fairly poor for the vehicle and given mods. A stock Discovery 2 Td5 when touring should be able to top 600 miles to a tank.

If it was fuelled up and being driven hard, then lower mpg should be expected. But driving it fairly sensibly with just some bolt on goodies shouldn't really being having such an impact.

Of course, it may be handy to know how the op is calculating mpg... as that can often be error prone.
Click to expand...
Someone said what I didn't wanna say, sometimes people take it the wrong way and just think you're being arsey in a message :p But 600 Miles in a stock TD5 sounds like doing 55 on a motorway everywhere, Whilst I say I'm not doing 80MPH everywhere, I do tend to sit at TRUE 65-70 (since realising the 8.9% speed increase as above) on the motorways in the morning, 60-65 on the ways home.

I am calculating by the following on a tank to tank brim, with a fresh odometer clock after logging the miles on a piece of paper

Fuel filled e.g. 85L / 4.5 (into gallons) = 18.88...

miles travelled / 18.88 I.e. 425 / 18.88 = 22.6MPG ?

and with confirmation of not adding that 8.9% by Wammers;

22.6MPG x 1.089 = 24.6MPG

So if that is correct, I'm a little bit more happier. Plus I've decided after my 2nd annual visit Off road venture in the Lakes on the 5th, I'm going to put my BFG A/T's back on till I really need the M/T's (mainly saving the £150 a piece tyres than the fuel)
 
It would be worth checking how accurate your speedo and odo are.

Plot a route on Google maps and drive the route, noting the mileage the vehicle claims vs what Google maps says. They should be pretty close.

Speedo is easy to check if you have a smart phone or SatNav, as both can display as GPS speed, which should be fairly accurate.
 
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