What's your fuel economy like in your L322 / P38?

Actually quite anal about tyre pressures, so doubt it's that.

Really? Rubber and air weigh more than metal? Who knew?

All I know is that, on more than one occasion, changing from a larger wheel to a smaller wheel, has seen my fuel economy increase. I always put it down to narrower tyre profiles, and less weight per corner, but I guess it's more likely to just be magic and snake oil then.

Saint.V8 said:

Will make no difference to MPG...rolling resistance will be exactly the same....

on 18's the P38 is a little wondery...have to get the tyre pressures spot on for a decent ride on 18's

Click to expand...

Really? Over here there is a fuel economy rating for different tyres as the rolling resistance varies greatly. Michelin claim some of the best figures but some of the cheapo far east ones are even better.

I've been getting between 10.5 and 16mpg on LPG depending on use for our '97 P38 4.6HSE. Much of my use is short local runs in heavy traffic and and hilly terrain - this cripples the economy as does off-road use. A good run on the motorway improves this significantly. I'm currently paying around 70ppl for LPG so can almost double the effective mpg in relation to cost. I've maintained a spreadsheet of running costs since getting the P38 and mpg has averaged an effective 22mpg.

As I'm working through 'niggles' this is getting better. Recently put a pair of GG ATs on the front and a pair of new Insa Turbo's on the back (brand new, given to me for the cost of fitting ) - I reckon these have saved 1mpg over the mismatched rubber that was on when I picked her up but not done any long runs yet. There's a bit of lumpiness on the engine, caused I think by a small exhaust leak. I've not found that yet as it doesn't do it on tickover. I think it's o/s manifold so will need to remove a few bits to investigate - suspect new gaskets or bolt tightening might be all that's required. Intend to do a full service in the next few weeks too and hopefully can squeeze another 1-2mpg...

Saint.V8 said:

Got feck all to do with resistance....unless they are of narrower width...then there will be a small reduction in resistance...but if maintaining same width and similar tread pattern and depth, there will be no difference in rolling friction....

You could argue Polar and Mass moments of inertia and etc, but can't be arsed gooooooogle it!

Click to expand...

Datatek said:

Really? Over here there is a fuel economy rating for different tyres as the rolling resistance varies greatly. Michelin claim some of the best figures but some of the cheapo far east ones are even better.

Click to expand...

As I mentioned in a later post Keith, it is dependant on tread pattern, depth, etc hence why some tyres are more economical than others....but changing from 18's to 16's will have no effect on rolling resistance on the basis the same make/model of tyre is used.....

Changes in momentum to get the tyre rotating due to differences in weight and inertia may have an effect, and yes, you are quite right, getting a good quality tyre designed with efficency in mind will be a good call too..

But generically just the change from 18 to 16 will not make the difference, the choice and width of tyre will...

Datatek said:

Really? Over here there is a fuel economy rating for different tyres as the rolling resistance varies greatly. Michelin claim some of the best figures but some of the cheapo far east ones are even better.

Click to expand...

They are sold here with a twin rating system. Wet weather performance and fuel economy. My current rubber is C for wet weather and D for fuel economy.

xs2man said:

Actually quite anal about tyre pressures, so doubt it's that.

Really? Rubber and air weigh more than metal? Who knew?

All I know is that, on more than one occasion, changing from a larger wheel to a smaller wheel, has seen my fuel economy increase. I always put it down to narrower tyre profiles, and less weight per corner, but I guess it's more likely to just be magic and snake oil then.

Click to expand...

Rubber reinforced with steel. Wheels are made of light weight alloys, tyres use stainless steel. Because of the increased side wall you require more rubber and steel to build the carcass. Weigh them if you want, the difference in weight will be non-existent if any at all.

Going from a big fat wheel to a skinny one has no bearing on the rolling radius and obviously weight. I run 255/65/r16. I could slap on 255/55/r18 and have no difference in fuel use. But I could also just get 18" wheels that are silly skinny and save fuel.

Saint.V8 said:

As I mentioned in a later post Keith, it is dependant on tread pattern, depth, etc hence why some tyres are more economical than others....but changing from 18's to 16's will have no effect on rolling resistance on the basis the same make/model of tyre is used.....

Changes in momentum to get the tyre rotating due to differences in weight and inertia may have an effect, and yes, you are quite right, getting a good quality tyre designed with efficency in mind will be a good call too..

But generically just the change from 18 to 16 will not make the difference, the choice and width of tyre will...

Click to expand...

There is a big difference in tyre wall flex between 15" & 18" tyres, I would have thought that would make a difference.

Bix said:

....My current rubber is C for wet weather and D for fuel economy....

Click to expand...

What tyres are you on if you don't mind my asking? I'm guessing they're aimed at the black stuff as I struggled to better an E or F for economy (GG ATs front and Turbo Rangers rear - both 255/55/18)

Datatek said:

There is a big difference in tyre wall flex between 15" & 18" tyres, I would have thought that would make a difference.

Click to expand...

possible, but if the tyre width is the same and the pressures are correct, the contact patch on the road will be the same, hence resistance/friction/drag will be the same! (aslong as the same tread pattern and depth is the same too!)

Wow, I appear to have opened a can of worms...

So, short version is I am basically better off checking fluids, plugs, and maybe a de-coke or at least some injector cleaner and some new filters than spending money on new wheels...unless I missed something?

Given the fact the previous owner bought a new EAS compressor to "sort" the suspension issues but was running the thing off a cheap halfords battery without the required CCA value and thus flogging the alternator (which he also replaced) to charge a system that was draining down more rapidly than it could charge, I am guessing his technical wizardry probably didn't extend to a decent service, just "top it up and pray no.lights come on"...

Then it is on to tackle the fact my vents only blow hot air regardless of the setting/direction and the HVAC book symbol just appeared...

Rangies, you gotta love 'em!

Rob4x40 said:

What tyres are you on if you don't mind my asking? I'm guessing they're aimed at the black stuff as I struggled to better an E or F for economy (GG ATs front and Turbo Rangers rear - both 255/55/18)

Click to expand...

Yeah, they were road biased. Anything remotely good in mud and you're looking at Es and Fs.

Turns out I was wrong though, just rechecked, they are C economy and E for wet weather. One too many :beer2: is fudging my memory.

Kumho Road Venture APT. I wouldn't use them again to be honest. Were good in snow, good on roads, good on dry mud/limestone roads. But the second you're in wet muddy mud, they just go "#### that noise".

Bix said:

Yeah, they were road biased. Anything remotely good in mud and you're looking at Es and Fs.

Turns out I was wrong though, just rechecked, they are C economy and E for wet weather. One too many :beer2: is fudging my memory.

Kumho Road Venture APT. I wouldn't use them again to be honest. Were good in snow, good on roads, good on dry mud/limestone roads. But the second you're in wet muddy mud, they just go "#### that noise".

Click to expand...

Thanks, I feel better buying Fs now then. Thought I might be missing a trick there... Only use about 5-10% off road TBH, but don't always know when and can't be messing around swapping wheels all the time. There's nothing worse than having to dig out in the rain however so went with ATs. They don't seem any worse on economy than the mish-mash of patterns and treads on when I got her and I don't drive like an idiot in the wet, so not overly concerned. Cheers.

Bix said:

They are sold here with a twin rating system. Wet weather performance and fuel economy. My current rubber is C for wet weather and D for fuel economy.

Click to expand...

They actually have a triple rating in Europe, fuel economy; wet grip; and noise level. the ones I just bought for the Transit are C; C; 71db, that's with the correct 113R load rating.
I doubt they will last long as they are far east cheapies but the grip is supposedly good and so is the economy and as the Transit is converting into rust at an alarming rate, long life was less a consideration than good grip. The Firestones it has on at present have bugger all grip in the wet.

Rob4x40 said:

Thanks, I feel better buying Fs now then. Thought I might be missing a trick there... Only use about 5-10% off road TBH, but don't always know when and can't be messing around swapping wheels all the time. There's nothing worse than having to dig out in the rain however so went with ATs. They don't seem any worse on economy than the mish-mash of patterns and treads on when I got her and I don't drive like an idiot in the wet, so not overly concerned. Cheers.

Click to expand...

In hindsight I should've got some ATs. The price difference was marginal (about £3 in it), but I was panicking because I needed some new tyres quickly and the place had the Kumho's in stock.

Datatek said:

They actually have a triple rating in Europe, fuel economy; wet grip; and noise level. the ones I just bought for the Transit are C; C; 71db, that's with the correct 113R load rating.
I doubt they will last long as they are far east cheapies but the grip is supposedly good and so is the economy and as the Transit is converting into rust at an alarming rate, long life was less a consideration than good grip. The Firestones it has on at present have bugger all grip in the wet.

Click to expand...

Oh yes, I forget about noise. The boots on my RR are 71db too. Was only 74db for some ATs which an't too shabby. Especially as wind noise is what I hear, rather than road noise.
Most van tyres are utter arse in the wet as they are usually a hard compound for extra life. The best thing I ever did on my van was put on some decent rubber. They wore out a bit quicker but the ability to turn in the wet rather than just understeer through a fence made them worth it.

Good to see Ford are addressing the rust issue though. The amount of 2-3 year old vans that go past my office has lead me to the conclusion that do not by a Merc or Ford van as they turn to rust very quickly. The Vauxhall/Nissan/Renault vans are rust free (visually anyway) at upwards of 8 years old. Even our 13 year old Berlingo is fighting off rust better than anything Ford have of the same age.