Bigger Wheels - Better MPG

[in_the_garage]

Hello,

I recently converted my van from a 1.9DG to a 1.9TD (AAZ?) and, just for good measure, swapped out my original auto box for a 5-speed box - it was *just* marginally cheaper to do than buying a new van

Now I've got a fantastic amount of low-down torque compared to before, so I want to make better use of it - I hardly use 1st gear, just pull away in 2nd everywhere...

I want to put wheels with a larger rolling circumference on, in a bid to get a higher MPG figure - the engine seems to be revving quite high at 65-70MPH, so bigger wheels would be kinder on it too, I would have thought.

The thing is, looking around and hunting through good old eBay, the most common wheel/tyre combo that seems to come up is 16" alloys wearing 205-55/16 tyres. Having too much time on my hands, I used lovely excel to work out the rolling circumference compared to my current 185R14's (which, apparently, have an aspect ration of 82 - it's just not stated for reasons best known to tyre manufacturers);

Tyre:__________Circumference (mm)________Revolutions/mile
185 (82) 14..........1594.......................................1009.6
205 55 16............1631........................................986.5
215 45 17............1660.5.....................................969
235 45 17............1689........................................953

If I assume (generously) I'm getting about 45MPG at the moment, even with the 235/45/17's, I'd get about 47.7MPG if you scale everything mathematically (obviously, I probably wouldn't, but it's a good rough guide).

So the differences in sizes and performance are pretty small - much smaller than I thought - is it really worth changing my wheels if all I'm interested in is better MPG? The tyre/wheel sizes I've used above are just the ones I've found for sale on the web, so it may be there are others I've just not come across yet - does anybody know of a wheel/tyre combo which will fit my unlowered van without having to grind away bits of the arches?

Anyway, fascinating, eh?

Any thoughts will be much appreciated,

Thanks

[ringo]

Having too much time on my hands, I used lovely excel to work out the rolling circumference compared to my current 185R14's

Sweet jesus - you have got to much time if you think excel is lovely

If I assume (generously) I'm getting about 45MPG at the moment, even with the 235/45/17's, I'd get about 47.7MPG if you scale everything mathematically (obviously, I probably wouldn't, but it's a good rough guide).

If was getting anywhere close to 45MPG i would be incredibly happy!

However, if your figures are correct, then yes, there doesnt seem much of an improvement (around 6%) - i suppose it all depends on how many miles you do. Considering new wheels and rubber will cost approx £300 - how long will it take to make that up?

Still, the other advantages like being able to cruise at lower revs and maybe a higher top end may well make this worth while. Its a damn site cheaper than sorting out the gearbox!

Is this the same gear box as the DG used?

BTW, on the wiki, there is some useful stuff - like a link to this tyre size calculator:

http://www.tyresave.co.uk/tyresize.html



Ringo

[in_the_garage]

you have got to much time if you think excel is lovely

Well...in the same way that I go to lovely work and pay lovely taxes

The 5-speed gearbox is "new" out of a 2.1 petrol - the DG had an autobox on it. 45MPG is really quite nice, although I'm not sure I'm actually getting that, it's just an estimate based on the MPG of the Golf the engine came out of...I've not done enough miles to know, yet.

My DG with a 3 speed auto box, though, managed an astonishing 25mpg on the motorway and just shy of 20mpg everywhere else. When I drove it from Cornwall to Scotland last Christmas, it would have been cheaper to hire a private jet than pay for the petrol I used

[in_the_garage]

Er...anybody spot the deliberate mistake, there?

Yep - all my wheel sizes assumed the tyre was completely flat...I'd only added on the height of the tyre to one side of the rim I wondered why the tyresave thing didn't match my sizes...I've actually been out and measured my wheels this time, so I know I'm right...

Tyre:__________Circumference (mm)________Revolutions/mile
185 (82) 14..........2024.......................................795
205 55 16............1985........................................810.5
215 45 17............1964........................................819
235 45 17............2021........................................796


So, in fact, it's even worse than I thought - all the sizes I quoted above are smaller than the standard 185R14's, even if you assume the lowest aspect ratio of 78%.

The only wheel/tyre combo which is better (that I've found so far) are the standard ones off a T4, at 195/70 15, which gives you a circumference overall of 2055mm and, in theory, 45.7MPG. Do T4 wheels fit T25 hubs?

Thanks

[mickymole]

hi i have a pair of wheels with 225/70/15 tyres fitted,i used them to gear my van up and they worked a treat.they are on merc rims that bolt straight on with a pair of vw trims.if interested bell me on 07828834567.
regards mick.

[CycloneMike]

Richard Jones has compiled this table which is useful for working out gearbox and speed stuff.
http://www.rjes.com/html/gearbox_types.html

[tonytech]

MPG isnt just about RPM. It's about RPM power, and torque.

If increasing the size of the wheels meant better MPG we'd all be riding round in cars with wheels taller than me, (Im a short a***)

Increase the wheel size and you will go further per engine revolution, but the engine will need to work harder to turn the wheel

I'm out of my depth, but would suggest studying the torque curve for your engine. and the wheel size, gearbox ratios of the donor vehicle, to determine the sweet spot.
T

[HarryMann]

Excel is lovely

I agree, very lovely, one of Microsoft's better productions - but it wasn't always so lovely particularly behind the scenes in the early versions!

If I assume (generously) I'm getting about 45MPG at the moment, even with the 235/45/17's, I'd get about 47.7MPG if you scale everything mathematically (obviously, I probably wouldn't, but it's a good rough guide).

Well, you won't be getting 45mpg with an AAZ, pretty sure of that... 30 would be a good figure in a T25. If 45 was possible, nobody would have a petrol engine in their vans anymore!
Go for about a 7-10% increase in rolling circumference, any more and you might find you're having to make the engine work hard for long periods on motorway inclines and into headwinds.

Indirect injection diesels can produce a lot of extra heat under those circumstances (revs + boost) and put most of it into the oil... so an oil cooler is a very worthwhile fitment. Having seen frightening oil temps for far too long, I fitted a 19 row cooler using 3/4" hoses which has also increased oil capacity to 7 litres - and has transformed its ability to take boost at speed and not send the oil temp gauge into orbit!

Also, see if you can find a Diesel binnacle with a revcounter replacing the clock, you should know quite how high those revs are, imagine its quite undergeared, however, don't underestimate how draggy a T25 is above 65 mph, they use loads of extra power to cruise at 70 or more, compared to 60... Loads!

I run 15" high aspect ratio tyres (Syncro), 205/75's or 195 80's, don't go too far above that unless you're always running light...

Good luck with the AAZ, make sure you do regular oil & filter changes with good turbo diesel oil, and take a look at the air-filter from time to time, they pass an incredible amount of air...

[HarryMann]

I'm out of my depth, but would suggest studying the torque curve for your engine. and the wheel size, gearbox ratios of the donor vehicle, to determine the sweet spot.

Yes tony, the actual curve needed is SFC Vs rpm and throttle setting (specific fuel consumption), usually a 'map' of curves as its against two variables.
You won't find one easily*...

Only if the SFC against load is fairly flat at cruising revs will things improve linearly... but within a small range its a not too unreasonable an approximation. The temptation is to cruise a bit faster rather than at lower revs though, nullifying any benefit.
AAZs like 3600~3800 for cruising, by 4000 you're definitely past the optimum point on the SFC curve! But by ungoverning they can be made to whip round to 4500 easily and (ask Mr Simpson here) round to 5500 in the gears
DI diesels (Tdis) hate high revs, and they are the ones that really need very high gearing, so don't confuse gearing for a TDi and an Indirect Injection Diesel - very different animals, and the TDi doesn't put so much heat into oil at speed, at all, at all.

*I've done at set for the Subaru 2.2, in lovely Excel, which on are Subaruvanagon forum (they're not pretty, but show what an excellent lump this is)

[in_the_garage]

Thanks for all of this

The 45MPG is only a guess, although I'd hope to be getting better than 30MPG! OK, the T25 isn't that aerodynamic, but its not got airbrakes!

I'm pretty happy that this engine develops a lot more torque than the old DG I had, but I've been wondering about the whole RPM vs Throttle Position with regard MPG - maybe somebody could answer me this? When faced with a hill, is it better (from an MPG POV) to drop to third and climb it at, say, two thirds throttle but higher revs or stay in fourth and chug up the hill at lower revs but with my foot to the floor.

Thanks for the tips - I've been asking about an oil cooler on the alternative engines forum and have been pointed at brickwerks and I'm currently watching a diesel instrument set on good old eBay. I actually have a petrol tachometer in my instruments at the moment...there's no way of picking up a rev-feed from a diesel engine with it, is there?

Thanks again

[toomanytoys]

That old conundrum...

Some engines are better than others at certain speeds.. peak torque is the most fuel efficiant spot in the rpm range, this is why most vehicles were geared for 56mph at peak torque (generalisation of course ), just where the government mpg tests were performed.

Armed with your gearbox code (if you know its correct) you could contact Aidan (link below) as he has done quite a few AAZ and TDi etc boxes now and knows what works and what doesnt.. (IIRC he has a good calculator that can factor tyres in too so may even suggest what size tyre could give you the best

The only true way to find out what mpg its really doing, is to brim fill the tank, drive and then rebrim the tank (pref at the same pump) recording the litres used and the distance covered, over several times... you need to check the speedo/mileometer for accuracy too..

Oh and gong from petrol to an AAZ if thats all the mpg I would expect to see.. then it makes a hell of a lot more sense to LPG the petrol engine... AAZ 30mp means the running cost would be nigh on the same... (bio/homemade etc, excepted)

[HarryMann]

The 45MPG is only a guess, although I'd hope to be getting better than 30MPG! OK, the T25 isn't that aerodynamic, but its not got airbrakes!

Effectively it has, certainly much over 60 mph!

Quite enough speculation on here about mpg
Fact is, until you record all the fuel over a substantial distance and through a season or two, you just don't know.... do you?

Whereas however bad, I at least know, to a pretty high degree of accuracy

As Simon says, repeatedly brimming and noting the odo reading on back of fuel VAT receipt is best way, then plot in Excel is the best way, then you see the trends and any filling inconsistencies too, or can judge a mad bad fast run against a few weeks taking it easy. You can also see when the engine or vehicle is deteriorating e.g airfilter getting clogged

P.S. Some diesel cars don't get a genuine 40 mpg, other than in the driver's imagination

The group that took their Tdi syncros (admittedly well-laden) to Morocco Christmas before last, despite cruising slowly, seemed to agree about 30~32 mpg. These were good condition intercooled Tdis. 2WD will do better, but AAZ IDI worse than a Tdi (but considerably more fun!)

[syncrosimon]

[quote="in_the_garage.... managed an astonishing 25mpg on the motorway and just shy of 20mpg everywhere else. When I drove it from Cornwall to Scotland last Christmas, it would have been cheaper to hire a private jet than pay for the petrol I used [/quote]

Just to be controversial, and not using excell.

700 miles @ 25 mpg on unleaded = 126 litres @ 1.16p = £146.16
700 miles @ 32 mpg on diesel = 98.4 litres @1.31p = £128.95

a saving of £17.21

[in_the_garage]

a saving of £17.21

I'm going to pretend I didn't read that, as it wasn't figured out in excel...

...well, I'll just have to spend that £17.21 on loose cars and fast women then, to make the very most of it

[toomanytoys]

a saving of £17.21

I'm going to pretend I didn't read that, as it wasn't figured out in excel...

...well, I'll just have to spend that £17.21 on loose cars and fast women then, to make the very most of it

At that rate, you need to cover 81348 miles to get the ave £2000 AAZ conversion cost back over a petrol engine..... not too much then......