Fuel Economy Puzzler

[MHig]

Hi All,

I'm new here - joined after buying an Alhambra.

There's a lot of experience and know-how on here so I am hoping someone may be able to help with my puzzling problem.

I bought a 130 TDI Alhambra to replace a 115 TDI VW Sharan. My puzzle is that the fuel consumption is about 10% worse, especially on faster journeys. The car had its 60k service just before I bought it so all should be well and it drives fine.
The official consumptions for the two cars are virtually the same. As I am doing the same trips over the same roads as before, I expected similar mpg.

I have checked the tyres for low pressure and signs of misalignment -all OK. A diagnostics check came up all clear.

Any ideas on possible causes and things to check would be most welcome. Thanks.

 

[90landrover]

Hi

Just a thought but are the tyre sizes the same as I would have thought that if the sizes were different they would lead the consumption computer to give you a false reading and also a wrong speedo reading.

 

[MHig]

Thanks 90L.

Standard fitment tyres for the Stylance, I think: 225/45/17.

The Sharan had 215/55/16 which gives a bigger diameter but only by a few mm - within the range of tyre wear.
Maybe the slightly narrower tyres gave the VW lower rolling resistance but I very much doubt it would account for the difference.

I am starting to wonder about the intake air filter, mass flow meter, etc..

 

[bmsc22990]

I have the stylance and the tyres are 215/55/16 . Mine is the auto but i'm sure tyres are the same throughout range.

 

[MHig]

17" SEAT rims are pretty standard as well - I saw them on 3 or 4 others before I bought mine and there were lots on Autotrader.
Anyway, the rolling diameter is virtually identical to yours.

 

[Slimy]

The only thing I will say is ignore the official fuel consumption figures, they're virtually impossible to achieve. They're done in strictly controlled conditions, and mean absolutely nothing in the real world.

So without those, you've got one car that has 115, and the other has 130. By my reckoning thats about 12% difference. How much did you say was your fuel economy change?

 

[MHig]

Even if the official figures are not realistic, two almost-identical engines having the same government figures seem likely to be very close on the road. (As it happens the old car was very close to the official figure in normal mixed motoring.)

I don't think the difference in power is the issue. If anything I would expect the higher-performance engine to be more efficient. There are quite a few posts on here confirming how remaps give both more power and better economy, for instance.

 

[100andthirty]

It's worth checking the tyre pressures. unladen they're supposed to be 42psi front and 40 psi rear. this sounds very high but these are the right pressures for the 225x45/17 tyres you've got.

 

[MHig]

One of the first things I checked, especially as most dealers etc do not know that the settings are so high (the sticker in the fuel flap states Front/Rear: 42/42 for light loads and 45/49 for heavy loads!).

Next chance I get, I plan to have the air filter and air flow meter checked.

 

[100andthirty]

thanks for reminding me of the right settings. I relied on memory to quote the values; clearly my memory is becoming unreliable!!

 

[MHig]

Update

The tyres were ready for a change so I took the opportunity to go for a slightly different spec: 215/50/17 instead of the standard 225/45/17.
The idea was to reduce the rolling resistance slightly due to the narrower tread and raise the gearing a little by increasing the diameter.
It has helped - fuel economy is closer to my old car. Also the ride is a bit smoother. I just have to remember that the speedo now under-reads by 4 - 5%!

 

[the_fbi]

I just have to remember that the speedo now under-reads by 4 - 5%!

Less than 2% difference between the 2 sizes.
225 45 17 has a Circ of 199.35cm
215 50 17 has a Circ of 203.28cm

Only 12.5mm difference in radius between the 2, which given an 8mm new tread depth (16mm diameter increase) is less than the difference between a new 215 50 17 and a, just about to be illegal, 225 45 17.

Tyre width is an important factor with regards to drag. Its a little known fact that the Calibra is the worlds most aerodynamic production car, but even less known is that it was the 8v model with the skinny tyres which made this record.

Oh, and drag squares with speed. So ignoring other factors like drivetrain losses, if it takes 5bhp to move a car at 10mph, to move it at 20mph will take 25bhp.

OK, clearly I need some sleep......

 

[MHig]

Thanks FBI; quite agree with you about the dimensions. The calculator I used showed the same difference: a new diameter of 647 mm versus the standard 634 mm. That is only a 2.03% difference, it told me.

The old tyres were down to about 3 mm tread so that knocks another 10 mm off the diameter which makes the difference more like 3.4%.

So I can only assume that the speedo was under-reading slightly from the start as I have now checked it carefully several times against motorway distance markers. If the old car's was over-reading - never checked - it would explain a bit more of the puzzle.

Having said that, the new tyres do look noticeably bigger which makes me wonder about the tolerances in the measurements for different brands.

Whatever! It has helped a bit.

Drivetrain losses do become more significant at higher outputs - it is interesting to look at rolling road plots. I use the "square rule" argument with my kids in the context of momentum when braking or, god forbid, if they hit anything.

 

[javorb]

tyres

as a tyre fitter/technician for 26 years i would ask ,why change the tyre size?what kind of tyres were on both cars were they the same ,with your added power you are probably accelarating more ,i can say /advise if you are looking for improved fuel michelin are the best tyres to fit ,you do get what you pay for with tyres,also a big factor as well in mpg is the condition of your air filter or even a maf sensor on its way out

 

[MHig]

Javorb, Thanks for your comments. I went for a larger diameter tyre with a higher profile because it would raise the overall gearing and lower the rolling resistance - both by small amounts. I went for NCTs - not a Michelin fan.

You mention a "panel" air filter elsewhere on these threads. What do you mean by that? The filter is another item I want to check.

 

[javorb]

panel filter

it is the air filter ,if you fit a performance one you get much better throttle response and can improve mpg as well

 

[MHig]

Thanks. Can you recommend a brand?

 

[Viking]

Larger diameter wheels will make a difference of about 3mpg. The 115 Sharan with 16 inch wheels will be more economical than the 130 Alhambra with 17 inch wheels because of this. Note, this has nothing to do with overall diameter of wheel / tyre selection.

 

[MHig]

Larger diameter wheels will make a difference of about 3mpg. The 115 Sharan with 16 inch wheels will be more economical than the 130 Alhambra with 17 inch wheels because of this. Note, this has nothing to do with overall diameter of wheel / tyre selection.

How does the wheel diameter on its own influence economy, noting that you are not referring to the overall rolling diameter?

 

[Viking]

Because the larger wheels are heavier than the smaller wheels, and the weight is moved further out from the centre of rotation. This means that the engine needs more power to turn them (meaning more fuel used) which reduces fuel economy. The wider tyres also add more rolling resistance which adds a little to the equation aswell.