Hi Ban,

I was meaning rolling radius and also footprint size. No reason why smaller overall diameter wheels couldn't be fitted, but would put the speedo out! There would definitely be a difference in weight, but maybe getting a bit ott I would admit

Yes, quicker acceleration can help, but I guess its a happy medium and one people need to try for themselves.

Drag increases proportionally to the square of the speed increase - hence it would be most unlikely that a car would be more economical at 70mph than 50. In my experience with my Bravo 1.9D 8v 120bhp I can get 60mpg at around 56mph whereas it reduces to about 53mpg at 70mph.

IMHO effects of low rolling resistance tyres or conversely driving with excessively low pressures have been overstated. Also weight does not have much effect on steady speed fuel consumption.

Also research by BMW has shown that accelerating to a steady speed as fast as possible then cruising is more efficient than accelerating gently to cruising speed.

madmalc said:

Drag increases proportionally to the square of ...

Click to expand...

… and to overcome drag we have thrust, & where there is conflict between thrust & drag it’s my experience that optimum efficiency is around 2,000 rpm & 70 mph. I’m not alone in finding this.

FIAT Croma 1.9 m-jet Road Test
The 110 mile test route covered twisty, undulating country roads, villages and also a stretch of motorway where the car settled down to a quiet cruise-controlled 70 on the clock at exactly 2,000rpm. Experience is beginning to teach me that this is the optimum engine speed for economy with a modern diesel, though the engine will need to see a few more revs now and again to keep its fuel system and injectors clean. HJ (08/2005)
Full text: http://www.honestjohn.co.uk/road-tests/fiat/fiat-croma-150-multijet-2005-road-test/

55 Croma Prestigio 1.9/16v150
182130Z

Are we sure that slimmer tyres necessarily have less area in contact with the road than thicker/wider tyres? Isn't it a function of the weight of the vehicle over the area of the tyre generating more pressure on the smaller tyres, hence spreading them out more? I'm fairly sure the physics isn't always quite what you think it might be...?

bigmacinspain said:

Are we sure that slimmer tyres necessarily have less area in contact with the road than thicker/wider tyres? Isn't it a function of the weight of the vehicle over the area of the tyre generating more pressure on the smaller tyres, hence spreading them out more? I'm fairly sure the physics isn't always quite what you think it might be...?

Click to expand...

But a tyre is round, if the correct pressure for size the same amount will be in contact with the road.

Exactly, So if you look at a cross section of a fat tyre compared to a skinny tyre...do they have the same amount of areal contact?

Most Easterly Pandas said:

But a tyre is round, if the correct pressure for size the same amount will be in contact with the road.

Click to expand...

I think a tyre is only round when on a wheel not attached to a car and not sat on the ground they are definitely not round when in use.......

rossocorsa said:

I think a tyre is only round when on a wheel not attached to a car and not sat on the ground they are definitely not round when in use.......

Click to expand...

Correct, but if at the correct pressure the contact surface area will be smaller than if the tyre is a lower pressure or flat for example.

I was following a Moggy 1000 the other day and I'd estimate his tyres were about 3" wide.
Not much rubber/tarmac interaction there.

http://www.mpgforspeed.com

Aerodynamics does play a factor, hence why some cars 'cut' through the air better than others. Ever noticed that a lot of scoobys almost hit a wall at higher speeds?

You are right - manufacturers used to put on skinny tyres to improve the official 'CD' figures quoted for their 'slipperiness' through the air...

Fergie said:

Hi Ban,

I was meaning rolling radius and also footprint size. No reason why smaller overall diameter wheels couldn't be fitted, but would put the speedo out! There would definitely be a difference in weight, but maybe getting a bit ott I would admit

Yes, quicker acceleration can help, but I guess its a happy medium and one people need to try for themselves.

Click to expand...

no diameter wouldn't effect your cars speedo reading,if you stuck to a size that gave the same circumference [one complete turn of tyre] thats why you find
cars that have the option of 15/16 /17 inch wheels ect ect, as you go up an inch in wheel size you get a wider diameter tyre but you also get a lower profile tyre too that way the circumference stays near as dam it identical, regardless of wheel size...

Fergie said:

Interesting you find it more efficient at 70mph - sounds like another test lol.

Click to expand...

Not quite the 50 v 70 mph `compare & contrast mpg’ demo I was looking for – but the following 50 v 65 mph clips maybe of interest …





202045Z

Of course we have no idea if the car was going up hill or downhill and if its the same peice of road or not

Fergie said:

Of course we have no idea if the car was going up hill or downhill and if its the same peice of road or not

Click to expand...

… & a one-off stunt could hardly be described as a trial.
(I think the Fifth Gear team did 6 runs when trialing mpg results on “posh” fuel).
But all I could find on youtube until someone puts-up something better.
Edit& presumably our hero is on US gals ... <4L

Speaking of a trial – how’s your blackbox assignment from #20 coming along & the 20 mile @ 50 mph trial?

230940

I made the videos you guys were talking about with my car...I do not have the speedlimiter so I tried to keep my foot steady. I calculated the speed from MPH to km/h the consumptions is in l.
I hope it helps. The consumption is higher due to brand new 225/45/17 tyres.

Sorry about the music on one of the videos, I did not notice it while driving

http://www.youtube.com/watch?v=voFTYLYOtjA&feature=youtu.be

110km/h or 70mph

http://www.youtube.com/watch?v=EFDCqjQoTvo&feature=youtu.be

speed 80km/h or 50mph

http://www.youtube.com/watch?v=8lm6lE5vwLI&feature=youtu.be

140km/h or 86mph




Does anyone know how to insert a youtube video like the one done by Caravadossi?

Interesting. Difficult to judge the first two averages:

The 80km/h /50mph average was around 6l per 100km, or 39.2 mpg.
The 110km/h /70mph average was around 7l per 100km, or 33.6 mpg.
The 140km/h /86mph average was around 8l per 100km, or 29.4 mpg.

I am attaching consumtion of our Golf 6 2012 1.4TSI 120HP Gasoline version, since some of the guys like to compare things.

http://www.youtube.com/watch?v=d0YgxNIKYP0&feature=youtu.be

This is the consumption at 71km/h

http://www.youtube.com/watch?v=IosbKzVy1mQ&feature=youtu.be

This is consumption at 110km/h

Average 25mpg mainly town driving (lots of hills) + very short motorway journey.

Motorways 40 mpg at 85-90mph, or cruising at 70mph 45mpg

08 bravo sport

I don't think that readings from the trip instant consumption prove much even the average mpg on the trip isn't all that accurate and the spot readings are a bit erratic.