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Where the rubber hits the road. Discussion about drive train; (locking) diffs; wheels; hubs; suspension and, of course, tyres

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  #11  
Old 28-10-17
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Originally Posted by rammypluge View Post
Larger tyres dont actually reduce fuel economy, they just make the odo and trip underread.
The larger tyres are heavier and take more power to turn them so they do increase fuel consumption, although only by a poofteenth. As you allude to, most forget to account for the change in odo accuracy!
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Old 28-10-17
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When it comes to sand driving, just remember the desert is full of sharp timber spikes that isn't on a typical beach, so sidewalls (LT) etc are more important
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Old 28-10-17
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Default Tyres and the Simpson

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Originally Posted by rammypluge View Post
Larger tyres dont actually reduce fuel economy, they just make the odo and trip underread.


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Actually they do....
Due to increased rolling resistance and increased weight. Sorry but you canít escape physics.
Gear ratios play a huge role too. Thatís why some people can get away with it more then others.

Last edited by green troll; 29-10-17 at 08:50 AM.
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Old 29-10-17
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Originally Posted by Nab View Post
The larger tyres are heavier and take more power to turn them so they do increase fuel consumption, although only by a poofteenth. As you allude to, most forget to account for the change in odo accuracy!
Larger diameters have lower rolling resistance, because imperfections in the road surface are more easily surmounted.

They also turn at lower rpm, and therefore reduce bearing friction.

So yes, they are a bit heavier, but there are plus sides on the economy spectrum. As well as other pluses.

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Old 29-10-17
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Actually they do....
Due to increased rolling resistance and increased weight. Sorry but you canít escape physics.
Gear ratios play a huge role too. Thatís why some people can get away with it more then others.
As mentioned larger diameters have less rolling resistance, as imperfections in the road surface are more easily surmounted.

By altering the gearing, larger diameters reduce rpm at highway speeds, which reduces friction in the engine. Generally, they have significant likelihood of reducing cruising fuel consumption, but if it significantly reduces torque converter lockup when cruising that could be a negative.

During most driving the engine will be operated within a certain rpm band and therefore the change in gearing will be of little consequence, with a balance of minor positives and negatives in specific circumstances.

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Old 29-10-17
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Originally Posted by rammypluge View Post
Larger diameters have lower rolling resistance, because imperfections in the road surface are more easily surmounted.

They also turn at lower rpm, and therefore reduce bearing friction.

So yes, they are a bit heavier, but there are plus sides on the economy spectrum. As well as other pluses.
Some aspects have a more significant impact than others. Bearing friction and road imperfections will be swamped by the increased energy required to accelerate a heavier tyre (which has a larger second moment of inertia, whether it's larger or not), and the increase in frontal surface area because the vehicle now sits taller (no, the "extra space" under the vehicle doesn't counter this - it's a turbulent area that creates drag).

Under some ideal conditions (i.e. gentle cruising on flat terrain with a tail wind) taller tyres may produce a marginal improvement in fuel economy. As a general rule, if you put a heavier tyre on your vehicle it will use more fuel. Who puts a larger but lighter tyre on their 4wd?
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Old 29-10-17
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Proven on the dyno lost approx 5hp going from a H/T to a A/T ... was same diameter tyre also ...

On the bike dyno machine when we are chasing horse power go with a track tyre .. ...
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Old 30-10-17
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Originally Posted by nj swb View Post
Some aspects have a more significant impact than others. Bearing friction and road imperfections will be swamped by the increased energy required to accelerate a heavier tyre (which has a larger second moment of inertia, whether it's larger or not), and the increase in frontal surface area because the vehicle now sits taller (no, the "extra space" under the vehicle doesn't counter this - it's a turbulent area that creates drag).

Under some ideal conditions (i.e. gentle cruising on flat terrain with a tail wind) taller tyres may produce a marginal improvement in fuel economy. As a general rule, if you put a heavier tyre on your vehicle it will use more fuel. Who puts a larger but lighter tyre on their 4wd?
The increase in weight of larger tyres is minor compared to the vehicle weight, and has almost no influence on economy during cruising.

Yes, they will adversely affect aerodynamics, but again, the difference is small, considering we are comparing say a 32" vs a 33".

The proof has been in my pudding because i have put larger tyres on almost every vehicle i have owned, including 2wd's, and having monitored fuel consumption before and after and correcting the odo and trip, have found either no difference or improvement.
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Old 30-10-17
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You need to consider engine RPM's at certain speeds, is the engine in the middle its peak torque band? Most modern 4wds are overgeared and this might be fine when empty and cruising on the flat but load it up with gear and or hitch a load on behind, add a few undulations and the effect of the standard overgearing is highlighted.

By increasing the overall tyre diameter this makes the overgearing worse combine this with poorly programmed automatic transmission control and the end result is more fuel consumption.

Any accessories you bolt on your 4wd, a suspension lift, larger diameter tyres, more aggressive treaded tyres and wider tyres all add to the weight of the vehicle and increase its air resistance therefore increasing fuel consumption.

If larger diameter tyres decreased fuel economy then manufacturers would be fitting the tallest tyre possible on their cars!

OJ
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Old 30-10-17
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Quote:
Originally Posted by old Jack View Post
You need to consider engine RPM's at certain speeds, is the engine in the middle its peak torque band? Most modern 4wds are overgeared and this might be fine when empty and cruising on the flat but load it up with gear and or hitch a load on behind, add a few undulations and the effect of the standard overgearing is highlighted.

By increasing the overall tyre diameter this makes the overgearing worse combine this with poorly programmed automatic transmission control and the end result is more fuel consumption.

Any accessories you bolt on your 4wd, a suspension lift, larger diameter tyres, more aggressive treaded tyres and wider tyres all add to the weight of the vehicle and increase its air resistance therefore increasing fuel consumption.

If larger diameter tyres decreased fuel economy then manufacturers would be fitting the tallest tyre possible on their cars!

OJ
I havent come across the "overgearing" yet.

My car has an 8spd, the gearing was a bit low even in 8th, perhaps partly because its a commercial vehicle, and it is now better with 33" tyres. It now runs about 1800rpm at 110km/h, and pulls easily. I havent wrecked the aerodynamics, and i dont take it over gvm.

Even if i put 37" tyres on it, it would lock up nicely in 7th when it needed to go down a gear. Modern autos lock up a lot more often, have a wider range between lowest and highest gears, have more gears (and change gear quicker), so the issues are less, not more.

As i mentioned, larger diameter tyres, all else being equal, reduce rolling resistance. Its simple physics. They roll over imperfections easier. There is a diagram thats shows it, but its not hard to draw.

To give an example, consider a skateboard with new high quality bearings, running on coarse asphalt. On the flat it will need constant input. How much better do they feel on new smooth asphalt, and even better on polished concrete! Contrast that with a racer bicycle. If you only go a comparable speed on the racer to a skateboard to have similar wind resistance, you could just do a very light pedal, occasionally, and it would be hard to not overspeed the skateboard.

I theorise that manufacturers dont fit larger tyres due to packaging constraints, due to more brake pedal boosting or larger discs being required, due to a lack of vision to do things better, due to slightly more expensive rims and tyres having to be fitted, due to the economy gains being relatively minor. When high economy concept cars are produced they typically have larger diameter (and narrower with higher pressure) tyres fitted.

Last edited by rammypluge; 30-10-17 at 08:25 AM.
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