mileage

[PolyTek]

I've had my 5 speed Delica for just under a month now, and after 4 fills, I'm getting 10 to 14 L/100km as well.

[torchard]

A post in this thread alludes to the effects of wider tires on fuel economy. This isn't spoken of much and I haven't seen a handy-dandy calculator that will give some comparative calculations, but all you cyclists know the huge difference between pedalling skinny road tires vs. fat offroad rubber. Similarily, I would expect that wider tires to have a significant impact on fuel economy.

[PolyTek]

If I'm not mistaken, the reference is to oversized wheel/tire combos that actually affect the accuracy of the speedometer and odometer in a way that must be compensated for in order to calculate accurate milage. ie. the tires do less rotations per kilometer because they have a larger circumferance than the stock tires, thereby logging lower speeds and odo readings.

Pardon me if I've misinterpreted the cues. It does seem obvious that wider tires and/or offroad treads will have an effect on milage, but not in a way that needs to be re-calculated. It will just show up as lower milage.

Cheers,
D

[Green1]

wider tires will result in worse fuel economy, Tires do have a rolling-resistance rating, which would help to figure out the amount, however there are many more factors than just the rolling resistance when calculating mileage, so how much of a difference it will make is much harder to gauge.

[mararmeisto]

The last couple of posts are talking exactly to the point I was trying to make: a 'bigger' tire will change fuel economy because of its rolling resistance, not because of its circumference. For example, one can see from the following link

http://www.miata.net/garage/tirecalc.html

that a 235/75R15 is practically the same tire as a 215/80R15 (a difference in circumference of 1.2% which is negligible). However, because the 235 tire is wider, more of it will be touching the ground, which means more effort will be required to overcome friction (more physics and a bit of math to explain further than is required on this forum).

[delicat]

I fully agree with Torchard, and I like to comparison with the bikes.

On a bike anyway, the difference of tire width is huge. I'm sure it'd be similar on a Delica. On a previous post I was asking on a very specific tire, a Yokohama Y742S which comes in size 7R15 that being 29.8" tall by 7.7' wide and 36lbs (yes, the weight of the tire will have a big difference as well as the centrifugal force takes effect). This Yoko compared to a 235/75/15 which are about 28.9" tall and 9.3" wide would be taller, skinnier (lowers RPM, reduces friction...) How much of a difference in fuel economy, that's what I'd like to know as well! (on the bad side the Yoko has very aggressive pattern.)

Those BFG sure look great but I'd be curious to know how much affect on MPG they have compared to a more "civilized" (and boring) city tire...

[Green1]

On a bike anyway, the difference of tire width is huge. I'm sure it'd be similar on a Delica

actually there's quite a difference here between a bike and a Delica such that it isn't as big a deal (I'm not saying it doesn't matter at all, because it does, just not as much as on a bike)

[JMK]

Also, I put the roof rack back on, so I imagine my aerodynamics went down a bit (not that these are a terribly 'slippery' as an object passing through moving air, but that's a subject for another forum).

We tested our consumption with and without our Yakima Ski Pod and roof rack between Radium and Banff on our Chrysler LHS (3.5l V6 gas engine) during 4 trips, 2 with it on, 2 with it off. For the round trip of about 270km, the effect of not having the roof rack appeared to save almost 5l of fuel each time it was off. Driving habits and conditions were pretty much consistent and I use cruise most of the way at the same speed of about 100 km/hr each trip going, and average 90 coming back at night due to all the deer on the road.

I recall watching a scientist on TV once remarking that even just leaving the bare roof rack rails on the roof of the car has a significant effect on fuel economy because it drastically modifies the intended aerodynamics that the designers intended in the first place. Roof racks shouldn't really be left on just for convenience when they're not being used, but I think we all do it.

[torchard]

I used bikes as an easy illustration. I'm sure you're right that the relative impacts aren't similar.

For anyone dying to know more, this report speaks to this topic: "Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance -- Special Report 286 (2006)."

12-20 percent of the energy from a gallon of gas gets to the wheels (most of the energy from a gallon of gas is lost to heat through the combustion process and friction through the driveline). One-third of this energy is consumed by rolling resistance. For most passenger vehicles a ten percent reduction in rolling resistance amounts to a one to two percent increase in fuel economy. The report acknowledges that "in one sense, rolling resistance consumes only a small fraction of the total energy extracted from a gallon of fuel. In another sense, a reduction in rolling resistance will reduce demand for mechanical energy at the axles. This will have a multiplier effect because it will translate into fewer gallons of fuel being pumped to the engine in the first place." It also acknowledges the many variables affecting these estimates including the characteristics of the tires themselves.

Find it here at http://books.nap.edu/openbook.php?recor ... 620&page=1

--T

On a bike anyway, the difference of tire width is huge. I'm sure it'd be similar on a Delica

actually there's quite a difference here between a bike and a Delica such that it isn't as big a deal (I'm not saying it doesn't matter at all, because it does, just not as much as on a bike)

[Schwa]

There's a way to calculate a vehicle's total rolling resistance, I'm not sure of the mathematics, but it involves bringing the vehicle up to a certain speed and then coasting in neutral on a flat surface, and no wind... the longer you go until you stop (or reach a certain speed?), the less rolling resistance, so you can see exactly what difference having the hubs locked makes, or compare tires, vehicles...

[delicat]

Interesting study Torchard.

10% reduction in resistance, 1% to 2% increase MPG doesn't sound much but it equals to a saving of 6 to 12 Gallons of fuel every year. This "small" 10% fuel saving would exceed the tire purchase cost over it's lifetime... In the USA alone this saving also equals to taking 2 millions to 4 millions cars and light trucks off the road! Add to that less used tires ending in landfill and the small saving isn't so small after all... But let's not forget, safe tires are more important than all the above!

[marsgal42]

Gumdrop came with 235/75 R15 tires, and I've confirmed the speedometer readings with GPS. It often feels like you're going a lot faster than you really are. But what the speedometer says is right.

Unless I add power, my maximum speed down the Cut is about 95 klicks. With no engine braking, it will be limited by rolling resistance and aerodynamics.

...laura

[Green1]

the longer you go until you stop (or reach a certain speed?), the less rolling resistance

that's not entirely the case, that measures resistance, but it combines rolling resistance of the tires, with the resistance of the axles and differential, as well as aerodynamic drag... changing the tires is only a small part of the total equation.

[torchard]

For what it's worth I do like the look and practicality of tall, but skinny tires - similar to what you sometimes see on landrovers. I'm not a big fan of the roller skate look of some vehicles. But to each his own. Another aspect often overlooked is the loads oversized tires and rims add to your axles and related components, particularly when they are offset further outboard.

[mararmeisto]

Generally the 'oversized' tires are on different rims. That is, wider rims because of the wider profile (235 vice 215). Putting wider tires on thinner rims is possible, but generally not good practice because the bead might not hold.

The factory rims are generally plain steel with diamond-shaped cutouts (5 or 6). The Moon Machine has "Beste auf der Weld" rims (I don't know if that is their proper name, but that is what is etched onto them), an aftermarket, dressed-up rim.

Skinnier, taller tires can give you better rolling resistance and a lesser likelihood of hydroplaning, while fatter, lower-profile tires can give you better traction (more footprint). And there are, quite literally, many other factors that will change enhance/lessen performance of the vehicle based on the wheels/tires. One of the simplest things on a vehicle, wheels/tires have, proportionally, one of the greatest effects.