Tyre size and fuel ecomomy

[Hoppy11]

It has been stated on here in the past when going to a bigger tyre the fuel consumption goes up, If diff ratio's were changed to bring the gearing back to standard would going up a few inches in tyre size increase the consumption??

Cheers
Hoppy

[chikoroll_]

the gearing would depend on the weight of the tyres, not just the size,
a heavy 30" is just the same as a lightweight 33" with the needed gearing


wind resistance would have an effect, allow 5% for that, avoid headwinds, crosswinds and high speed to save some

moment of inertia, and rolling resistance, allow 5% for that
when in stop/start, keep the wheels moving at all times to save it (creep at the lights, however much it might piss off everyone else)

tyre pressure is a major factor... over pressurize to get better fuel economy (gain 5-10%), but sacrifice tyre life and ride quality

[Slunnie]

It has been stated on here in the past when going to a bigger tyre the fuel consumption goes up, If diff ratio's were changed to bring the gearing back to standard would going up a few inches in tyre size increase the consumption??

Cheers
Hoppy

I found that changing from 29's to 33's that my consumption went up slightly around town, and went down slightly on the highway. It also became more variable. When I changed to 33" open pattern tyres then the fuel economy worsened. When I changed the diffs to suit the 33's the fuel economy became more regular, but it did not improve. These at least are my experiences with a TD5 Discovery.

[KiwiBacon]

If you don't calibrate your odometer then larger tyres show less km's driven and hence higher mpg.

That said, higher gearing can reduce fuel consumption as engines are more efficient with higher load and lower rpm (there are exceptions). But the "bigger" tyres people fit are often wider and chunkier with greatly increased rolling resistance so there are other reasons fuel economy turns to crap.

Tyre weight is only a problem with start/stop driving like around town. On the open road it makes little difference.

[whiteknight]

When I put 33's on my 2.8D Hilux with AM Turbo I got an extra 80km out of a tank consistently. I did loose power on take off etc but the lower RPM at highway cruising offset this I think as most of my driving was as 100km/h.

Depends on how you drive I think...

[dumbdunce]

... engines are more efficient with higher load and lower rpm ...

so I should run the absolute biggest tyres and tallest ratios I can get my hands on?

engines are most efficient at or near their peak torque rpm, and assuming your engine has sufficient power at peak torque to maintain your chosen cruising speed, your gearing choice should be for the cruising rpm to be close to the peak torque rpm. If your engine does not have sufficient power at peak torque then you will have to run a lower (numerically higher) ratio, and run the engine closer to its power peak (which is always a higher rpm than torque peak) to maintain your chosen cruising speed. this obviously sacrifices economy. if your engine lacks sufficient power even at peak torque (eg 2.8 turbo patrol with 35's and 4" lift) then your only choice is yo revise your cruising speed or find some more torque/power.

In answer to the original question, Slunnie is on the money. I also find that tyre pressure has a far greater effect than tyre size on economy, but by far the biggest influence is driving style.

Match the revs to the speed you want; you might find if you have fitted significantly larger tyres (eg a hilux with 35's up form 29's) without changing the final drive ratio, then 4th gear will be near (probably slightly taller) than where 5th used to be, and it becomes the cruising gear of choice, returning better economy than lugging the engine in 5th.

[KiwiBacon]

... engines are more efficient with higher load and lower rpm ...

so I should run the absolute biggest tyres and tallest ratios I can get my hands on?

If you're after max fuel economy, then yes.
[/quote]

engines are most efficient at or near their peak torque rpm, and assuming your engine has sufficient power at peak torque to maintain your chosen cruising speed, your gearing choice should be for the cruising rpm to be close to the peak torque rpm.

It's a little more complicated than that, here's a BFSC plot (that's Brake Specific Fuel Consumption) for a VW 1.9L diesel. The lines and islands represent the efficiency of that engine at different points.



This engine is most efficient at 1750rpm, producing 200Nm and 50hp. The fuel consumption there is 197 grams of diesel per kilowatt hour.
But you need to suck 50hp from the engine to get it in that sweet spot.

If we only need 25hp from that engine to cruise, the best point is found at 1400rpm where the best BSFC we can hit is 205 g/kwh.
If we geared it as you suggest to still run at max torque rpm of 1750 then your BSFC has dropped to 250g/kwh. You're burning 22% more fuel than necessary.


That's for a diesel, petrols hit their best efficiency at rpm far lower than peak torque. Here's a plot from an audi W12.


Best efficiency is around 2000rpm and 3/4 throttle. Peak torque is at roughly 2700-4700rpm.

Bottom line, higher gearing almost always gets you better fuel economy. The exceptions are rare.

[-Scott-]

Bottom line, higher gearing almost always gets you better fuel economy. The exceptions are rare.

So car manufacturers don't gear their vehicles for optimum fuel economy?

[KiwiBacon]

Bottom line, higher gearing almost always gets you better fuel economy. The exceptions are rare.

So car manufacturers don't gear their vehicles for optimum fuel economy?

They compromise drivability and fuel economy.
Most people hate vehicles which can't drive up a slight grade in top gear, which is what you get if you gear for maximum fuel economy at cruising speeds.

[-Scott-]

Bottom line, higher gearing almost always gets you better fuel economy. The exceptions are rare.

So car manufacturers don't gear their vehicles for optimum fuel economy?

They compromise drivability and fuel economy.
Most people hate vehicles which can't drive up a slight grade in top gear, which is what you get if you gear for maximum fuel economy at cruising speeds.

Thank you - that was the bit of the theory I was missing.

[alien]

Alirght smarty pants - try this one =)

Sierra SWB soft top - 1.6L weber carby engine, SPOA on 31x10.5s (at 20psi for comfort) geared with calmini 5.14 tcase gears, which bring gearing back to withing 1% of stock gearing... now, before the SPOA and 31's i got 280km from 30L doing city driving - but now with 31's and the extra height i get 200 on the odometer (once you allow for tyre incorrectness its 220km)... the only variables are:
- weber used to be tuned to run on 98ron fuel, now running 92)
- 31's
- lift

Thoughts?

[KiwiBacon]

Alirght smarty pants - try this one =)

Sierra SWB soft top - 1.6L weber carby engine, SPOA on 31x10.5s (at 20psi for comfort) geared with calmini 5.14 tcase gears, which bring gearing back to withing 1% of stock gearing... now, before the SPOA and 31's i got 280km from 30L doing city driving - but now with 31's and the extra height i get 200 on the odometer (once you allow for tyre incorrectness its 220km)... the only variables are:
- weber used to be tuned to run on 98ron fuel, now running 92)
- 31's
- lift

Thoughts?

The rolling resistance of your new tyres is greater than your old ones, especially running them at 20psi.

BTW "city driving" is full of variables, you've changed the fuel, retuned your carby's, changed tyres and lift.
Sounds like badly controlled experiment.

[thehanko]

ever ridden a road bike, then a mountain bike? tread and tyre width make a huge difference, not to mention driving on borderline flat tyres.

Also your sitting higher so more wind resistance - though this is only a factor over about 70 km/hr.

Are you finding your wearing your outside edges from tyre roll as well?

Have a play with your car and let us know.

try a tank with harder tyres.

try a tank with your old rims (if you still have them)

try a retune. carbies are hit and miss anyway. to keep them at their best they need to be readjusted fairly regularly.

with each change run it like that for a tank and see.

[Hoppy11]

A can of worms!!

We just got back from a trip from Geraldton to Port Headland in the X5 4.4 V8, we drove up there on the coast road, using 98 fuel, I set the cruise control to 120, at 2500 rpm the trip computer stated that i would get 715km's from the tank at 12.4 lt per 100km, it said this the whole way. On the way back we took the inland road (to see Karajini), again the return in economy was the same, 160km south of Newman we could only get 91 fuel, not too long after filling up, on the open road cruise set on 120, the computer said I would get 835km from the tank and the Lt per 100 started to drop to mid 11. We then filled up again in Mount Magnet with 98, got back out on the raod at 120 and the computer went back to 715 and the consumption crept back up to 12.4.

I thought that 98 would go further than 91.
Any thoughts
Hoppy

[-Scott-]

Alirght smarty pants - try this one =)

Sierra SWB soft top - 1.6L weber carby engine, SPOA on 31x10.5s (at 20psi for comfort) geared with calmini 5.14 tcase gears, which bring gearing back to withing 1% of stock gearing... now, before the SPOA and 31's i got 280km from 30L doing city driving - but now with 31's and the extra height i get 200 on the odometer (once you allow for tyre incorrectness its 220km)... the only variables are:
- weber used to be tuned to run on 98ron fuel, now running 92)
- 31's
- lift

Thoughts?

When you're talking city driving you're spending very little time at this mythical "optimum" driving speed.

You're using much more fuel as you accelerate away from every intersection/corner, and during that acceleration you're spinning up larger and heavier tyres. All of this requires power, and that power comes from the fuel you burn.

[-Scott-]

I thought that 98 would go further than 91.
Any thoughts
Hoppy

I would have thought the same. It's got me farked! No headwinds at all?

[Hoppy11]

I thought that 98 would go further than 91.
Any thoughts
Hoppy

I would have thought the same. It's got me farked! No headwinds at all?

Na no head winds, just pulled in, filled up with 91, cause thats all they had and away we went. I have never put 91 in the X5, the book says it's ok to run it, it recomends 98
Hoppy

[dumbdunce]

octane rating has nothing to do with the available energy in the fuel; in many cases a higher octane fuel has a lower energy density. if you engine management and/or tuning and/or engine design is capable of taking advantage of a higher octane (ie slower burning) fuel, then it may be possible to realise efficiency gains by using a higher octane fuel. seems strange in such a vehicle that cheaper fuel gives a loss in perceived efficiency. interesting to see what the available power is on each fuel.

as for other variables of your journey, unless you do the comparison with the engine on a flywheel dyno in a controlled environment, you can't say for sure if there was any head/side winds, variations in air temperature, pressure, an overall gain or loss in elevation over the entire trip etc - these things all contribute.

[-Scott-]

if you engine management and/or tuning and/or engine design is capable of taking advantage of a higher octane (ie slower burning) fuel, then it may be possible to realise efficiency gains by using a higher octane fuel. seems strange in such a vehicle that cheaper fuel gives a loss in perceived efficiency.

Yes. My own personal prejudice expects that a modern BMW engine would be at the forefront of such technology.

[cloughy]

if you engine management and/or tuning and/or engine design is capable of taking advantage of a higher octane (ie slower burning) fuel, then it may be possible to realise efficiency gains by using a higher octane fuel. seems strange in such a vehicle that cheaper fuel gives a loss in perceived efficiency.

Yes. My own personal prejudice expects that a modern BMW engine would be at the forefront of such technology.

Forefront?, knock sensors have been around for years

[dumbdunce]

if you engine management and/or tuning and/or engine design is capable of taking advantage of a higher octane (ie slower burning) fuel, then it may be possible to realise efficiency gains by using a higher octane fuel. seems strange in such a vehicle that cheaper fuel gives a loss in perceived efficiency.

Yes. My own personal prejudice expects that a modern BMW engine would be at the forefront of such technology.

Forefront?, knock sensors have been around for years

if the compression ratio and valve timing just aren't there to take advantage a longer burn, there is only so much that advancing the ignition timing is capable of.

other variables were involved, it's a very specific instance, the so-called high octane fuel may indeed have been crap, and vice-versa. sun spots, solar flares, pictsies, there are a million possible variables.

[-Scott-]

if you engine management and/or tuning and/or engine design is capable of taking advantage of a higher octane (ie slower burning) fuel, then it may be possible to realise efficiency gains by using a higher octane fuel. seems strange in such a vehicle that cheaper fuel gives a loss in perceived efficiency.

Yes. My own personal prejudice expects that a modern BMW engine would be at the forefront of such technology.

Forefront?, knock sensors have been around for years

And I expect BMW charges more for them than most.

[Gwagensteve]

Too many variables to call the one off experience science. Average economy figures aren't predictive, they're based on the previous activity. Fuel quality with 98 is very, very critical - it has a very short shelf life.

The last two cars I've owned have been cheaper to run and have achieved better economy on 98 than 91.

Steve.

[Hoppy11]

It's just what I observered from sitting behind the wheel for hours on end, every time i filled up I calculated the fuel consumption myself to see if it was matching the on board computer, and it was, it was the only time on the trip that we used 91 ($2.09 a litre mind you). I actually think I filled up in Carnarvon with 95.
I did'nt notice any difference in power, not that it would be that easy to detect?
Hoppy

[Gwagensteve]

Nah, you wouldn't feel the difference. You certainly wouldn't pick the difference between stale 98 and decent 91, as an example.

Steve.

[alien]

to answer the question about the outsides of the tread - nope, its wearing the same all over.... 20psi gives me a nice flat contact patch on the road without wobble or lean in the tyres - they barely even bag out going over bumps... my zuk as it stands and a soft top full of water weighed in at 1040kg... i'd say the rims and tyres is about 150kg of that too... lol