bigger tyres and fuel economy

Submitted: Tuesday, Jul 26, 2005 at 12:46
ThreadID: 25062 Views:1937 Replies:10 FollowUps:15
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i fitted some bigger tyres to the jack last week 265/75/16 wich are about 2inches bigger than standard.
before tyres were fitted i was using around 22 litres per hundred week after week.
fitted the tyres and filled the tank.
i refilled the tank after doing 400 ks 300 city and 100 highway it took 61 litres thats close enough to 15 per 100.
after filling up i have now done 750ks mostly highway with around 4-5 hrs low range and plenty of slow up hill climbs and turns and took 110 litres so thats around.15 as well.not to bad i recon.
cheers
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Reply By: Wombat - Tuesday, Jul 26, 2005 at 12:56

Tuesday, Jul 26, 2005 at 12:56
You're not calling Bendigo 'city driving' are you Jim?
AnswerID: 122107

Follow Up By: Diamond (Vic) - Tuesday, Jul 26, 2005 at 21:17

Tuesday, Jul 26, 2005 at 21:17
well maybe theres quite a few cars up here.
cheers
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FollowupID: 377360

Reply By: Brian B (QLD) - Tuesday, Jul 26, 2005 at 13:04

Tuesday, Jul 26, 2005 at 13:04
Hi,

Is it possible that the improved economy is now because your odometer recording is out because of the larger tyres?

I am not sure of this but thought I would raise it as a possible reason for the ? improved economy.

Have a good one.
AnswerID: 122110

Follow Up By: Truckster (Vic) - Tuesday, Jul 26, 2005 at 13:09

Tuesday, Jul 26, 2005 at 13:09
you havent seen Mrs D driving ;)
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FollowupID: 377265

Follow Up By: Mainey (WA) - Tuesday, Jul 26, 2005 at 18:30

Tuesday, Jul 26, 2005 at 18:30
The speedo will not be registering the same figures as it did previously, eg you will be speeding maybe 106 and seeing only 100 Kph same with the MPG etc, you will not have the same numbers because the speedo has not been calibrated to the new diameter of the wheels :-)

However on the positive side you may now be getting better MPG because you are now driving at closer to the maximum torque figure for you engine, thus returning you better fuel economy.
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FollowupID: 377329

Follow Up By: Diamond (Vic) - Tuesday, Jul 26, 2005 at 21:20

Tuesday, Jul 26, 2005 at 21:20
from what ive read here the aproximate spedo error with 2 inch bigger tyres is around 10% so if my spedo is reading 100 im actually doing 110.
therefore when my odometer says ive traveled 700ks ive really done 750.
i took this into consideration on my figures.
cheers
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FollowupID: 377361

Follow Up By: Gerhardp1 - Wednesday, Jul 27, 2005 at 10:19

Wednesday, Jul 27, 2005 at 10:19
If your error is +10% and your ODO reads 700, you have done 770 not 750
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FollowupID: 377437

Reply By: Member - Tonester (VIC) - Tuesday, Jul 26, 2005 at 14:04

Tuesday, Jul 26, 2005 at 14:04
Diamond, you got 22lt per 100km previously? Thats heaps. Am I missing something...? I'm just suprised. I got 14lt or so in my old Jack with stock tyre size. Mine was petrol, maybe there's a difference if yours is diesel, not sure. But anyway, guess it doesn't matter what was before, but its great you got better economy now.
AnswerID: 122124

Follow Up By: Diamond (Vic) - Tuesday, Jul 26, 2005 at 21:22

Tuesday, Jul 26, 2005 at 21:22
mine is a 3.2 petrol auto.
yes it is very thirsty.
but most of the driving is very stop/start and not much time in top gear.
cheers
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FollowupID: 377362

Reply By: fourplayfull - Tuesday, Jul 26, 2005 at 18:07

Tuesday, Jul 26, 2005 at 18:07
As consumption = dist divided by litres your bigger tyres will show less distance therefore worse economy .
Maybe the improvement is due to less engine revs per km.
AnswerID: 122157

Follow Up By: Diamond (Vic) - Tuesday, Jul 26, 2005 at 21:25

Tuesday, Jul 26, 2005 at 21:25
if my spedo was showing less distance wouldnt that mean im getting better economy?
i did allow a 10% factor because ive read hear that 2 inch bigger tyres equates to around 10% change.
cheers
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FollowupID: 377363

Reply By: Outnabout David (SA) - Tuesday, Jul 26, 2005 at 18:56

Tuesday, Jul 26, 2005 at 18:56
Am I reading this wrong.

22 litres /100k is only 12 MPG . that is worse than a Petrol Landcruiser Auto.

You did say you had a Jackaroo. Wow they are gas guzzlers.
AnswerID: 122166

Follow Up By: Diamond (Vic) - Tuesday, Jul 26, 2005 at 21:29

Tuesday, Jul 26, 2005 at 21:29
lol no it wasnt a typo yes 22 per 100.
mines an auto with most of my weekly driving is short distance in 40/50/60 speed limits with heaps of stopping.
funny thing with the jack auto is they are a 4 speed auto with lock up converter.
at 60 ks in 4th im doing around 2200 revs.
the converter locks up at around 70 ks and its then doing about 1800 revs go figure.
cheers
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FollowupID: 377365

Follow Up By: Gerhardp1 - Wednesday, Jul 27, 2005 at 10:18

Wednesday, Jul 27, 2005 at 10:18
Your Jack must have some sort of unusual problem.

I have a 3.5 auto Jack and I have never drunk 22 per 100.

There is no way that at 60ks in lock up 4th gear that you are doing 2200rpm - that's just not possible - 2200 in 4th is over 90k.

When you fit larger tyres, you would normally expect to have a slightly increased consumption around town, because the gearing is "taller" and you need more power to get the unit rolling.

I reckon you must have taken the handbrake off now that you have bigger tyres!!!!
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FollowupID: 377435

Reply By: BenSpoon - Tuesday, Jul 26, 2005 at 20:33

Tuesday, Jul 26, 2005 at 20:33
I'd recommend checking the figures again. It sounds like 22L/100km is too high.

Think about it: with larger diameter tyres, your speedo shows you going slower than you actually are, as well as your odometer counting up the Km's slower than you actually do them. End result is you think you have travelled 100km, but its more like 110km.

Add to this the increase in rolling resistance, the extra weight you are making rotate, the extra petrol being burnt to get the low down torque you now rely on more....

Do you still reckon you are getting that 32% increase in economy??
AnswerID: 122183

Follow Up By: Diamond (Vic) - Tuesday, Jul 26, 2005 at 21:33

Tuesday, Jul 26, 2005 at 21:33
yes im 100% sure of the figures.ive checked and double checked them.
when we bought the jack for the mrs we were shocked in the amount of fuel it used.we were using around $120 per week for just running around for the week.
now mainly use the jack for going away towing ect.
bought the mrs an old laser now she using about $35 per week.
cheers
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FollowupID: 377366

Reply By: Member - Bradley- Tuesday, Jul 26, 2005 at 22:03

Tuesday, Jul 26, 2005 at 22:03
Strange buddy, yes the autos do have a thirst, i would have expected 17-18 tops around town.

I went from 245/70 to 265/75 and it was 106% increase.

My figures went from 12/100 on 245/70 at pattern to 14-15/100 265/75 mtr's. and you know i'm not shy of giving her some stick.
AnswerID: 122206

Reply By: Member - Roachie (SA) - Tuesday, Jul 26, 2005 at 23:35

Tuesday, Jul 26, 2005 at 23:35
Stop/start driving is not a great way to measure economy....too many variables.
IMHO you need to travel a given route (eg: Bendigo to Melbourne) and start with a full tank each time and see how much fuel it takes to re-fill at the end of the trip. Do this with the smaller tyres, then the bigger ones. Still not real scientific cos temperatures have an effect, wind etc.
Cheers
Roachie
AnswerID: 122222

Follow Up By: Wombat - Wednesday, Jul 27, 2005 at 12:48

Wednesday, Jul 27, 2005 at 12:48
. . . or Bendigo to Birdsville, then change the tyres and come back!
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FollowupID: 377458

Reply By: Member - Tim - Wednesday, Jul 27, 2005 at 00:50

Wednesday, Jul 27, 2005 at 00:50
When I went from 265/70 to 265/75 the economy got worse by about 1 litre per 100k. According to the GPS the speedo is now almost perfectly accurate so it must have been out before (didn't have the GPS then). There should be no way that your economy will get better with bigger tyres as the engine is working harder to get the car moving. I also went from HT to AT and that affects the economy as there is more rolling resistance.

Seems like a big difference with no reason so something is strange.
AnswerID: 122229

Follow Up By: Gerhardp1 - Wednesday, Jul 27, 2005 at 10:22

Wednesday, Jul 27, 2005 at 10:22
Why is there more rolling resistance between HT and AT?

That's a new one on me, I thought rolling resistance was affected by tyre pressure - the higher your pressure the lower your rolling resistance.
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FollowupID: 377438

Follow Up By: Member - Tim - Wednesday, Jul 27, 2005 at 11:52

Wednesday, Jul 27, 2005 at 11:52
This may well be something that I am wrong about and I would appreciate comment from anybody who knows for certain but I have always believed that the more aggressive tread pattern of an AT or MT would create more rolling resistance. One reason could be related to the fact there are more edges and holes that the tyre has to be pushed past. Like trying to roll a square rather than a circle. This is why there is more noise with the more aggressive tread as well.

Tim.
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FollowupID: 377446

Follow Up By: Azo082 - Wednesday, Jul 27, 2005 at 17:06

Wednesday, Jul 27, 2005 at 17:06
The larger and more aggressive tread knobs on an AT tyre will move and deform more compared to that of a HT tyre. When the knobs deform, energy is lost, therefore more energy is required to move the tyre a set distance.

An easy comparison would be to ride a mountain bike on the road with full tread tyres, and then swap the tyres for ones that are 'slick' or with minimal tread.

Also, with all things being equal, a tyre with a tube will also have more rolling resistance than a tubeless tyre as there is friction between the tube and tyre.

HTH

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Follow Up By: Gerhardp1 - Wednesday, Jul 27, 2005 at 18:30

Wednesday, Jul 27, 2005 at 18:30
I can accept the argument that the deformation of knobby tread blocks may increase the rolling resistance, but that could also work the other way depending on how stiff the AT rubber is compared to the HT - very stiff AT might deform less than soft HT.

Probably a greater impact on rolling resistance would be the width of the tyre - a 245/70 HT would have less resistance than a 265/70 of the same tyre.
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FollowupID: 377523

Reply By: Member - Collyn R (WA) - Wednesday, Jul 27, 2005 at 11:00

Wednesday, Jul 27, 2005 at 11:00
Rolling resistance is affected by low tyre pressures, but with tubeless radial ply tyres running at the correct pressures, raising pressure further makes only marginal difference.

Fitting larger diameter tyres may reduce consumption very slightly in some circumstances (the cooling fan rotates marginally less fast and thus absorbs slightly less energy, and there are marginally less friction losses ) but any apparantly observable difference in fuel consumption is far more likely to be due to slight changes in driving habits, and/or experimental error.
Collyn Rivers
AnswerID: 122260

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