Big tyres & fuel consumption

Submitted: Thursday, Nov 07, 2013 at 19:44
ThreadID: 105038 Views:5361 Replies:14 FollowUps:6
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A lot of people say that bigger diameter tyres on a 4WD will result in heavier fuel consumption of 1-2 l/100 km above stock tyres. But I wonder if anyone is correcting their odometer reading to allow for the larger tyres.

My SST Max 33s are 841 mm diameter and the old Geolander tyres were 804mm. So I multiply whatever fuel consumption figure I now get by 804/841 to get the correct figure.

When I do that, I get the same mileage out of every litre of fuel over the last 5,000 km that I did with the smaller tyres.

Disregarding progressive tyre wear, does than make sense?

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Reply By: mikehzz - Thursday, Nov 07, 2013 at 20:03

Thursday, Nov 07, 2013 at 20:03
It does to me. It also means when you go to sell the car, it has 5-10% less kilometres on the clock depending on how big the tyres are. There would be a slight increase in rolling resistance but not as much as most seem to think.
AnswerID: 521055

Follow Up By: mikehzz - Thursday, Nov 07, 2013 at 20:17

Thursday, Nov 07, 2013 at 20:17
Let's list some factors-
If the tyres are the same width then the resistance of the tyre and the road will be the roughly the same but the bigger diameter tyre might have a slightly longer footprint so slightly more resistance.
A bigger tyre has more mass so takes more energy to get to the same speed. It will also hold momentum longer.
A bigger tyre will make the gearing taller so the engine will be revving slower for the same real ground speed.

The only real way to tell is measure actual kms travelled by not using the odometer and actual fuel used at the pump.

This all assumes you haven't put a bullbar and winch or roof racks on at the same time. :-)
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Reply By: Nutta - Thursday, Nov 07, 2013 at 20:09

Thursday, Nov 07, 2013 at 20:09
Im hopeless with math but I know when I went up to 33s on the patrol it used a fair bit more juice.

After going up 40mm on the colorado the speedo is identical to the gps which is great, not sure if it uses more juice though, would have to though, odometer, not sure?

Power didn't suffer to much on either of them though, the collie still flies!
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Reply By: snow - Thursday, Nov 07, 2013 at 20:27

Thursday, Nov 07, 2013 at 20:27
Yes was warned/advised of the same some tears ago when I fitted 31" ham kooks to my LN65 Hilux but I also found that I had absolutely no negative impact on fuel consumption. I did find that it was a little slower off the mark and that braking was a tad less responsive. I later fitted a turbo which helped with other areas.
AnswerID: 521057

Reply By: Ross M - Thursday, Nov 07, 2013 at 20:32

Thursday, Nov 07, 2013 at 20:32
Generally a vehicle fitted with bigger tyres will use a bit more fuel.
It all depends on how the manufacture set the gearing for the torque and also the way the injection happens at that speed to get the torque.
At some speeds it may be worse and at others a bit better because the use of the fuel energy relative to distance travelled is the critical factor. This combined with various loads also has an effect.
Manufacturers settings & sizes are a compromise for all round normal use, once outside that it could go either way.
AnswerID: 521058

Reply By: 08crd - Thursday, Nov 07, 2013 at 21:27

Thursday, Nov 07, 2013 at 21:27
It is a very interesting subject.
I put larger diameter tyres on my car, which made the speedo exactly correct and the odometer 6% slow i.e when I do 100klm I actually have done 106klm.
I found a good way to check the odometer was by checking it against measured distances, between small targets, like the klm posts on the roadside.
Travelling from Perth to the North or the East, gives heaps of opportunity to check the speedo, also kills a bit of time.lol
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Follow Up By: Member - KeithB - Thursday, Nov 07, 2013 at 21:42

Thursday, Nov 07, 2013 at 21:42
My speedo and odometer both read 4% low against a measured 5km distance of the freeway. So it really does look like the bigger tyres make little or no difference.
The tread on these is much more aggressive which I'd expect to increase fuel consumption. But the carcass is a lot less flexible which may compensate.
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Follow Up By: 08crd - Thursday, Nov 07, 2013 at 22:36

Thursday, Nov 07, 2013 at 22:36
I meant more like a distance of 100 - 200 klm, not 5km. The error will become more accurate over a longer distance.
We travel from Perth to Kalgoorlie quite a lot, so I zero the odo at a klm post, eg Kal 160klm then check the odo as the mile post count down. After about 100klm you start to get a definite error.
I also do the same process when going across the nullarbor or when heading North, it passes the time and confirms your assumptions.

A tire size calculator such as this one will give you basic info on speedo correction.

http://www.1010tires.com/Tools/Tire-Size-Calculator
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Reply By: AlbyNSW - Thursday, Nov 07, 2013 at 21:59

Thursday, Nov 07, 2013 at 21:59
Another aspect is that often with the bigger tyre comes a more aggressive tread pattern with consumes more fuel another issue is lifting of the vehicle which effects its aerodynamics
AnswerID: 521065

Reply By: Bazooka - Thursday, Nov 07, 2013 at 22:51

Thursday, Nov 07, 2013 at 22:51
Too many variables to be certain Keith but the extra weight of four larger tyres, and changes to the factory set parameters in your engine mgt system (primarily gearing I guess) will often result in increased fuel consumption according to my mechanic bil. The difference is probably greater for smaller vehicles with less power/torque and mass (a few extra kilos makes a bigger difference). Of course increased fuel consumption is just one possible side effect of bigger tyres. If you go to extremes as some do the additional stress on your suspension and steering is significant, hence the need to beef some components up goes hand in hand, or should.
AnswerID: 521069

Reply By: CSeaJay - Friday, Nov 08, 2013 at 00:08

Friday, Nov 08, 2013 at 00:08
Keithb
I did correct my odo in my calculations and yes there is a 1to 2 l/100km difference based on real distance travelled.
This has been confirmed by several people on Lcool. But is must also be said that the 200's consumption is susceptible to change. This includes bullbar roofrack etc.
It may depend on the car you drive as to how much difference, but it will be more: more power needed to accelerate, bigger rolling resistance, heavier as well,
CJ
AnswerID: 521072

Follow Up By: 08crd - Friday, Nov 08, 2013 at 00:20

Friday, Nov 08, 2013 at 00:20
Not necassarily a lot will depend on the final drive gearing. If for example your car has a low final drive gearing that doesn't complement the engines torque curve, it may benefit fron taller tyres.
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Follow Up By: Bazooka - Friday, Nov 08, 2013 at 10:17

Friday, Nov 08, 2013 at 10:17
In theory that's true 08crd but you would have to know exactly what the primary considerations by vehicle manufacturer were when they designed the vehicle and programmed the ems. The ems will (should) have been optimised for "all round" performance with the standard tyres and wheels so any changes you make will result in some "inefficiency dividend", again in theory.
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Reply By: Batt's - Friday, Nov 08, 2013 at 01:29

Friday, Nov 08, 2013 at 01:29
I have been using a GPS for my speedo and for calculating fuel economy for a few yrs and ignoring the cars speedo because of larger tyres. I had a 4.2ltr petrol GQ and found that 32" A/T tyres gave me the best fuel consumption any bigger it uses more fuel and my current 4.2 ltr turbo diesel GQ gets it's best fuel economy running 285/75 A/T tyres. I don't use the cars speedo because it's 6kph out and even a vehicle like my neighbours 2006 stock standard Mazda BT50 is reading 4kph out gps reads 100kph cars speedo reads 96kph. My wife's elantra is 1kph out so even the cars computer gives incorrect fuel economy reading when I checked it against the GPS so how many people out there are getting incorrect figures on the same note I hope that my GPS isn't lying to me and I've been getting it right.
AnswerID: 521075

Reply By: The Bantam - Friday, Nov 08, 2013 at 08:57

Friday, Nov 08, 2013 at 08:57
The OP does well to bring up the odometer error introduced in the calculations, that I am sure many do not consider.

As others have said there are so many factors.

Size, weight, type of motor and how well the vehicle was optomised in the first place.

Then there is terain.
A steeper gear ratio due to larger tyres would advantage a vehicle running on flat terain and or a lighter loaded one and or one that is lighter with a smaller motor.

I expect to see less difference in a diesel than a petrol motor.

If the terain was hilly, particularly consistently uphill, I'd expect larger tyres to be more of a disadvantege, particularly in a heavier vehicle or heavier loaded.

Out on the highway, where acceleration and enertia are less of a cost there will be less difference, but in the city with lots of acceleration and deceleration, the overgeraing may cost some vehicles considerably.

Then there is the question of how much increases in size....modest size increases are more likley to make bugger all difference...where as substantial tyre and wheel size increases are likley to make far more difference.

cheers
AnswerID: 521087

Reply By: guy007 - Friday, Nov 08, 2013 at 10:48

Friday, Nov 08, 2013 at 10:48
Hi all,
When I purchased my 80 series it was fitted with 10R 15 tyres. These are undersized at about 30.5". Truck went very well but revved too high and I could not keep up with my mate with the same vehicle on 33"s when cruising on the open road. Fuel consumption was very good however.
I then changed to 285 or 33" tyres for a slight loss of performance and a slight increase in consumption when not towing. It was better in the rough stuff however. A big bonus however came when towing the caravan and where 5th gear was a no no. 4th gear then became the perfect towing gear when pulling the camper trailer and later the caravan. It enabled a higher cruise speed for the same revs.Fuel consumption was little changed when towing.
regards
Guy
AnswerID: 521098

Reply By: Member - Noldi (WA) - Friday, Nov 08, 2013 at 12:51

Friday, Nov 08, 2013 at 12:51
Likewise, personal experience, Years ago I went up 2 sizes on my Jackaroo and noted little difference in fuel consumption or take off acceleration.
Did the same with a 1.3lt Jimny and found the only difference was I could cruise at 95 without feeling the pistons were going to pop out of the cylinders. Will be looking for new tyres for the Cruiser shortly and will look at going larger. The only thing is of course is insurance coverage and speedo error
AnswerID: 521108

Follow Up By: Member - KeithB - Friday, Nov 08, 2013 at 13:25

Friday, Nov 08, 2013 at 13:25
Polaris has a nice little heads-up GPS speedo that I am going to put on mine. Saves doing maths all the time.Polaris heads up display
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Reply By: greywiki - Friday, Nov 08, 2013 at 18:40

Friday, Nov 08, 2013 at 18:40
Diameter of tyre equates to distanced travelled per revolution, your 804 diameter tyres carry you 2526mm per revolution where your 841 diameter tyres will carry you 2642mm per revolution. The larger diameter tyres are fine on nice on long straight roads as it gives a higher ratio drive the same higher ratio drive also applies going uphill which means the engine has to work that little bit harder. Conversely if you put smaller tyres on you lower the drive ratio which is fine going uphill but on the level you need more revolutions to maintain the same speed. Basically larger tyres gives you slightly better ground clearance and worse fuel economy in hilly country, so no major gains only more expensive tyres.
AnswerID: 521138

Reply By: mynance - Saturday, Nov 09, 2013 at 09:14

Saturday, Nov 09, 2013 at 09:14
About 2 litres extra on mine going from the same size HT to AT BFG.
Would be a lot worse if I increased the tyre size as my car is an auto and would not lock up the torque converter till a higher speed was reached.
New car has 18 inch rims so will have to buy some new 17 inch rims when I put off road tyres on.

Myles
AnswerID: 521156

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