Larger Tyres - better or worse fuel economy ?

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WayneMan
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Thu Sep 26, 2013 1:12 pm

I recently replaced my standard tyres of 245/75/15 with 31" Khumos on my 2005 KZTE. By my calculations, this equataes to about 1.5" taller tyre or 0.75" effective lift.
Because they are slightly larger, I would expect the fuel enonomy to be slighlty better because of the larger circumfrance, and also have slightly lower revs.
I also exprected the power to be a lot worse off.

My conclusion and findings:

1) the power loss is hardly noticable in town, but on the open road on the up hills it is a lot worse, sometimes having to drop to 4th or 3rd gear, depening on the speed limit, where as I would probably never have to have done it previously.
2) the revs have dropped ever so slightly, and probably the reason for the lack of power on the open road.
3) fuel economy seems to be my biggest problem. I honestly expected to have a better economy on the open road.

On a recent trip from Joburg to the Sentinel Peak in the Drakensberg, and back again I found that traveling at a constant 120km/h the economy was 13l per 100km. On the return trip I drove at a constant 110km/h and hte economy increased to about 13.75l/100km.

Before I got the new tyres, my economy was much better, in the region of around 11 ro 11.5 l per 100kms.

What is the reason for this higher fuel economy ? Any thought or suggestions ?
Should i be reverting back to the original size?
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Thu Sep 26, 2013 1:18 pm

The 31" tyres are about the equivalent of a 265/70/15 which is wider and heavier, resulting in higher rolling resistance, and therefore higher fuel consumption. Should not be too much higher though than with the 245's
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Thu Sep 26, 2013 1:22 pm

your engine has to work that much harder to turn your tires. eg on my 2.8d sfa i had 30 9.5's and i got 10 L 100km when i moved to 31's it went to 11.5/100km and now i have 31 muds and its at 12.5/13 L to 100km.

normal driving i would revert back, but if you lots of serious off roading the 31's are nice.
the question is are you ready to loose power and pay and extra R 25 per hundred km just for having bigger tyres :?:
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Thu Sep 26, 2013 1:24 pm

I also put 31" on my previour KZ.

I found that I got much better consumption and on the open road the slight drop in RPM made it a much nicer cruising speed.

I would suggest you look at an intercooler an chip. Mine had the SAC on and that made a huge improvement over the standard setup.
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WayneMan
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Thu Sep 26, 2013 1:57 pm

Ok, so i got 5 new tyres for sale then. done 9000kms.
KL 78 khumo.

Good tyres, but i think i need to go back to the original size. I will still buy the same KL78 Khumos

Make me an offer....
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Thu Sep 26, 2013 2:25 pm

Throw it over to the "for sale" thread and add some pics so that we can start twisting your arm.
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Thu Sep 26, 2013 2:36 pm

WayneMan wrote:I recently replaced my standard tyres of 245/75/15 with 31" Khumos on my 2005 KZTE. By my calculations, this equataes to about 1.5" taller tyre or 0.75" effective lift.
Because they are slightly larger, I would expect the fuel enonomy to be slighlty better because of the larger circumfrance, and also have slightly lower revs.
I also exprected the power to be a lot worse off.

My conclusion and findings:

1) the power loss is hardly noticable in town, but on the open road on the up hills it is a lot worse, sometimes having to drop to 4th or 3rd gear, depening on the speed limit, where as I would probably never have to have done it previously.
2) the revs have dropped ever so slightly, and probably the reason for the lack of power on the open road.
3) fuel economy seems to be my biggest problem. I honestly expected to have a better economy on the open road.

On a recent trip from Joburg to the Sentinel Peak in the Drakensberg, and back again I found that traveling at a constant 120km/h the economy was 13l per 100km. On the return trip I drove at a constant 110km/h and hte economy increased to about 13.75l/100km.

Before I got the new tyres, my economy was much better, in the region of around 11 ro 11.5 l per 100kms.

What is the reason for this higher fuel economy ? Any thought or suggestions ?
Should i be reverting back to the original size?
Keep in mind that the circumference of the tyre is longer. So it has to turn less times to complete the same distance than before. Now keep in mind that your speedometer was "calibrated" on the 245's so the odometer is not reading the same distance anymore. So if the speedometer shows that you now how have done 120km's it previously might have shown that you did 128km's.

So if you did not do you measurements with a GPS distance, you cannot compare the figures unless you use a correction factor.

So in short... with 245's your odometer showed you did 100km and when you fill up you used 10L
Now with the 265's you traveled the exact same distance but the odometer shows you only did 90km but you still used 10L
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WayneMan
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Thu Sep 26, 2013 2:47 pm

Thats a very valid point. Perhaps I must check the actual distance, and not rely the readings from my odometer.

Kepp up the good work lads :)

Cheers
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Thu Sep 26, 2013 3:42 pm

WayneMan wrote:Thats a very valid point. Perhaps I must check the actual distance, and not rely the readings from my odometer.

Kepp up the good work lads :)

Cheers
If you have a GPS, use it - the easiest way to confirm true distance :thumbup:
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Thu Sep 26, 2013 10:07 pm

i bought my truck with 31"s and changed to 33"s. i found fuel consumption to be the same but with a loss in power.
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Thu Sep 26, 2013 10:36 pm

Your economy will be more accurate because your size have been increased. So it means that your consumption was previously showing a better l/100km, but actually it was doing worse. Now with the bigger tyres, it brings your consumption to reality.
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WayneMan
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Fri Sep 27, 2013 8:47 am

Thanks for the input.....

I think I have figured out my mistake.

Originally I did my calculations based on my odometer vs the number of liters put into the tank.

I never took into account that larger tyres equates to less tyre revolutions, and hence the shorter distance on the odometer.

I logged onto maps.google.com and also checked my tracker system for actual kilomenters traveled, and then recalculated.

The figures were vastly different - it now calculates at about 11.5l/100 instead of the +- 13.5l/100

Makes lots of sense now, and I slept a lot better last ngiht too :)

Thanks gents !
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Fri Sep 27, 2013 9:03 am

:thumbup: :thumbup: :thumbup:
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WayneMan
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Fri Sep 27, 2013 3:36 pm

Anyone know if it is possible to calibrate you rodometer reading to be more accurate?

I heard there are tiny little gears in the cluster and these cannot be changed or modified ??
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CasKru
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Fri Sep 27, 2013 3:48 pm

WayneMan wrote:Anyone know if it is possible to calibrate you rodometer reading to be more accurate?

I heard there are tiny little gears in the cluster and these cannot be changed or modified ??
You can by speedo recalibration boxes that do the correction on the speedo.

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Mon Sep 30, 2013 8:33 am

WayneMan wrote:Anyone know if it is possible to calibrate you rodometer reading to be more accurate?

I heard there are tiny little gears in the cluster and these cannot be changed or modified ??
Uhm, if I understand this dicussion correctly, the bigger tyres cause a direct decrease in ODOMETER kilometers travelled, correct? Actually, fitting 31x10.5x15 tyres to a 2005-2008 Hilux Vigo model would cause the speedo to be almost spot on with GPS. Travelling at 120km/h on the odometer would be 120 on the GPS, whereas with the original tyres there is around a 10% difference in odo to GPS. To take it one step further, if 2 vehicles travel the exact same distance, one (1) with original OEM tyres on and one (2) with bigger tyres fitted (31x10.5x15), after vehicle (1) travelled 100 000km, vehicle (2) would be on 90 000km, although they travelled to exact same distance, the odometers would be different.

Thus, a person who fit 31x10.5x15 off the showroom floor would extend the 'shelf-life' of his vehicle by 10%? Is this a deliberate ploy by Toyota to reach the magical 90 000km trade-in mark 10% sooner than the "real" kilometers travelled, or am I missing the technical understanding of this?
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Mon Sep 30, 2013 8:44 am

No. All vehicles odometers over read by about 6%. The percentage also differs at different speeds. At 60km/h it could be 3% and at 100km/h it could be 6%.

It is more a safety factor in order to protect you
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Tue Oct 01, 2013 8:56 am

I agree with Cassie but what you are saying Dewald is actually correct. If you used the original tyres the Odo would be indicating more kms travelled than the vehicle actually did. It would be out by the average factor that your speedo over reads.
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Tue Oct 01, 2013 1:07 pm

Ek weet op ons Amarok's kan jy dit op computer regstel.....
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coert
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Tue Oct 01, 2013 3:30 pm

Thanks manne het self geworstel met die ene 30x9.5x15 of 31x10.15 gaan die 31's opsit. Goeie stukkie info die.
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Fri Oct 04, 2013 6:55 pm

:thumbup: thanks vir die waardevolle info. baie interresant
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