Bigger Tyres - 265/70 tried on the 3 door

[PradoJim Member Find all posts View Profile Close PradoJim]

I changed my stock Grandtreks for BFG KO2 AT 265/70R17.

I did some fuel tests before and after. I figured that the only real practical fuel test was to fill it up to the brim, drive it and then fill it back up again (I didn't consider weighing the fuel "practical". So it was just a volume test).

I have found the car fuel use computer entertainment only. Once the big tyres went on, it was even more useless and not even good for comparative readings.

I won't bore you with the details of what I did to keep these tests as comparative as possible. I went to a lot of trouble, including filling up on the same side of the same bowser at the same garage (as just one example). All fuel use is based on the Google distance (which is pretty accurate) so the change in odo reading with tyres is irrelevant. I just put the odo in for interest.

I drove all tests with economy in mind, stuck to posted speed limits and used the GPS as a speedo, not the car's.


Expressway test. Car fully warmed up before refuel at service centre on the M4. A/C off. Drive at 100 kph west on the M4 (110 was too difficult to maintain consistently), take an offramp and drive back east, offramp and return west to the same garage. Total distance on the fixed route 53.0

Standard tyres at 32 psi (placard figure + 10 %- a realistic everyday use figure):

Fixed route distance by Google 53.0 klm. Odo read 51.8. Actual fuel consumption 6.32/100, 44.89 mpg

BFG KO2's 265/70R17 also at 32 psi:

Odo read 50.0. Actual fuel consumption 9.21/100, 30.82 mpg (!)

This figure was so bad, I had to try it again. Some increase in fuel use is expected even with same size tyres due the more aggressive tread pattern, but not this much. This time I upped the pressure to an unrealistic (I thought) 45 psi.

Odo read 49.9. Actual fuel consumption 6.98/100, 40.65 mpg.

Country Drive Test: This next test involved a 173.3 klm loop on very quiet, narrow, undulating extra urban roads where the speed limit varied a lot between 60, 80, 90 and 100 kph. About 33 klm of it was also expressway.

The fuel use was so bad with the BFG's at 32 psi that I stuck with the 45 psi. Once again, the car was fully warmed up prior to the refuel.

Standard tyres at 32 psi:

Fixed route distance by Google 173.3 klm. Actual fuel consumption 6.25/100, 45.41 mpg

BFG KO2's 265/70R17 at 45 psi:

Odo read 165.6. Actual fuel consumption 6.64/100, 42.72 mpg

Interestingly, I did the "4 psi rule" test. After an hour of driving I tested the pressure. Cold 45 psi, hot 50 psi. By the 4 psi rule, I'm slightly underinflated. Unfortunately though the ride at 45 psi is firm. The car almost vibrates over the small irregularities on our dismal back roads.

There is no doubt that the car is working harder. I figured that the given the 3 door SX is nearly a quarter tonne lighter than the 5 door but with the same gear train, it might even like the slightly higher ratio (rpm at 100 kph drops from about 1980 to about 1920). You're within 10% of stock fuel use at the higher pressures but the ride is not nice. Also, I will be towing a van of around 2.6 tonne and I think my fuel use will be disproportionately worse.

Expensive experiment, but someone's going to win when I market week old BFG KO2's with less than 500 klm on them.

PS They're actually quieter than my half worn Grandtreks recording a consistent 68 - 69 dba at 110 kph vs 69 - 70.

[GUV Junior Member Find all posts View Profile Close GUV]

If that is 6.25 litres per 100 km you must have the hybrid Prado, even your 9.21 LP 100 would make 99.9% of Prado owners envious.
It'll be a fun ride when you hook up the 2.6 t on the back, really, put the calculator away and go enjoy your holiday.

[BiLLz0r Advanced Member Find all posts View Profile Close BiLLz0r]

I was thinking the same thing. Those figures don't add up. There's no way 70 profile tyres (arguably the most common upgrade) made that much difference.

[PradoJim Member Find all posts View Profile Close PradoJim]

Simple. That's the amount of fuel that went in after driving straight west on an expressway and turning around (no lights, offramp, cross the M4, onramp) and coming back and parking up at the same side of the same bowser. I've done this test dozens of times on different vehicles. It cannot be wrong in any significant way.

No, that economy is not unrealistic. I'm driving economically under flat straight near perfect conditions. It's also a 3 door which is 10% lighter. No aircon, no baggage, 1 driver. It's also not the best economy that I've achieved. It's also nowhere near the worst.

Slightly larger tyres on another 4wd I had also made a big difference, returned economy on this same run- Stock tyres 43.84 mpg, BFG's of a larger size at the same pressures 34.34 mpg. In that case I found that I needed to run the larger tyres at much higher pressures to get better economy. Very similar to what I found with the Prado. So the figures make perfect sense (albeit disappointing). I air down when I get where I'm going offroad.

Thanks Guv, I will.

[Richoson Avid PP Poster! Find all posts View Profile Close Richoson]

I also found the difference with the larger tyres to be around 1.5 - 2lts/100, albeit a more aggressive pattern as well. Interestingly I recently put Goodyear Duratracs on after MT ATZ 4 ribs and noticed a further increase in consumption by about 05.-1lt/100, these have a squarer profile and more aggressive tread.

I kept the same pressure for bitumen at 36psi cold as I find this to be the best compromise between ride and economy.

See ya on the tracks, Richo.

[PradoJim Member Find all posts View Profile Close PradoJim]

Thanks Richo. I was wondering if I should try some compromise pressures before I throw these on Gumtree. I'll give 36 a go. I can wear a bit more consumption but how much is that 13 mm extra under the diff worth ? (I'll know next time I ground out under the sump guard).

Cheers.

If anyone doubts my math, here's what actually went in the tank. Please remember, this is idealised economy, you won't achieve this during normal use. It's just to give a comparative baseline:

[brogers Advanced Member Find all posts View Profile Close brogers]

i'd try around 40psi personally

[brogers Advanced Member Find all posts View Profile Close brogers]

Thanks Richo. I was wondering if I should try some compromise pressures before I throw these on Gumtree. I'll give 36 a go. I can wear a bit more consumption but how much is that 13 mm extra under the diff worth ? (I'll know next time I ground out under the sump guard).

Cheers.

If you want to check my math, here's what actually went in the tank. Please remember, this is idealised economy, you won't achieve this during normal use. It's just to give a comparative baseline:

32 psi....honestly, I'd run slightly higher than that on "big gibber" gravel roads. That pressure is way to low for highway driving.
despite what tyre placards might say, 32psi is too low for those tyres on that car.
In fact 36psi is still too low.

[PradoJim Member Find all posts View Profile Close PradoJim]

Well, the fuel tests certainly show that. I just used 32 to standardise with the stock tyre and it was obviously way too low. My other car is much lighter, this is my first experience with a "heavy" 4wd. I'll throw 40 into the mix and give that a go as well.

Thanks.

[PradoJim Member Find all posts View Profile Close PradoJim]

Delete - post made in error.

[Spilsy Avid PP Poster! Find all posts View Profile Close Spilsy]

Arguably, the distance travelled (and therefore the fuel consumed) are too little to give truly meaningful results. Unfortunatley though, its a bit difficult to put in a 1000km trip, twice, to check fuel consumption...
It is an interesting read. As others have suggested, something in the 38-40PSI range would probably be about right.
My 5 door is giving me 11.27L/100km since May last year (actual litres and actual distance as calculated through my lease). The screen in the car goves me about 10L/100km.