285's may be a bit big.

Submitted: Tuesday, Jul 29, 2003 at 23:24
ThreadID: 6253 Views:1965 Replies:13 FollowUps:13
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Swapped the 265/70's for 285/75's on my GU and the difference is easy to feel. Far from the "barely noticable" impression that I gained form reading past posts. The rev range changes at 50, 60 and 80 km/h have all meant changing the gears that can be held comfortably. Whether this will result in any significantly different fuel usage is to be seen I guess. They look great and have definitely lifted the vehicle but I wonder if the half way step of 265/75's may have been a better choice?
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Reply By: Andrew(WA) - Wednesday, Jul 30, 2003 at 00:09

Wednesday, Jul 30, 2003 at 00:09
Hilly

keep us posted on the fuel usage side of things. I will be interested in knowing the results.

What do you mean by "have all meant changing the gears that can be held comfortably"???

The way I read it, you mean you don't change gear as much but I would have thought going up in tyre size would have meant same gear changes but at different speeds/revs??

cheers
Andy
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Follow Up By: Hilly - Wednesday, Jul 30, 2003 at 00:34

Wednesday, Jul 30, 2003 at 00:34
For example I meant that where you previously coasted a wide corner in third at about 30 -40 kmh, on the 265/70's you would be able to accelerate away still in third because the engine revs were still high enough to do so. On the bigger tyres, although the ground speed is the same, (real speed not speedo) the engine revs are lower and you need to change down to second. Same thing with 4th gear and 60 kmh, any sort of small incline now needs a blat in third instead of holding 4th.

On the brighter side, 4th is better at higher actual ground speeds than before and 5th gear rpm is dropped for highway cruising.
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Follow Up By: Andrew(WA) - Wednesday, Jul 30, 2003 at 00:42

Wednesday, Jul 30, 2003 at 00:42
Do you know what your RPM's are at 'true' 110km/h in 5th

The standard tyres, 265/70's, at 'speedo' 110km/h on mine is 2750rpm

Can you give any comparisons???

cheers
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Follow Up By: Hilly - Wednesday, Jul 30, 2003 at 00:46

Wednesday, Jul 30, 2003 at 00:46
I'll check them for you tomorrow versus my GPS and post back then.
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Follow Up By: Waynepd (NSW) - Wednesday, Jul 30, 2003 at 11:13

Wednesday, Jul 30, 2003 at 11:13
Hi Guys,
I have Cooper s/t 265/75/16
at 110km/h i am around 2500rpm by my tachoCheers
waynepd
%
%:-)
%
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Follow Up By: Hilly - Wednesday, Jul 30, 2003 at 14:56

Wednesday, Jul 30, 2003 at 14:56
Here we go 265/70 285/75
4th gear 70kmh 1800 1750
80 2150 2000
90 2400 2250
100 2500 2300

5th gear 90 2150 1900
100 2300 2150
110 2600 2350

These figures reflect the fact that my speedo was correct vs GPS with the 265/70's and now reads about 8% low. Actual speed is higher than indicated. My daughters note taking also meant these figures were a little awkward to decipher but should give a reasonable approximation. Wow she can scream.

I think Darren worded it better than I did in his explanation which is why I work for an Oil company instead of writing for Playboy.
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Reply By: joshinthecity - Wednesday, Jul 30, 2003 at 00:39

Wednesday, Jul 30, 2003 at 00:39
Hilly,
What is the RPM at 110 now? I am thinking of fitting some 285/75/16 Cooper ST's to my GUIII with this being of real interest to me as I do alot of high speed running between offroad adventures

Josh.
AnswerID: 26303

Follow Up By: Hilly - Wednesday, Jul 30, 2003 at 00:46

Wednesday, Jul 30, 2003 at 00:46
Same as for Andy, Josh. I'll post them tomorrow.
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Reply By: joshinthecity - Wednesday, Jul 30, 2003 at 00:50

Wednesday, Jul 30, 2003 at 00:50
Personally Hilly,
I'm not nessisarily interested in the GPS reading, more a case of .. what was it, and what is it now would be fine for me.

Josh
AnswerID: 26304

Follow Up By: Hilly - Wednesday, Jul 30, 2003 at 01:05

Wednesday, Jul 30, 2003 at 01:05
The speedo and tacho say the same revs and speed as they did before Josh. I need to use the GPS to get a true ground speed of 110 ...which should be reading less on my speedo... and see what the revs are then. Prior to the swap the instruments matched perfectly.
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Reply By: joshinthecity - Wednesday, Jul 30, 2003 at 01:43

Wednesday, Jul 30, 2003 at 01:43
That's very surprising Hilly,
I would have thought that with bigger diameter tyres, you would be pulling less RPM for the same speed on the speedo ( regardless of actuall road speed )
eg: Car was showing 2900RPM and 110kph.
Car now showing 2750RPM and 110kph.

Josh.

AnswerID: 26309

Follow Up By: Steve from Drive Systems Victoria - Wednesday, Jul 30, 2003 at 08:39

Wednesday, Jul 30, 2003 at 08:39
What you have to get your head around, old son, is that the tacho(engine revs) is still connected to the speedo(tailshaft revs), nothing has been altered between them.
The relationship between the speedo and the wheel speed(ground speed) is what has been altered. Comprendi??
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Follow Up By: Member - Bonz (Vic) - Wednesday, Jul 30, 2003 at 20:16

Wednesday, Jul 30, 2003 at 20:16
Laughing all the way to the ARB outlet, yes josh, nothing would change tacho to speedo,only the wallopers would see a difference in theor speed gun if you use the speedo to measure your velocity

all the bestSo many places to go!
So much work to do :0(
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Reply By: joshinthecity - Wednesday, Jul 30, 2003 at 01:47

Wednesday, Jul 30, 2003 at 01:47
From your original post:
"The rev range changes at 50, 60 and 80 km/h have all meant changing the gears that can be held comfortably."

I would have assumed that if there are rev changes at 50, 60 and 80, there would have been an incrementally larger one for 110 ( or at least some change )

Josh * correct me if I'm wrong...*
AnswerID: 26310

Reply By: joshinthecity - Wednesday, Jul 30, 2003 at 01:58

Wednesday, Jul 30, 2003 at 01:58
Ok, I have found the source of Andy and my confusion I think. Below are quotes from your above posts.

"5th gear RPM is dropped for highway crusing"
then..
"The speedo and tacho say the same revs and speed as they did before Josh"

I'm going to have to send this in to Rove as a "what the?" Hilly.

I've got a headache thanks to you and am going to have to have a Becks and a good lie down. Talk to you boys in the morning.

Josh * when in doubt.. beer. *

AnswerID: 26311

Follow Up By: stillthinkinaboutit - Wednesday, Jul 30, 2003 at 08:41

Wednesday, Jul 30, 2003 at 08:41
Josh,

This might help,

The tacho is reading engine RPM and the speedo reading is derived from tailshaft speed. Nothing has changed here.
The real difference is that the diameter of the tyres has increased.
This alters the distance travelled per revolution of the tyre, by an amount of around 10% I believe for these tyres.
( Same RPM but greater distance is travelled.)
This means that the speed indicated on the speedo is incorrect, hence the need to verify with a GPS.
By the way, most speedometers read higher than actual road speed, usually between 3 - 10%. The odometers are usually spot on. What you may find now is that the speedo is reading very close to road speed, but the odometer is now out by about 10%.

I am also looking at the 285/75/16's for my GU1 4500, so very interested in the comments about driveability,low range gearing is not the best now, it will get worse with 285/75/16's fitted. May need to fit rockhopper gears or change diff ratio's.

Regards,
Mark
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Reply By: DARREN - Wednesday, Jul 30, 2003 at 08:39

Wednesday, Jul 30, 2003 at 08:39
I got 285/75/16 coopers on my gu from new a few months ago and (whilst I do not have standard tyres to compare with) the reduced rpm at 110 on the freeway can only be a good thing however i must say driving to work each day the tyres do make 60 kmh a bit awkward i.e. revvy in third but almost lugging in 4th. 4th gear really likes to sit closer to 70kmh. Something to think about but no great hassle and otherwise really happy with them.
AnswerID: 26319

Reply By: Member - Russell - Wednesday, Jul 30, 2003 at 09:02

Wednesday, Jul 30, 2003 at 09:02
I'm not going to be of any help whatever on the 285/75 front, but you did wonder about the 265/75 and I can provide some info on that. I just put a set (Goodyear Wrangler MTR) on the Prado and then compared it with our Falcon. At 95 on the Prado, the Falcon was doing 103, which is actually 100 km/h in real road speed. That's near enough for me. I used to have 265/70 BFG AT, and they seemed to give a speedo reading that was pretty close to actual road speed, so there seems to be about 5% difference. Might check with the GPS on Saturday.Russell S
Prado RV6
AnswerID: 26323

Reply By: Member - Bill- Wednesday, Jul 30, 2003 at 11:58

Wednesday, Jul 30, 2003 at 11:58
Hilly,

I went the 265/75 route a couple of months back, admittedly, I've got an Auto (GUIII 3.0Di) but did notice a definite difference from the 265/70's in the urban cycle (acceleration, braking) but not too pronounced and certainly liveable. They seem a good compromise for 'round town and off-road touring. I looked at the 285's briefly but came to the conclusion that most blokes that go for them are on the more extreme end of the user band (no offence intended) and go for mud patterns also. They don't tend to really care about everyday driveability and noise etc. You will also find a lot of 285 drivers go for the Ditronic to increase power to compensate for loss of performance due to gearing.Regds

Bill
AnswerID: 26334

Follow Up By: just - Wednesday, Jul 30, 2003 at 12:19

Wednesday, Jul 30, 2003 at 12:19
Bill, What's a "Ditronic"?
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Follow Up By: Member - Bill- Wednesday, Jul 30, 2003 at 14:26

Wednesday, Jul 30, 2003 at 14:26
The power up chip, increases HP and torque by about 20%Regds

Bill
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Follow Up By: just - Thursday, Jul 31, 2003 at 13:26

Thursday, Jul 31, 2003 at 13:26
Wow! where do I get such a chip and how much? What's the down side to them?
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Reply By: just - Wednesday, Jul 30, 2003 at 12:15

Wednesday, Jul 30, 2003 at 12:15
For those mathematically oriented there is a 6.77% increase in circumference between the 265/70 and 285/75 which can be used to translate to power and speed. Similarly the difference between 266/75 and 285/75 is 3.4%.
I'm toying up going up to the 285s and with my auto trans and torque converter the difference should not be too noticeable - I hope!
AnswerID: 26335

Reply By: simon - Wednesday, Jul 30, 2003 at 15:05

Wednesday, Jul 30, 2003 at 15:05
I run 305/70/16 procomps my fuel econ is normally average around 13L/100km best was 10.5L/100km
so running 285/75 should make bugger all difference
note i have a 4.2l turbo int
AnswerID: 26353

Reply By: diamond (bendigo) - Wednesday, Jul 30, 2003 at 22:46

Wednesday, Jul 30, 2003 at 22:46
with outsounding like a stick in the mud.
in most australian states that size tyre requires engineers certificatenew job coming so no fraser this time:-((
AnswerID: 26404

Reply By: diamond (bendigo) - Wednesday, Jul 30, 2003 at 22:46

Wednesday, Jul 30, 2003 at 22:46
with outsounding like a stick in the mud.
in most australian states that size tyre requires engineers certificate
ask your insurernew job coming so no fraser this time:-((
AnswerID: 26405

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