tyre size

Submitted: Sunday, Jun 27, 2004 at 13:51
ThreadID: 14180 Views:2387 Replies:5 FollowUps:5
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Im currently running 265/75/16 BFG ATs on my patrol, want to also get a set of MTs but am considering upping size to 285s...what problems could i have if any or is there anything i need to know.
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Reply By: Member Eric - Sunday, Jun 27, 2004 at 14:09

Sunday, Jun 27, 2004 at 14:09
speedo will be out , If the vehicle is at standard hight , might rub
AnswerID: 65336

Follow Up By: Brian - Sunday, Jun 27, 2004 at 16:09

Sunday, Jun 27, 2004 at 16:09
The Speedo is actually more accurate! and the tyres don't rub!
The main difference that I found (4.2td Gu Wagon) was the change in gearing .
I went from 265/70-R16 to 285/75-R16 and found that the change in gearing was very noticable,especially when towing. The other difference was a loss in fuel economy mine was 1 km/l less!!!!
If I was to do it over again I would go no larger than 265/75-R16

Just my experiences!!

FollowupID: 326348

Follow Up By: gonebush - Sunday, Jun 27, 2004 at 16:43

Sunday, Jun 27, 2004 at 16:43

Are you sure that your fuel economy was actually 1km/l less because with the larger tyres the speedo will read about 7% less km travelled .........giving the appearance that your fuel economy dropped dramatically. e.g. say you were getting 8km/l with the smaller tyres you would 'appear' to get about 7.4 km/l with the bigger tyres. If after fitting the bigger tyres you were then getting say 7km/l, the actual drop in economy would be 0.4km/l.

When I went to the bigger tyres I actually lost a very small amount of economy that can also be partly due to a more aggressive tread pattern (Cooper STs)

FollowupID: 326354

Follow Up By: Brian - Sunday, Jun 27, 2004 at 17:00

Sunday, Jun 27, 2004 at 17:00
I would have thought that it would be 7% more not less?????

I have checked it against the Navara on same trips before and after tyre change and the economy has droped coupled with the earlier gear changes I still think that the economy has droped!

FollowupID: 326357

Reply By: gonebush - Sunday, Jun 27, 2004 at 14:12

Sunday, Jun 27, 2004 at 14:12
285/75s will bring your speedo reading and actual speed to almost the same. With 265/75s your speedo is reading about 7% above actual i.e. speedo reads 100kph and actual speed is about 93kph.

This will also similarly affect the kms travelled and trip meters

Ensure that you notify your insurance coy and they records the change in tyre size.
AnswerID: 65338

Reply By: lralfi - Sunday, Jun 27, 2004 at 18:17

Sunday, Jun 27, 2004 at 18:17
what is the reason for all these tyre-types. I use only 100 % height MT's like 8.25/16, 9.00R16, 255 x100. These tyres are a little less wide than a 285 or 265 but they are higher. That is an advantage in Mud and esp. in soft Sand.

In the moment I prefer Michelin XZL, XS and Pirelli Dakar.

If you increase the Tyre size, the following can happen. The speedo will show you a wrong speed (slower) the Tyres can hit the body or the frame of the car. The gears become longer, you loose torque and you crawling speed increases. Another problem can be that your axles brake earlier, esp. if you use Difflocks. This is known for older LR and Jeeps, also for Toyota LC but not typical. The brakes loose power.

AnswerID: 65393

Reply By: Phil G - Sunday, Jun 27, 2004 at 20:41

Sunday, Jun 27, 2004 at 20:41
Just to add that your brakes will be 7% less effective.

The increased fuel consumption can be due to the heavier tyres, wider tread and increased rolling resistance with the LT tyres. My MTRs are each 6kgs heavier than grandtreks, meaning I'm turning around 24kgs more tyre. My fuel consumption is about 8% increased (GPS corrected) when I went from 265/70 Grandtreks to 265/75 MTR.
AnswerID: 65416

Reply By: Nigel (WA) - Thursday, Jul 01, 2004 at 18:57

Thursday, Jul 01, 2004 at 18:57
Hi all,

Well i read alot about what you guys are saying, but i got a point to argue here a set of 265/75/16 BFG ATs and 285/75/16 BFG ATs should not affect the speedo, why you ask, well all that is different is the width of the tyre not the height of the tyre, your wheel shouldn't be covering greater distance and effecting you speedo.

Sorry can some tell me, how the width of a tyre can effect a speedo

AnswerID: 66004

Follow Up By: Muddy 'doe (SA) - Thursday, Jul 01, 2004 at 20:33

Thursday, Jul 01, 2004 at 20:33
The 75 refers to the height as a percentage of the width. 75% of 285 is more than 75% of 265!

FollowupID: 326866

Follow Up By: Member - Rohan K - Friday, Jul 02, 2004 at 21:16

Friday, Jul 02, 2004 at 21:16
Ergo, as you increase width, you increase overall diameter (2x2 radius) so the tyre is taller/higher, whatever you want to call it.

To calculate overall diameter use:

(width mm x profile % x 2) + rim mm
(265 x .75 x 2) + 406.4 = 803.9 mm diameter
(285 x .75 x 2) + 406.4 = 833.9 mm diameter

Percentage change is:

(833.9 - 803.9) / 803.9 x 100 = % change
33/803.9 x 100 = 4.1% larger diameter.
FollowupID: 327037

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