Tyre Size vs Gearing

Submitted: Sunday, May 02, 2010 at 21:29
ThreadID: 78173 Views:3211 Replies:6 FollowUps:1
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Hi All

I have a 2004 Nissan GU Patrol with standard Bridgestone 693's 165/75/16LT and need to replace soon. Thinking about going to taller tyre 185/75/16LT as my speed is incorrect according to Garmin which says I am actually traveling at 94km/ph when speedo says 100km/ph.

Going from tyre OD of 808 mm to 838 mm.

1) Will that size tyre marginally or grossly affect my gearing? I currently tow a 750kg trailer and with partner and four kids along, can take most hills in 5th gear at 100km/ph.

Thanks in advance
Rob and Donna
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Reply By: Madfisher - Sunday, May 02, 2010 at 21:36

Sunday, May 02, 2010 at 21:36
Rob if you want your gearbox to last you should be changing down to 4th on hills anyway, overdrive gears are not ment for heavy lugging.
cheers Pete
AnswerID: 415148

Reply By: Member - Roachie (SA) - Sunday, May 02, 2010 at 21:43

Sunday, May 02, 2010 at 21:43
I presume you mean "265" and "285" respectively as "165" and "185" would be very skinny tyres...hahaha

Having said that, if your rig can cope with hills in 5th gear (regardless of whether it is a good idea or not to actually tackle them in 5th gear), then the additional gearing impost created by the the larger tyre will not cause you much discernable difference.
AnswerID: 415149

Reply By: ian zzr - Sunday, May 02, 2010 at 21:58

Sunday, May 02, 2010 at 21:58
hi there, you are pretty well right gu speed reads 100km but only 95 on my gps with 265 size tyres i also run 285 mt"s and speedo is then same as gps . I do notice a difference on hills but not to bad . My rig 4.2td 5 on board .
AnswerID: 415152

Reply By: Member - Alastair D (NSW) - Monday, May 03, 2010 at 07:41

Monday, May 03, 2010 at 07:41
The rolling diameter is different to the nominal OD but the change will be similar. Ratio of ODs is 1.04 ie 4% bigger. Speedo will now read 97.5. I would think that you will feel the difference in the gearing but not a big change.
AnswerID: 415174

Reply By: westozal - Monday, May 03, 2010 at 20:48

Monday, May 03, 2010 at 20:48
Hi,
I had a 3ltr St Patrol man 2004 model running 285/75/16 Mud terrain Hankook tyres.
The vehicle had no problem powerwise with the tyres and I also towed a caravan with this set up.
The man Patrol already runs reasonable rpm at highway speeds if I remember about 2700 at 110kmph in 5th so does not suffer from a loss of power.
My previous vehicle was a 1996 auto 80 series cruiser which was quite doughy running the 285's as in over drive I found it did not have enough rpm's on board especially towing the same caravan.
The Nissan was better tower of the two and more economical to boot even with Pet/gas conversion.
In my opinion you can run 285's no problem.
The only problem I had with the Nissan were the spooky engine stories and potential expense which is why I sold it in the end.
I now tow with an Isuzu Dmax dual cab 4x4 whch is better again than either the above with more fuel efficency. However not as good offroad. Again just my opinions.
Regards Alan.
AnswerID: 415262

Follow Up By: westozal - Monday, May 03, 2010 at 22:08

Monday, May 03, 2010 at 22:08
Hi,
I wrote my reply in a Hurry before,it was the 80 series that had the pet/gas on it.
Alan
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FollowupID: 685452

Reply By: OREJAP - Tuesday, May 04, 2010 at 09:16

Tuesday, May 04, 2010 at 09:16
I had a GU,same as yours with the 265's & my speedo indicated the same. changed to BFG T/A's 285's without any dramas to rev's or gearing. Speedo also read accurately. The BFG T/A IMHO is still one of, if not the best tyre for 4WD's on the market.
AnswerID: 415304

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