Larger tyre less torque?

Submitted: Sunday, May 25, 2008 at 16:55
ThreadID: 57982 Views:3506 Replies:4 FollowUps:2
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I need to replace my original Wrangler tyres (only 40,000km) on my Auto diesel Pathfinder and am thinking of an LT A/T tyre for towing a van and also our proposed trip up north & west next year (if I can afford fuel). So far I have only found a 265/70/R17 A/T tyre in LT construction and was told I could expect some loss in performance (ie torque) and fuel economy because of the larger diam tyre (approx 40mm extra). Does anyone have comments/experience with this and would it be noticable, especially if towing or in hilly country. I think there may be a 245/70/R17 in some brands but do not know if this is LT. Is an LT construction recommended or is standard A/T OK if not planning serious offroad work. I tow a 16ft poptop with extras (about 1600kg total). Apart from our planned trip, most of the driving is on bitumen or good gravel roads. Next question is of course what brand of tyre. There are many adverse comments on Cooper tyres in forums and most seem to prefer BFG's. Are Toyo any good as I have heard they are underrated? I need something that would be OK for my frequent trips over the Black Spur to Melbourne and would not be too noisy on bitumen as well as suitable for an outback trip with not much rough-road planned.

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Reply By: CRD-01 - Sunday, May 25, 2008 at 17:41

Sunday, May 25, 2008 at 17:41
Remember your engine is normally driving your wheels, not the opposite.
The vehicle's gearing hinges on the tyres. Gear ratios in the differentials, and in the gearbox and transfer case are selected to provide, adequate take-off torque at the wheels in the first gear high, and adequate cruising speed with suitably low revs, at 100km/h in fifth, while maintaining enough torque to nudge up a gentle incline. If you increase the diameter of your tyres, however there will be less torque at the wheels in every gear. Brakes are designed to operate on fixed rolling diameter. Increase the tyre diameter and the brakes are forced to work harder in and emergency stop. And even the vehicles Speedo will be affected.

Taller rubber lifts the lot, diffs included

But now, the good news! Taller tyres are far better at rolling over obstacles, and they also boost ground clearance. You will achieve an increase in ground clearance equivalent to half the increase in tyre diameter (i.e., the tyre radius), without having to touch a spring or a shocker. The best thing about this clearance is that even the axle tubes and diff housings get it. Compare that to lifting the suspension, the body goes up, but the diff and axles retain their original ground clearance.

Further, taller tyres not only boost ground clearance, but also approach, departure and ramp angles. Result? Fewer scrapes and graunches in really rough broken terrain. The downside of this clearance upgrade is that the centre of mass (a.k.a. the centre of gravity) goes higher. Result? More likelihood of a rollover seeing as how the vehicle is easier to tip over. ‘More likelihood' doesn't mean ‘ deadest dangerous' it's a question of degrees.

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Reply By: Nomadic Navara - Sunday, May 25, 2008 at 18:52

Sunday, May 25, 2008 at 18:52
Torque is the force produced at standard distance from the centre of a shaft. If you measure the force at a radius greater than the standard measuring distance you will measure less force for the same torque figure. With your bigger tyres you will not change the torque produced but you will have less tractive effort.

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Reply By: Member - Doug T (FNQ) - Sunday, May 25, 2008 at 20:23

Sunday, May 25, 2008 at 20:23
Your vehicle has a gear box , use it , the vehicle has no feelings , it's all in your mind , and yes you will notice a small difference , as for the 40mm , what's the law state about that, I read something along those line a few weeks ago,

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Follow Up By: Member - Doug T (FNQ) - Sunday, May 25, 2008 at 20:56

Sunday, May 25, 2008 at 20:56
The rim diameter may be varied from the standard size
but the overall diameter of the tyre must not vary by
more than +15mm or -26mm.
These limits have been set for a number of reasons.
Varying the overall diameter of tyres affects ground
clearance, centre of gravity, brake effectiveness, steering
geometry, performance and speedometer accuracy.
Generally, to meet these limits, 60 series tyres are fitted
to rims with a diameter 26mm larger than standard and
50 series tyres are fitted to rims with a diameter 50mm
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Follow Up By: StormyKnight - Tuesday, May 27, 2008 at 22:06

Tuesday, May 27, 2008 at 22:06
Yep its a darn silly rule for 4WDs but I still have the engineering certificate required in NSW so that I can run 265/75R16s on my Prado (1 size up from standard 265/70R16s)

The new tyre is 3.4% larger than original. There is less acceleration & usually worse economy but don't forget your economy is going to change from a road bias tyre to an AT or MT anyway, any increase in tyre size I believe is not as significant although, more fuel will be used to accelerate the car, but at the same time, the car will be reving less at cruise. Depending on the car this may be an advantage.

By the be legal in NSW at least, your speedo cannot underread the true speed of the vehicle.....

You need to work thru the pros & cons of going to bigger tyres before you head down to the tyre place.

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Reply By: stevie1947 - Sunday, May 25, 2008 at 20:40

Sunday, May 25, 2008 at 20:40
In my opinion Cooper LT A/T tyres are great. I run them on my 2002 Pathy and this is my 2nd set after having done 90,000 ks on the first. I tow a 18' caravan and have just returned from a 12 month trip around OZ and never even had a flat. I do a fair amount of off road (not real hard stuff) and never had any problems.
The only gripe is that they pick up small stones but I suspect that this would happen with all off road tyres.
Bloody good tyres.
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