Larger tyre less torque?
Submitted: Sunday, May 25, 2008 at 16:55
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jolo
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.
Kel
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