Gear ratio

[dloughrey]

I ordered my new truck with the 20" wheels and a 3.42 rear. Anyone know what the effective gear ratio will be with the 20" wheels?

Tires are LT 285/60R20E

 

[Hoefler]

The overall diameter of the tires should be the same no matter what the wheel diameter is. The effective ration should not be any different than any other stock size.

 

[cerberusiam]

The effective gear ratio is whatever is installed in the differnetials. Gear ratio is the difference between popellor shaft and axle shaft speed, it is not impacted by tire size.

 

[PC12Driver]

The effective gear ratio is whatever is installed in the differnetials. Gear ratio is the difference between popellor shaft and axle shaft speed, it is not impacted by tire size.

Not so. The term "effective gear ratio" references the perceived change in the final drive ratio by a tire diameter change. A quick Google search will come up with a bunch of Effective Gear Ratio calculators, and every one involves tire size.

But semantics really don't help the OP, do they?

Handy info would be that his 285/60-20 tires have a diameter of about 33.5"/622 revolutions-per-mile, and the standard 275/70-18 tires are around 33.2", for about 630 revolutions-per-mile. So like Hoefler says, it's a wash. Either one will make for seriously low cruising RPMs with 3.42s.

 

[cerberusiam]

Effective gear rato is a manufactured term to describe fun with numbers, nothing more. It is totally meaningless in this context. :-laf

Gear ratio, effective of ineffective, is about leverage and the only way to affect that is the difference between the proppelor shaft and axle shaft. The leverage doesn't change when tire size chnages, only distance traveled over time. Furthermore, a ratio requires 2 distinct points as the basis of calculation. Gear ratio, effective or ineffective, of a 3.42 differential with a 285/60/20 tire is 3.42, no way to calculate any more than what is there.

Gotta know where you been to find out where you are going.

 

[RustyJC]

Actually, larger diameter tires reduce the accelerative force applied to the road for a given propeller shaft input torque. This is a function of the distance from the center of the hub to the road, and the length of this lever arm increases as tire diameter increases. That (in addition to higher weight and rotational inertia) is why acceleration and braking performance are degraded with much larger tires and wheels. In terms of performance, the larger tires act like a higher (lower numerical) axle ratio.

To illustrate, torque equals force x distance. If I want to generate a given torque at the axle hub, the force I apply at the end of the lever decreases as the lever length increases. It works the same way for a constant torque at the axle hub being applied through the lever arm of the tire diameter; as the tire diameter (length of the lever arm) increases, the accelerative force applied from the tread to the road decreases.

The only way to get back to something approaching the initial accelerative force for a given propeller shaft torque is to go to a lower (higher numerical) axle ratio to yield a higher axle hub torque as tire diameter (lever arm length) increases, and you'll never get back the losses due to weight and rotational inertia increases.

Rusty

 

[PC12Driver]

Eh, it's only meaningless if you're not in circles that use it. Use that term in a group of off-roaders, for example, and there will be no question as to what you're talking about.

"When you tell somebody somethin', it depends on what part of the country you're standin' in... as to just how dumb you are."
-Bo Darville

 

[Wertles]

Just for comparison and discussion sake, my 06 had stock tires and 4.10's. When I put my 19.5's on, my effective ratio lowered to 3.89.

 

[NIsaacs]

Just for comparison and discussion sake, my 06 had stock tires and 4.10's. When I put my 19.5's on, my effective ratio lowered to 3.89.

And that was only in direct The OP's Aisin effective ratio in first (3.75) with 3.42's will be 12.82, 6th at 0.63 the effective ratio will be 2.15, then when you figure in the low range of the transfercase and torque converter....

Nick

 

[dloughrey]

WOW... thanks for all the information! Maybe a little too much information. But I do appreciate it.