Another Gear Ratio Question

[glundblad]

Hypothetically, suppose I had two identical '87's with 4.0/5speed. One had 3.08 gears and the other had 4.10. At 60 mph, the 3.08 would run at 2200 rpm in 4th gear and the 4.10 would run at 2200 rpm in 5th gear.

 

Are they both generating the same torque?

 

I realize off the line, the 4.10 would be much faster (except for the fact that I would need to let off the accelerator all the time to shift) but I was curious about highway driving. My feeling is 2200 rpm is 2200 rpm regardless of whether it is the axles or the transmission causing the gearing.

 

Any thoughts?

[mvusse]

Assuming the rpms are the same, as you state, yes, you'd have the same amount of torque.

[dasbulliwagen]

Same amount of torque at the engine, but the amount of torque available at the wheels would be more with the 4:10 due to the gearing difference... it actually should be more torque at the wheels in all gears just given the engine will have to turn more times to make up the same distance... does that make sense?

[jeepcoma]

60 mph is 60 mph, doesn't matter what gearing you have. It takes a certain amount of power applies to the wheels to overcome the various frictions at that speed, regardless of how you're making or transferring that power. So yes, torque at the wheels will be the same, at any constant speed. A more interesting question is which scenario would accelerate quicker, for that you'd need to know the 4th gear and 5th gear ratio.

 

I'd rather be running 4th gear at 2200 rpm than 5th gear at 2200 rpm if you're routinely accelerating hard at this speed or towing, mountains, stuff like that. 4th as a 1:1 can handle more torque without breaking, any overdrive gear puts a lot more stress on the transmission (think how much stress it puts on your legs when you try to bicycle at a slow speed in high gear).

[Eagle]

Same amount of torque at the engine, but the amount of torque available at the wheels would be more with the 4:10 due to the gearing difference... it actually should be more torque at the wheels in all gears just given the engine will have to turn more times to make up the same distance... does that make sense?

No ... not for this question. You are overlooking the fact that in one instance he's using 4th gear, in the other he's using 5th gear. If the speed is the same and the RPMs are the same, irrespective of how it got there the overall final drive ratio is identical and the rear wheel torque is identical.

[johnj92131]

Eagle is correct.

 

Your assumption is that we have the same engine rpm, at the same road speed, under both setups. That means that 4th gear and a 3.08 final drive is exactly equal to 5th gear and a 4.10 final drive. So, same over all gearing and the same engine rpm equals exactly the same results. Don't change the over all gears and put your foot to the floor and you will get EXACTLY the same results.

 

Comes down to one of Newtons Laws of Motion like F = M x V squared

 

JJ