3.15 vs 3.55 gears in a 2022 automatic GT

[PoCoBob]

Using Wallace's calculators all things being equal tires weigh the same, there is no difference in power to the ground.

I assume that's from the Wallace Racing website? It makes no sense at all, I spent some time playing with this calculator this morning. The only changes that make any difference are the HP and Weight fields, change those and you get a difference result. Change any other field and nothing changes except RPM, I tried 2 shifts through 7 shifts, no change. I tired 26,30,32 and 34 inch tires, no change. I tried 4.56 rear gear and 3.55 rear gear, no change. I would say that calculator is totally useless.

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[K4fxd]

I would say that calculator is totally useless.

It may be.

 

[Jstang23]

The more important factor is rotating mass of the larger tire. If you take the stock 17 inch wheels and compare them to the OEM 19 inch performance pack wheels the 17 inch will make more power and perform better because it's smaller but they ~roughly~ weigh the same. But if you get some really nice race forged 19 inch wheels that weigh less than the 17 inch wheels the reduced rotating mass will actually outperform the 17 inch wheels in the real world (not on a dyno). One thing I've learned over the years is that horsepower is just a number and more things that I can count factor into how fast a car is.

 

[Cory S]

Axle torque is axle torque. Standing force applied to the ground at a stand still doesn’t change with tire diameter. Once the vehicle is moving however, a numerically higher ratio will indeed get an object moving quicker. There’s also diminishing returns once the final axle ratio is increased too high. On the average 3850-4150lb vehicle, going over 13:1 axle ratio will be slower to a given end point. The 10R80 cars prove this, with the matching of a 3.15 rear gear being the ideal ratio in most cases for acceleration. 9/10 times, a 2nd gear launch when equipped with a 3.55 gear, will be the quickest to the 660/1320’ mark. 2nd gear (2.98:1) launch puts the starting line ratio at 10.6:1 with 3.55’s.

 

[Jstang23]

This is exactly what happened with my CBR. Increased the ratio and increased the effective top speed. Stock gearing had the bike at too low of an RPM on the top end of 6th gear to over come wind resistance. The higher gear increased the RPM which put the bike back into its power range and allowed it to push past what was the previous top speed. In theory it should have reduced the top speed, but the change in RPM range and the reality of air resistance gave an effective higher top end and faster acceleration off the line. Not saying the same happens with a mustang, but I've seen it 1st hand on a bike.

For a 6 speed car/bike I can see how increasing the gear ratio might increase the effective top speed. But the 10 speeds never let the engine out of its top power band. So effectively, regardless of what gear ratio you have, the car will have enough power to overcome that aerodynamic drag. So like I've been saying the 3.15 will have the higher top speed. In fact on the top end 3.15 cars pull on 3.55 cars with the 10 speed (real world scenarios).

Also a huge factor that I think everyone is missing is the ratio of the transmission gears. The 6th gear on the manual in 3.15 is basically a useless overdrive gear, the 3.55 will allow it to be used more effectively (this I can agree on). The 10 speed is the great equalizer here, the fastest accelerating 10 speed NA cars are running 3.15s.

 

[K4fxd]

What is the best real world gear for the 6R80?

 

[Cory S]

What is the best real world gear for the 6R80?

Very weight, torque, and application dependent.
In most applications, 3.15-3.31 would be the ideal ratios when running a tire between 26.8-28.2” in diameter.

 

[Fred Larson]

I just bought a new 2022 GT that came with the 3:55 gears. The first few gears are extremely short. In fact, during light acceleration, the car routinely skips 2nd gear on its own, and I likely would not notice but for the digital display. I don't know if you get different ratios in the transmission, as it is still just a shade over 2,000 RPM at 75 on the interstate. With hindsight, if I was ordering a new car today with what I know now, I would get the 3:15, as I am not a street racer, nor will I be on the track.

 

[TeeLew]

The difference in performance will be marginal between 3.15 and 3.55. Ran two 21 GT autos one with 3.15 and the other 3.55 stock and the 3.55 was .08 second faster in quarter mile. If you went from 3.15 to 3.73 you would notice a large difference. Also the lower the gear the higher top speed, the higher the gear the faster acceleration. So, the question is what's important to you?

3.15 might be easier to control as its smoother with its power delivery, but you may find if autocrossing you'll be in between gears for power. Most people who build their cars for drag go with the 3.15 because its fewer gear changes.

3.15 will have much better gas milage on the highway, about the same in the city. Off the line the 3.55 will be faster, but it'll be harder to launch without spinning.

I'm with you until the autocross comment. Taller gears generally help AutoX, because of fewer shifts. Gearing might be pretty sporty with the auto and a 3.15. The problem on this front is that you can't get the Torsen differential with the 3.15. The Torsen is only an option in 3.31 gears on up. If the 3.31 is still an option, that might be a place to look? The gears on the Auto box are so close that the rear end gearing becomes a pretty marginal change. You're much more limited by tire grip than gearing on the launch. The difference becomes what RPM it puts you through the lights. In a stop-light grand prix, this doesn't really come into play.

 

[TeeLew]

Effective gear ratio.

Still the car with numerically higher gear ratio will have a mechanical advantage, even if the effective ratios are the same. If you have 100 Lb/Ft of torque and a 3.15 rear gear you put 315 Lb/Ft to the ground. Assuming 1 to 1 transmission ratio. If you now have a 3.55 gear it is 355 Lb/Ft to the ground.

Tire size only affects engine or drive shaft RPM.

That is the simple explanation.

Sure, but you can't just apply gear until you're fast. Acceleration on the launch is limited by grip, not gear. You can have 2 cars with different rear end ratios and tire sizes that effectively balance the drive force at the contact patch (which is our end result of engine/gearing). In that regard, tire size is an integral part of the gearing.

 

[IPOGT]

Keep in mind the top speed of the automatics are capped substantially lower than the Tremec 6 speed manual or the MT-82.

 

[TeeLew]

I just bought a new 2022 GT that came with the 3:55 gears. The first few gears are extremely short. In fact, during light acceleration, the car routinely skips 2nd gear on its own, and I likely would not notice but for the digital display. I don't know if you get different ratios in the transmission, as it is still just a shade over 2,000 RPM at 75 on the interstate. With hindsight, if I was ordering a new car today with what I know now, I would get the 3:15, as I am not a street racer, nor will I be on the track.

I have a 2020 HPP with a 6-speed, and I see the same thing. I shift 4 times getting to 50 mph. This is not necessary. I'm changing my car from a 3.55 to a 3.15.

 

[IPOGT]

What is the best real world gear for the 6R80?

Any. That’s a great transmission.

 

[stannypack]

I have a 2020 HPP with a 6-speed, and I see the same thing. I shift 4 times getting to 50 mph. This is not necessary. I'm changing my car from a 3.55 to a 3.15.

Isn't that fun lol? I liked being able to bang through all the gears on my ecoboost, curious what the shift points on my new GT will be.

 

[Jstang23]

I'm with you until the autocross comment. Taller gears generally help AutoX, because of fewer shifts. Gearing might be pretty sporty with the auto and a 3.15. The problem on this front is that you can't get the Torsen differential with the 3.15. The Torsen is only an option in 3.31 gears on up. If the 3.31 is still an option, that might be a place to look? The gears on the Auto box are so close that the rear end gearing becomes a pretty marginal change. You're much more limited by tire grip than gearing on the launch. The difference becomes what RPM it puts you through the lights. In a stop-light grand prix, this doesn't really come into play.

I apologize I meant to say 3.55 is better for autocross!

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