Why isn't 6th gear super tall?

[chefdave]

My 2.0 petrol auto is geared to 28mph per 1000 rpm in top. It is a four speed. Ratios must be quite wide apart. Getting between 37 and 40 imperial mpg. Which is what manufacturer says is maximum mpg under ideal conditions. I drive far from ideal conditions. Hills strong winds and sub zero temps for 3 months of year.

[Baltothewolf]

Originally Posted by cRiPpLe_rOoStEr

Most of the times a 6th gear is worthless at all, and sometimes a 5-speed transmission may actually lead to a better mileage compared to a 6-speed. But anyway, instead of a taller 6th, if I had the choice I'd get a wide-ratio transmission with a higher gear spread and match it to a higher differential.

Yea I'm doing the opposite in the Miata. I'm doing a really short geared 6-speed swap (gears 1-6 are actually shorter than the 5 speed 1-5), but I'm pairing it with the tallest rear end available for the NA Miata (3.63 instead of 4.1) So my 1-5 is the same ratio as the 5-speed with 4.3 rear end, but 6th gear is a true overdrive and makes driving the car a dream. My RPM will drop about 500 over the 5-speed with 4.1 (from 3636 at 75 to 3150(ish), and 750rpm over the 5-speed with 4.3.

[Hersbird]

I used to skip shift my 6 speed and start in 2nd until I was told that was bad overall for mileage and wear and tear. This is on a Cummins diesel with 600 ft-lbs of torque on tap. If anything would work well with a 2 speed powerglide this would. I went with a tad taller tire 33" vs 31.5" stock) to drop freeway rpm. Now I run about 2000 rpm at 70mph and that seems to keep exhaust temps pretty low. It does get better mpg at 60 but the EGTs go through the roof towing with any throttle under 2000 rpm. So any acceleration or hill I always downshift and just use 6th for steady crusing. The NV5600 starts with a 5.63 in first and ends at 0.73 in 6th but it is a beast. Probably heavier than most 4 cylinder engines at 360 pounds.

[Daschicken]

Manufacturers don't put in tall top gears because they are concerned people will think the car is slow when they floor it in top gear and don't bother shifting down. "Gotta have good top gear roll-on performance, guys, nothing else matters. Its not like there are 5 other gears to choose from."

Originally Posted by Baltothewolf

The new Civic Si has an amazing 6-speed. 1-3 is as short as the old NA Miata's (VERY short). Then 4-5 is fairly tall, and 6th is really tall. I believe 6th at 80mph is 3000rpm. I know 6th at 70 is 2700, which is about what the honda insight is in 5th. Guess that's why it's rated for 41mpg highway.

1st gen insight is at 2392 RPM at 70. New civic Si is at 2626 RPM at 70. Almost spot on Balto, 80 mph is 3001 RPM!

My accord V6-6mt does 2312 RPM at 70 with my 25.7" tires instead of 25.5" stock which does 2330 RPM.

A 2017 Base model Vette 7 SPEED MANUAL does 1256 RPM at 70. WANT.

Meanwhile my CBR 250R did 9289 RPM at 70 stock, now at 8773 with 3 teeth down on the rear sprocket.

[gone-ot]

Q: Why?

A: Look up the definition of "lugging" as applied to the Internal Combustion Engine.

[freebeard]

Lugging>>sweet spot>>valve float.

[Baltothewolf]

Originally Posted by Old Tele man

Q: Why?

A: Look up the definition of "lugging" as applied to the Internal Combustion Engine.

Yea but no car is going to lug at 2500-2750rpm. Even going up a 10% grade in top gear at WOT. Even the insight won't.

[cRiPpLe_rOoStEr]

Originally Posted by Hersbird

I used to skip shift my 6 speed and start in 2nd until I was told that was bad overall for mileage and wear and tear. This is on a Cummins diesel with 600 ft-lbs of torque on tap.

I don't know the specific case of your truck, but anyway, skip-shifting is not so bad depending on traffic and terrain conditions.

[SmellyCat]

Great stuff! If I'm going down hill or on level road I often skip shift. 1st 2nd 4th 6th. If the car is rolling at 2 mph, I will stick it in 2nd and gently accelerate up to speed.i don't think starting from a stop in 2nd is a great idea, 2nd gear seems like the most abused and worn out gear in my older cars. And clutch wear will be heavier starting in 2nd.

I like the idea of trying taller narrower tires to lower the rpm.

[SmellyCat]

Here is a,link about Lug and Coast
https://www.caranddriver.com/columns...-lug-and-coast