5th Gear for Highway Cruise and "GAS" Mileage

[mikedsilva]

My 1979 SC sits near 2800rpm (according to the tacho) at 100kmh or 60mmph. Our speed limits are generally 100kmh on open highway. Some are up to 110kmh.

I'd like to be doing considerably lower rpm when cruising.

I drove a friends car and his seems to be maybe 2600-2700 at the same highway speeds but I dont know what diff is in his box or the ratios.

Looking at the ratios that came with these (according to the CMS website) I am guessing my car has a 28:23 (0.82) and my friend has a 29:21 (0.72).

Looking at the Albins site, they offer
0.79 - 29:23
0.76 - 29:22
0.72 - 29:21
0.68 - 31:21
0.59 - 32:19

That's quite an expansive range.. has anyone used any of those ratios?

Using motoristo.com and plugging in my diff ratio, (8;31) tyre size etc, 275/40/17 and with my ratio of 0.82 I'm at 2750rpm for 100 kmh.
Here's the numbers for the other ratios.. 100km/h - and resultant rpm

0.82 2750
0.79 2600
0.76 2500
0.72 2350
0.68 2250

Engine is a 3.0SC converted to turbo and efi.. do we think 2250 is too low rpm to be cruising?

[Showdown]

I (Tony) put a taller 5th (TM = 29:22 = .759)
in my 915 and it’s been really great on the highway in the fast flat Midwest.

At 80pmh I’m at about 3200 RPM which is a big change from the stock gear which had me at 4500 RPM for 80MPH

I’ve never driven on the highway at 60 in the Midwest


Sent from my iPhone using Tapatalk

[Joesmallwood]

Originally Posted by Showdown

I (Tony) put a taller 5th (TM = 29:22 = .759)
in my 915 and it’s been really great on the highway in the fast flat Midwest.

At 80pmh I’m at about 3200 RPM which is a big change from the stock gear which had me at 4500 RPM for 80MPH

I’ve never driven on the highway at 60 in the Midwest


Sent from my iPhone using Tapatalk

Exact same setup for me. 29:22 fifth in an '81 SC (w/915). Absolute game changer for interstate cruising.

[Showdown]

FWIW, I got my 5th from EBS, it was like $800, I tried to find cheaper but it was a waste of time... it is what it is...

Second data point: at 80mph I can get anywhere from 22-28mpg, more if there's no wind or I'm drafting a larger (any) car... For frame of reference, my 2021 Audi Allroad gets about 30mpg at that speed. My old Nissan Murano got maybe 20 on a good day...

[proporsche]

does your friend has the same size of tires?

[Ryan_Cunningham]

Originally Posted by Showdown

I (Tony) put a taller 5th (TM = 29:22 = .759)
in my 915 and it’s been really great on the highway in the fast flat Midwest.

At 80pmh I’m at about 3200 RPM which is a big change from the stock gear which had me at 4500 RPM for 80MPH

I’ve never driven on the highway at 60 in the Midwest


Sent from my iPhone using Tapatalk

Roughly how much did that cost? I've got a 3.6 VRam waiting to go into my 1981 and am scared 1st will now be essentially unusable and 5th will be way too short.

[Joesmallwood]

Originally Posted by Showdown

FWIW, I got my 5th from EBS, it was like $800, I tried to find cheaper but it was a waste of time... it is what it is...

Second data point: at 80mph I can get anywhere from 22-28mpg, more if there's no wind or I'm drafting a larger (any) car... For frame of reference, my 2021 Audi Allroad gets about 30mpg at that speed. My old Nissan Murano got maybe 20 on a good day...

Adding another data point: two road trips, to NC and back to Indiana (300+ miles), I averaged 22.5 MPG there and 23.8 mpg on the return trip. This is with 255x40r17 (25" dia) rear tires and a 3.55 liter with ITBs. Cruising at 80 is 3200 rpm and cruising at 85~90 is no trouble - unless the locals with radars disagree.

[Joesmallwood]

Originally Posted by Ryan_Cunningham

Roughly how much did that cost? I've got a 3.6 VRam waiting to go into my 1981 and am scared 1st will now be essentially unusable and 5th will be way too short.

I have a 3.55 liter in an '81. First gear does go by quick when giving it the beans, but for normal driving, it's still fine and useable. I added the taller fifth to drop the rpms at cruise and with the added cubic inches, it works wonderful - zero issues pulling hills and cruising anywhere between 70 and 90. Cruising between 60 and 70 on flat state roads or county roads is no problem either. With hills, it's better to simply shift down.

29:22 was perfect for my setup. Some folks go taller with 29:21, but I feel that might be too tall.

[GH85Carrera]

Here is a earlier thread about my 5th gear changes

https://forums.pelicanparts.com/porsche-911-technical-forum/1094959-highway-5th-gear-915-a.html

My reply then:

I ended up doing just that in my 85 Carrera. I went to a 915 TL gear or the .724 (29:21) gear. I think it is perfect for highway cruising in the 75 MPH speed limit days. If I get down to 60, I pretty much have to downshift to 4th. 55 is the minimum speed for 5th on level ground and I usually just downshift.

At 80 it is at 3,000 RPM and just sings along for mile after mile.

Since I often do long road trips, I was willing to spend the money to have it put in. I still have my original 5th gear and would be happy to sell it, but there is evidently not much demand for the stock 5th. I did not drop the engine and transmission just to replace 5th. I just took the opportunity to open the transmission and replace some syncros and do a minor refresh.

The TL gear is a little noisier, but the engine is a lot quieter. The stock 5th has a treatment to reduce noise a little. The TL gear does not have that.

[porschedude996]

Originally Posted by Joesmallwood

I have a 3.55 liter in an '81. First gear does go by quick when giving it the beans, but for normal driving, it's still fine and useable. I added the taller fifth to drop the rpms at cruise and with the added cubic inches, it works wonderful - zero issues pulling hills and cruising anywhere between 70 and 90. Cruising between 60 and 70 on flat state roads or county roads is no problem either. With hills, it's better to simply shift down.

29:22 was perfect for my setup. Some folks go taller with 29:21, but I feel that might be too tall.

Nearly stock 1983 911SC 3.0L with 29/21 5th gear set. No problem at all carrying 5th. I had the transmission rebuilt and used with some “Experienced Gears” and 5th is a little whiny. My plan is to drop the eng/trans and replace the set. 5th gear is easy to replace. I am considering an even taller gear set.

[mikedsilva]

Originally Posted by proporsche

does your friend has the same size of tires?

Good point. His rears are 255 40 17... not sure how that compares in rolling diam but I think they are close to my 275 35 17

[proporsche]

Originally Posted by mikedsilva

Good point. His rears are 255 40 17... not sure how that compares in rolling diam but I think they are close to my 275 35 17

i think you might have your answer there..try his on yours and see before you start spending money on parts-))))

Ivan
I used to have 245/50/16 and since i could find them in Europe i went to 245/45/16 ..i could see the difference on speedo ...
then again i am not an expert on tires maybe Bill will pitch in

[PeteKz]

Several of my comments are in the thread that Glen linked, and I agree with him. If you want a highway gear, go as high as you can on 5th. I also turn about 3000RPM at 80MPH and cruise easily in the 70-80MPH range. .

[mikedsilva]

Originally Posted by proporsche

i think you might have your answer there..try his on yours and see before you start spending money on parts-))))

Ivan

Not really... 255/40/17 and 275/35/17 are actually fairly close in rolling diameter. Works out to maybe 2km/h difference at 100km.

[mikedsilva]

Originally Posted by PeteKz

Several of my comments are in the thread that Glen linked, and I agree with him. If you want a highway gear, go as high as you can on 5th. I also turn about 3000RPM at 80MPH and cruise easily in the 70-80MPH range. .

Normal highway limits here are 100kmh or 60mph and up to 110kmh or just under 70mph.
I'm thinking 2500rpm at 110kmh which would be a 0.69 ratio. But mostly 100kmh so I'm leaning toward the 0.72 ratio of 29:21.

I'd be doing 2400 rpm at 100kmh and 2600 rpm at 110kmh..

[Otter74]

2400 at 100kph sounds pretty relaxing to me.

Also, 100kph speed limits sound tragic!

[mikedsilva]

Originally Posted by Otter74

2400 at 100kph sounds pretty relaxing to me.

Also, 100kph speed limits sound tragic!

yep, and they book if you you go even just 2km/h over.

[GH85Carrera]

Originally Posted by Otter74

2400 at 100kph sounds pretty relaxing to me.

Also, 100kph speed limits sound tragic!

Wow, yea that is horrible. In most all the areas outside of a city our speed limit is 75 and in some places it is 80. I still have to grin with joy as I pass a State Trooper on the side of the road, as I roll past at 80 MPH. And I so remember the bad old days of the national 55 MPH speed limit.

With my 5th gear (TL gear or the .724 (29:21) I really don't like going below 60 MPH on flat ground. If there is a hill, I downshift. It would not be a good gear for a 62 MPH speed limit.

I can drive 500 miles on one tank of gas while cruising at 75 to 80 MPH, AC cranked up to ice cold at hurricane velocity.

Last summer I got to drive on Talladega Super Speedway. The track has a 33 degree banking and we were just cruising along at 125 MPH, AC on with my wife in the passenger seat. At 100 MPH I had to steer hard uphill as gravity wanted to push us down the hill. At 125, it was very neutral, and fun.

[Matt Monson]

It’s been a while, but if you go with that .68, be sure to ask Albins if it requires machining the tailcone. They also used to make a .65 and .62, which I’m sure require machining. I don’t remember about the .68, but they might still be making it specifically because it doesn’t require machining.

[shoooo32]

Agreed with Matt - a .72 has very little clearance.

Another data point that I've picked up running a tall 5th gear is the heat generated by this ratio. 75w90 mineral oil breaks down quickly if you do a lot of sustained 80+mph highway driving. There's a reason Porsche added trans coolers to the German spec (read: Autobahn) cars. I combat this with 85w140 GL5 hypoid oil (swepco 210 or Belray gear saver) and frequent changes (10-15k miles). Oil is a lot cheaper than a rebuild!

Here is a box I just rebuilt that had a tall 5th that saw coast to coast road trips and rallies. The 90w swepco was a burnt smelling mess, with all surfaces coated in slime and the bearings were toast. This is a well cared for car with a fresh gearbox rebuild and tall gear added in 2022: