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Discussion Starter #1 (Edited)
Although I am posting this data, it comes only as a result of lots of meticulous work by Dan O'Toole.

For those that don't know, Dan is also the one who is converting each chapter of "The SSR Experience" as I release it, from Microsoft Word format to Acrobat pdf format, so that the file is smaller and can be opened by folks who do not have Word on their computers.

Dan bought his 2005 6-speed brand new (ok, one of GM's technically used "show vehicles") just a couple of weekends ago, and rove it from the dealershp in Dallas to his home in Toronto, Canada. Before he made the trip to pick it up, Dan asked me if there was any testing he could do on the trip that would help the board get some useful data.

I asked him if he would be willing to do some gas mileage testing, using a strict procedure described in detail in a much earlier posting this spring (search for "mileage" or "4.56 gearng"). THat procedure ensures that the results are very accurate and very representative of real world gas mileage at 60 mph.

I asked Dan to do the testing alternating between 6th gear and 5th gear. The reason I did this is to try to get a feel for how much the gas mileage would suffer for anyone who does the conversion from 3.73 to a higher rear axle ratio. I need to explain beforehand why the results are so encouraging.

The Tremec 6-speed has a 6th gear ratio of 0.50 (i.e. engine speed is cut in half by the transmission in overdrive 6th) and a 5th gear ratio of 0.74. THis means that runnin gin 5th gear versus 6th is equivalent to running a 5.52 rear axle ratio!! THis is WAY, WAY beyond what anyone would actually do as a rear axle change. In other words, the results are going to be MUCH worse than what anyone would actually encounter in reality with any sensible rear axle change,

You might ask how it is even possible to run at 60 mph in 5th, when the engine rpm is going to be 48% higher than in 6th. The answer is that the stock gearing is pretty ridiculous. The engine is only turning at 1284 rpm at 60 mph. This is so low as to be labeled "engine abuse" for an engine that redlines in the area of 6500 rpm! I told the technical folks at the SSR Homecoming yetserday that they should be ashamed of themselves even doing that! (They grinned sheepishly). Because of that low rpm in 6th, 5th gear at 60 mph is only 1901 rpm, which is almost 300 rm LOWER than I cam currently running on my 2004 5.3 liter LM4-based engine, and I am getting 19.6 mpg right now on the highway even after my major mods at MTI this week (more power now at the rear wheels than a stock 2005 SSR that has 13% more displacement!). THis is nothing to cry about . . .

Dan, bless his heart, took me VERY seriously, and did LOTS of testing, even getting results with top up, top down, AC on and AC off! Here are the results of his hard work. Because this posting medium does NOT support tabs, I have had to use "/" as the separator, which makes reading the results a little difficult, but if you bear with it, this is a gold mine of impressive data.


6th Gear / 5th Gear / Notes / Mileage in 5th vs 6th
23.5 / 20.8 /No Air Top Up / 0.89
24.7 / 21.1 / No Air Top Up / 0.85
24.9 / 19.8 / Air / 0.80
24.5/ 19.9 / Top Up Air / 0.81
26.8 / 23.3 / Top Up Air / 0.87
27.1 / 22.7 / Top Down Air / 0.84
27.3 / 19.8 / Top Down Air / 0.73
25.9 / 21.1 / Averaged results for ALL of above / 0.82

Combined summarized results (all at 60 mph):

With the top up, but no air, mileage in 5th was 87% of mileage in 6th.
With the top up, and air on, mileage in 5th was 83% of mileage in 6th.
With the top down, and air on, mileage in 5th was 78% of mileage in 6th.
Overall average was that mileage in 5th was 82% of mileage in 6th
Total Distance Driven 1,550 miles
Average MPG for trip 22.81 (!!)

6th gear ratio =0.50, rpm = 1284
5th gear ratio =0.74, rpm = 1901
Difference =1.48
Running in 5th versus 6th is equivalent to a 5.52 rear axle ratio!

So, what we have done is "simulated" running a 5.52 axle ratio, 48% stiffer gearing than stock, on the highway! We have found that even that absurd ratio STILL gives you 82% of the gas mileage of the stock 3.73 axle ratio! And that includes running top down with air on for part of the averaged results!

Running a 4.56 axle ratio like I am cost me only an actual 7.4% drop in fuel mileage (My new mileage was 92.6% of mileage with stock gearing), nd I got all of that back via a good dyno tune at MTI Racing.

You guys with 6-speeds will NOT be hurt much in gas mileage by changing ratios to anything sensible (4.56 recommended).

A couple more explanations:

Gas mileage with the top down is worse than with the top up because of poorer aerodynamics with the top down.

Gas mileage with the air on is worse than with the air off because of the horsepower used by the air conditioning system to make you comfortable. The AC draws a LOT of power.

The loss in fuel mileage in 5th versus 6th is mostly because you are spinning ALL the accessories and the driveline FASTER for the SAME road speed, so you are creating proportionately more driveline friction and driving the AC, the power steering, the alternator, etc, faster than you really need to. You would recover most of that loss (all but the increased driveline friction) by changing to underdrive pulleys (ONLY after stiffening the gearing!! NOT with stock gearing!), as I did at MTI this week also (It was a BUSY and dramatic 3 days at MTI, but that's a separate posting I will try to post later today or Sunday).

And yes, the SSR routinely beats the EPA highway fuel mileage rating unless you have a lead foot. :)

Jim G
 

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Discussion Starter #4
You mean the 2006 6-speed SSR has stiffer gearing than the 05?

If true, that is DEFINITELY a step in the right direction!!!

If true, the .57 6th gear ratio would pull engine rpm up to about 1450, or about equiavlent to a 4.25 axle on an 05 SSR. (Still too low, but, hey, GM is a big conservative company. :)

Is Chevy listenign to us on this website? :)

Jim G
 

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All this hard, detailed work shows vividly the answer to your question for the GM technical guys that made them sheepish. They are mandated by people we send to Washington to force gas mileage from our vehicles and a .50 6th does that quite well and you just proved it again. I really like reading the '06's are geared a bit lower, though because I would put bigger tires on back and actually think I just might be able to pull off getting a black over silver 6 speed '06 by Spring. ......... JAustin, Did you notice if you Build & Price using that new SSR site, it does not allow for a 6 speed? Also, if you click on the silver over black box choice you get a black over silver pictured. If you choose black over silver, it shows silver over black.......... is it me? It also charges nothing for two-tone.
 

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Mileage test questions

Jim G,

Thanks for another great test! I do have a couple of questions, and have reread your post a couple of times, so please bear with me.

Near the end you say"Gas mileage with the top down is worse than with the top up because of poorer aerodynamics with the top down." This is what I would have expected, but when you look at the table above, it looks like the MPG with top down and air on in two runs in 6th gear was actually the highest experienced at 27.1 MPG and 27.3 MPG. Is this correct?

My ride is a 2005 6 speed (tecnically a double overdrive 4 speed), and I would make the following observations after 2,000 miles of driving. You are right about the tall gearing at 60 MPH in 6th, but at 80 on the interstates, it feels pretty good here in the Southeast on comparatively long flat stretches. At these speeds, the bigger engine with 6 speed delivers over 20.4 MPG, and I am pretty pleased with that. Can't help but wonder if the new gearing on the 2006 won't hurt that a bit.

Thanks again for a great post!

PCBeach
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Danko: Didn't try to do a Build & Price; just looked at the Specifications page. I had noticed some other minor changes in the '06, so I when I read Jim's write-up, it just occured to me to check the '06 specs.
 
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homecoming road trip

19.1 mpg for the entire trip. over 2200 mi, a few mi. on woodward friday eve at 1.78 mph :lol all idleing. Drove mainly 75 to 85 mph. the whole way. 105 a time or two coming back from Lansing to warren. I can live with that on the 05 auto tranny gas mileage stock rearend. :seeya


My foot..... It's as heavy as the rest of me. :lol :lol
 

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Discussion Starter #10
PCBeach: EACH of the tests described above was done in BOTH 5th and 6th gear, ALTERNATING between 5th and 6th gear every 3 miles.

Therefore, the great mileage you noted was achieved in BOTH 5th and 6th, with the top down BUT with some other favorable factor. The favorable factors could have been one or a combination of:

tail wind
Flat versus hilly road
Better atmospheric conditions for good engine efficiency (i.e. colder air temperature, NOT humid, etc)

Do NOT think that you get better mileage with the top down! You won't!

Jim G
 

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JimGnitecki said:
PCBeach: EACH of the tests described above was done in BOTH 5th and 6th gear, ALTERNATING between 5th and 6th gear every 3 miles.

Therefore, the great mileage you noted was achieved in BOTH 5th and 6th, with the top down BUT with some other favorable factor. The favorable factors could have been one or a combination of:

tail wind
Flat versus hilly road
Better atmospheric conditions for good engine efficiency (i.e. colder air temperature, NOT humid, etc)

Do NOT think that you get better mileage with the top down! You won't!

Jim G
Hey Jim, another anomoly that may have affected the results was getting tucked in behind a transport truck a couple of times (less wind resistance). Something else to consider is the effect of the WindsuppreSSR as the reduction of wind turbulance in the cockit was noticeable after installation (all readings with the top down were made after the WindsuppreSSR was installed).
 

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Unscientific results

My round trip mileage was pretty good considering the hit the cruise took. Two tanks at just over 20mpg and one at 18 (two hours of crawling on woodward did that).

The DIC instant mileage in 6th at 70-75ish was in the 22-23mpg range when on reasonably level ground (Ohio's good for that on the western side). If I was in 5th at the same speed, it was down around 19-21 range. My entire trip out and back was pretty much kept between 70 and 75 as long as I wasn't stuck in traffic. The long-haul parts of my travel was done with the top up and AC on as well.

I wonder if I could somehow make the tranny fail if they'd replace it with the new '06 re-geared version? :rolleyes: Either that, or I'll consider swapping out to a 4.10 in the diff.
 
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