Lower Gears Higher Highway Mpg?

Prokopchuk_9 · December 26, 2010

I don't understand why everyone is stating that the 3.08 is leaving the truck underpowered. I think you guys are putting a little too much on the 3.42 ratio. With the 6-speed tranny, the 3.08 is like the 3.73 with the 4-speed, that has been stated by many people. Even if you tow, the 3.08 can still pull trailers. I see no problems with the 3.08, my truck gets up and goes!

not as much get up and go as 3:73 or 3:42

Matt_ · December 26, 2010

You say you're never going to tow anything.... are you going to put bigger tires on though? If so, then get the 3.42s or you'll never see 6th gear on the highway.

If you're not going to lift the truck at all, aren't going to race it, and aren't planning on towing... then 3.08s (and a tune) is a fine combo.

wicked ace · December 26, 2010

There was a big conversation about this recently I believe we decided the 3.42s were the best choice for mileage. "Most" agreed the 3.08's left the truck feeling underpowered.

Not to start an argument but I find the 3.08 rear to do nicely in my '10 Silverado w/5.3. The engine has plenty of power and the Six speed transmission does a good job putting the power where it needs to be. I never get below 4th going up a 2 mile long 10% grade doing 50. If I were doing some regular towing I would go for 3.42 gears at least.

nickvrebel · December 27, 2010

I don't understand why everyone is stating that the 3.08 is leaving the truck underpowered. I think you guys are putting a little too much on the 3.42 ratio. With the 6-speed tranny, the 3.08 is like the 3.73 with the 4-speed, that has been stated by many people. Even if you tow, the 3.08 can still pull trailers. I see no problems with the 3.08, my truck gets up and goes!

Yes the truck will "get up and go" thats where the six speed comes in. Makes the vehicle accelerate very well with a numerically low ratio (ie 3.08)

Top gear performance without forcing a downshift is where the numerically higher ratio will improve performance and in many cases improve fuel mileage. ie. 3.42 or even better 3.73

The overdrive ratios we have in modern transmissions put the cruise RPM too low and out of the torque peak when combined with 3.08 ratio. Get those RPM's up and improve mileage without excessive downshifting

if that is your line of thinking then you completely missed the whole point of the 6 speed transmission. When it downshifts from it's second overdrive to fifth gear, it's nothing like a four speed shifting from 4th to 3rd, it's supposed to do this as often as needed while when not under load achieving excellent economy.

Babyhauler · December 28, 2010

What is engine rpm at 70 mph with 6 speed and 3.08 vs 3.42?

Babyhauler

wicked ace · December 28, 2010

What is engine rpm at 70 mph with 6 speed and 3.08 vs 3.42?

Babyhauler

About 1600 RPM with 3:08. 3.42 I would guess about 2000 RPM.

Stevens03 · December 28, 2010

Quite frankly I'm running 3.73's with the 6-speed and wouldn't have it any other way. At 70mph, I'm just under 2000 RPM with those gears.

FargoZ71 · December 29, 2010

3.42's get my vote, less effort and strain to get the truck upto speed. And if your not gonna use it at all as a truck, ever think about a suv?

kdixer · December 29, 2010

What is engine rpm at 70 mph with 6 speed and 3.08 vs 3.42?

Babyhauler

See attachment for rpm's @ 70mph

If you run mainly highway you will see good mpg benefits from 3.08 gears. Other than that, I really do not think it will matter much. In hills or city, the lower rpm's will be negated by slightly heavier throttle and more shifting. Remember that efficiency is low during a shift because the engine is running but little power is being transferred to the wheels, even if it is for a split second.

I have said many times, 3.42 is a great compromise between performance and fuel mileage with the 6spd.

3.08_versus_3.42.pdf

wicked ace · December 29, 2010

What is engine rpm at 70 mph with 6 speed and 3.08 vs 3.42?

Babyhauler

See attachment for rpm's @ 70mph

If you run mainly highway you will see good mpg benefits from 3.08 gears. Other than that, I really do not think it will matter much. In hills or city, the lower rpm's will be negated by slightly heavier throttle and more shifting. Remember that efficiency is low during a shift because the engine is running but little power is being transferred to the wheels, even if it is for a split second.

I have said many times, 3.42 is a great compromise between performance and fuel mileage with the 6spd.

Looking at the chart I would have to agree. Less than 1800 RPM with 3.42 rear is still pretty good. I had a 3.42 rear in my GMT 800 and would turn about 2000 RPM at 70 so I was using that as a reference. The 6 speed is a game changer.

butzers09silverado · December 29, 2010

the shorter the gear (i mean 4.10) versus a taller gear (say 3.08) keeps the 4 cylinder mode in longer but you spin higher rpms.... i wish i had 4.10's with my 4spd, and wish they had 3.73's for the 6spd. I can't imagine the 6spd stays in 4 mode when it shifts, if that is the case, then get the shortest gear you can to keep the truck from shifting. I'm not positive on the 6spds, but on the 4spd the tcc unlocks or at leasts slips when it shifts down.

Babyhauler · December 30, 2010

I need the 3.23, not my 3.73. I don't want them. When I hit 1100 miles in a week every now and then, I would save about $12.00 a tank @ 2.75 gal. That's $624 per year at one tank per week. We had few company trucks,ECSB w/ 5.3, 4 sp and 3.23 and they hit 20-21 regularly. They would pull 4000-6000 lbs trailers often. Not as fast as a 2500 w/ 3.73, but they worked okay. My mpg jumps from about 15 to 18 when I go from interstate driving @ 2000 rpm 70 mph to 55 mph at the lower rpm. All I haul now is kids to school and me to see customers. Maybe the dog, dirtbikes, appliances etc, nothing heavy or long hauls. These numbers are based on all fill ups recorded and calculated. I know I need a car but I know a gear change would increase my highway mileage as proven by my sales staff. It would save some tire wear by eliminating a few "entertainment" launches also. This is my experience from several trucks and Tahoes over the years. This might help someone with their decision. It ain't for everybody, but it is for some of us.

butzers09silverado · December 31, 2010

with the afm, the higher rpm's make it easier to keep the truck in afm mode because there is less load on the engine when you approach high winds or hills... believe it or not. with a taller gear (3.08) it is a lot easier to reach the load limit versus a shorter gear. you will turn more rpms but you will stay in 4 cylinder mode more often.

wicked ace · March 10, 2011

There was a big conversation about this recently I believe we decided the 3.42s were the best choice for mileage. "Most" agreed the 3.08's left the truck feeling underpowered.

I wouldn't go on to say underpowered. I have a '10 Silverado with 3:08 rear gears and it definatly has more spunk than my '03 Silverado had. I just recently pulled a 3000# trailer with a 1500# load and had no difficulty. I would not nor do I intend to pull a larger trailer though.

On the other hand if I were to do it again I would look for a truck with 3:42 gears. The trade off in milage is minimal. At 70 mph you would turn 1800 rpm vs. 1600 rpm with a 3:08 rear.

wicked ace · March 10, 2011

Lower gearing would mean lower RPM's at speed, so yes, you would get slightly higher MPG's. Personally though, I would still go with the 3.42 gearing.

Hmm. Here we call a 4:10 a low gear and 3:08 is a high gear. No tow.. go with the 3:08. The six speeed makes up for the rear gear being so high. JMHO

thats the way it is...i deal with gearing, and making gears all the time gotta remember ur driver vs driven...lower the first number in the ratio, the less the pinion has to spin to turn the ring once, giving you a higher ring speed. just like any other. the rear end of a car is just about on the same principle as almost any other bevel gear. the only thing that differs with hypoid gears is that the pinion is offset to the center of the ring. the lower the first number in the ratio the higher its geared. higher=lower gearing

long story short, 3:08 is pretty high...ive heard guys with it and they shift all the time. for the amount its gonna save you in gas vs. 3:42 is it really worth all the annoying shifts every little lump on the road????

Long story short....I have to be going up a pretty steep incline for my truck....with 3:08 gear to downshift cruising at highway speeds. It dosen't shift all the time.

Unless....and I just recently discovered it I use E85 (That's another topic). My thought is the lower torque resulting from the fuel. Using E10 @ 87 octane I can run the 75 miles to my mountain house with no drama whatsoever.