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Think I found out why 3.08's and 2.73's were so common

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by dyeager535, Feb 5, 2004.

  1. dyeager535

    dyeager535 1 ton status Premium Member

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    Think I found out why 3.08\'s and 2.73\'s were so common

    "How are fuel economy estimates obtained?

    The fuel economy estimates are based on results of tests required by the U.S. Environmental Protection Agency (EPA). These tests are used to certify that vehicles meet the Federal emissions and fuel economy standards. Manufacturers test pre-production prototypes of the new vehicle models and submit the test results to EPA. EPA re-tests about 10% of vehicle models to confirm manufacturer's results. The vehicles are driven by a professional driver under controlled laboratory conditions, on an instrument similar to a treadmill. These procedures ensure that each vehicle is tested under identical conditions; therefore, the results can be compared with confidence.

    There are two different fuel economy estimates for each vehicle in the Fuel Economy Guide, one for city driving and one for highway driving. To generate these two estimates, separate tests are used to represent typical everyday driving in a city and in a rural setting. Two kinds of engine starts are used: the cold start, which is similar to starting a car in the morning after it has been parked all night; and the hot start, similar to restarting a vehicle after it has been warmed up, driven, and stopped for a short time.

    The test used to determine the city fuel economy estimate simulates an 11-mile, stop-and-go trip with an average speed of 20 miles per hour (mph). The trip takes 31 minutes and has 23 stops. About 18 percent of the time is spent idling, as in waiting at traffic lights or in rush hour traffic. The maximum speed is 56 mph. The engine is initially started after being parked overnight. Vehicles are tested at 68 F to 86 F ambient temperature.

    The test to determine the highway fuel economy estimate represents a mixture of "non-city" driving. Segments corresponding to different kinds of rural roads and interstate highways are included. The test simulates a 10-mile trip and averages 48 mph. The maximum speed is 60 mph. The test is run with the engine warmed up and has little idling time and no stops (except at the end of the test).

    NOTE: To make the numbers in the Fuel Economy Guide more useful for consumers, EPA adjusts these laboratory test results to account for the difference between controlled laboratory conditions and actual driving on the road. The laboratory fuel economy results are adjusted downward to arrive at the estimates in the Fuel Economy Guide and on the labels seen on new cars, light trucks, and vans. The city estimate is lowered by 10% and the highway estimate by 22% from the laboratory test results. Experience has proven that these adjustments make the mileage estimates in the Fuel Economy Guide correspond more closely to the actual fuel economy realized by the average driver."

    Our numbers aren't based in the real world

    So in other words, they put a vehicle on a dyno, crank it up to 55MPH, and check MPG. Then they figure that doesn't simulate real world very good, so they add a certain percentage on. Of course, they make no differentiation based on coefficient of friction, so a Prius and a K5 both have the MPG ratings dropped by the same percent.

    Thats the gov's info there, so if they left something out that would make more sense, it's their fault. But from what I read there, the makers can easily screw the government/consumers by putting drivetrains in the vehicles that can't possibly be as efficient on the road as in a laboratory because they don't figure in drag for each vehicle. With a K5, drag is huge, so you test a 305 and 700R4 with 3.08 gears, to get the best mileage possible, (helsp the CAFE numbes) without having to factor in how much gas it *actually* takes to propel the vehicle with drag added in.

    This is probably why my 305/700R4/208/3.08/31 '83 got worse mileage (slightly) on the freeway than my 350/465/3.42/33" '86, and probably holds true for most of the OD equipped 2.73/3.08/3.42 trucks. Each drivetrain combo has to be tested by the EPA, so obviously the 3.73 or 4.10 geared truck will get worse mileage than the 2.73 vehicle, but once you factor in how much drag is on the vehicle, I almost guarantee the 3.73 truck equals out to the "laboratory" 3.08 combo.

    I tend to fly off the handle in these kinds of issues, anyone want to comment? I can't see anything in that article that takes into account drag though.

    Hey, put a Nascar vehicle up for testing, I bet based on EPA calculations, top speed is 600MPH!
     
  2. 83ZZ502_Jimmy

    83ZZ502_Jimmy 1/2 ton status

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    Re: Think I found out why 3.08\'s and 2.73\'s were so common

    That is a great find Dorian. It really explains alot on how vehicles are tested. No wonder I got 2.73 gears.. /forums/images/graemlins/smile.gif

    No disputes from me /forums/images/graemlins/whistling.gif /forums/images/graemlins/laugh.gif

    John
     
  3. CyberSniper

    CyberSniper 1/2 ton status Premium Member

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    Re: Think I found out why 3.08\'s and 2.73\'s were so common

    I'm living proof that 3.42 gears are better than 2.73 gears even with stock (235x75r15) tires. I get 16mpg on the highway/expressway with my 3.42s on 33x12.50 rubber and I get 15mpg on the highway/expressway with 235x75r15 rubber. Even on my best day with my 2.73s I never exceeded 14mpg.

    I'm an oddball, I get to take 500 mile day trips that are virtually all highway driving several times a year so I can get a pretty accurate estimation of fuel mileage. With the 3.42s and 235 rubber the RPMs are too high for economy cruising. At 70mph I'm turning 2400rpm, which is about 400rpm too high for ideal economy.

    If I had to guess, for economy purposes:
    3.08 gears with 235x75r15 tires
    3.42 gears with 31x10.50r15 tires
    and 3.73 gears with 33x12.50r15 tires

    That's for a pickup, I don't know about a K5.

    No matter what, airplane gears never belonged in any fullsize. I think there's a reason why no IFS truck that I'm aware of got gears taller than 3.42s.
     
  4. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    Re: Think I found out why 3.08\'s and 2.73\'s were so common

    Makes sence to me...2.73' are a joke in a fullsize to me. 3.08's are useless too unless your running the base model rubber which was like a 27 or 28" tire. It seems whenever we get a "fuel mileage" type thread going, it's the guys with 3.73 with stock 31's that seem to be yielding the best mileage in town and on the highway.

    This is making me wan't 4.88's bad because with my rubber and 4.88's, that would be like having a stock pickup with 4.10's, OD, and 31" tires. That in my opinion is about the best combo for a mix of best in town economy/towing/and still managing decent highway economy. /forums/images/graemlins/thinking.gif
     
  5. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Re: Think I found out why 3.08\'s and 2.73\'s were so common

    My best ever mileage with the 350 was 16.4 mpg...back when it had 2.73's and 33's with the 465. 1870 rpm at 65.

    It was the rough equivalent to a K5 with 3.73's and a 700R-4.

    I swapped to 3.73's and 35's...best ever with that combo was 12.9 (cruise rpm = 2325 @ 65)

    I swapped to 4.56's and 36's and mileage dropped to a best of 10.5 (cruise rpm = 2770 @ 65)

    I swapped in the 6.2 diesel and bumped up the tire size to 39.5's and my mileage went up to a best of 17 and an average 16.2. (cruise rpm = 2550 @ 65)
    If I can get my cruise rpm down to 2000 or so I should gain another 1.5-2 mpg.

    Dorian, I don't understand why a test track couldn't have been used to figure mileage estimates. It would have been a lot more 'real world' and aerodynamic drag would have been much more accurate, as would rolling resistance...combined with a more true test of each particular drivetrain combo's efficiency.

    Rene
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: Think I found out why 3.08\'s and 2.73\'s were so common

    Just like carmakers complaints (especially domestics) that economy rules would "kill them", the dyno testing method would appear to make all vehicles get better mileage than actual, since the MPG numbers are definitely skewed and not right.

    I can only guess then, that real world testing has been, or was fought, tooth and nail by the auto manufacturers. If it wasn't, had the EPA said your CAFE (Corporate Average Fuel Economy) numbers need to be 16/25 city, except we are now going to test "real world", GM's (for instance) CAFE numbers would have decreased without any other chaneg except in test procedure (just like the change in the 70's from gross to net HP ratings) and that would have meant that somehow, economy would have to increase to meet those numbers.

    At the time, that would have meant the automakers (domestic in particular) would have had to get off their butts, and actually spend some money developing better technology. In effect, these regulations are why fuel injection and Vortec/Fast Burn heads exist, but it costs the automakers money to get there.
     
  7. jayccb1

    jayccb1 Registered Member

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    Re: Think I found out why 3.08\'s and 2.73\'s were so common

    I run 3.42's and have 35 in tires, I get 10mpg on a good day, would lower gears help the gas mileage? Might I have another problem. I have an 89 with a 350.
     
  8. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: Think I found out why 3.08\'s and 2.73\'s were so common

    Depends on if you are talking around town mileage, or cruising on fairly flat ground at 60MPH for extended periods of time.

    Only time you are going to get much better than 10MPG is on the freeway. (with a gas rig) However, if the speedometer is wrong, and you use the odometer to figure mileage, then your MPG calculations are wrong too.
     
  9. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    Re: Think I found out why 3.08\'s and 2.73\'s were so common

    [ QUOTE ]
    I run 3.42's and have 35 in tires, I get 10mpg on a good day, would lower gears help the gas mileage? Might I have another problem. I have an 89 with a 350.

    [/ QUOTE ] I would say your mileage would increase a bit if you had 4.10's or 4.56's. It seems that the guys that have a little bit lower ratio than what you have are yielding a little better mileage. That includes me with 4.10's and heavy 35" boggers. I get about 8 or so in town and about 12 on the freeway, but thats feeding a thirsty 406 though too.
     
  10. 88sub4x4

    88sub4x4 1/2 ton status

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    Re: Think I found out why 3.08\'s and 2.73\'s were so common

    I'm running a stock 350/700R4 with 4.10's and 35" A/T. I get 10 around town, and 15-16 highway. When I had my 3.42 and 31" tires I was 10 and 12-13 highway. I got one of those vornado things (It was free, I would never spend money on something like that) and I dropped to 8-9 around town and 13-14 highway. I took it out and MPG's went right back up. I think it just caused restriction in the intake.
     
  11. R72K5

    R72K5 Banned

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    Re: Think I found out why 3.08\'s and 2.73\'s were so common

    2.73 was std equip in at least 81-up and such is listed right in gm specs book sheets in 81

    it was all about economy and such in the 80s, not performance, but we all know that, 2.73 is most common youll find in any 10 bolt trucks '80's at least

    compare this to the std equipment ratio of 3.73 in at least 72 and older.. back thne it was all about work performance, real trucks built right /forums/images/graemlins/laugh.gif
     
  12. Blue85

    Blue85 Troll Premium Member

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    Re: Think I found out why 3.08\'s and 2.73\'s were so common

    You would think that Trucks/SUVs would get one set of "scew factors" and cars would get another. It must be based on the average of all vehicles sold. If so, then trucks would tend to get mileage below what their window stickers say and cars would tend to get mileage above EPA ratings. I'm not sure that's consistent with what we see everyday, though.

    The biggest difference should be seen in the highway cycle, as wind drag wouldn't be a big factor in the low speed city cycle.
     
  13. mr_fujisawa

    mr_fujisawa 1/2 ton status

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    Re: Think I found out why 3.08\'s and 2.73\'s were so common

    [ QUOTE ]
    2.73 was std equip in at least 81-up and such is listed right in gm specs book sheets in 81

    it was all about economy and such in the 80s, not performance, but we all know that, 2.73 is most common youll find in any 10 bolt trucks '80's at least

    compare this to the std equipment ratio of 3.73 in at least 72 and older.. back thne it was all about work performance, real trucks built right /forums/images/graemlins/laugh.gif


    [/ QUOTE ]

    yup. i have an '88 and it came stock with 31's and 2.73's
    :P
     

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