20.9 mpg at 69 mph -98 Suburban-1510 mile 69mph trip

Discussion in 'GM' started by phoebeisis, May 20, 2008.

  1. phoebeisis

    phoebeisis Well-Known Member

    I have been very surprised at the 20.9 mpg I got from my 200,000 mile 1998 Suburban.On the trip-1510.5 miles,67-70 mph,72.42 gallons from New Orleans to Flagstaff AZ we got 20.9 mpg.This is with no hypermiling techniques other than 42 psi tires.The tires are worn a bit( probably helps mpg) I use Mobil 1 5w-30, but that is it.

    I set the CC to 67-68 for the 1st 600 miles,and set it at 70 mph the last 900 miles.

    From what I had heard,and what my scan gauge "told" me I expected about 17 mpg.I didn't bring the scan gauge because I didn't want to concentrate on it.
    I just set the cc and pointed.Maybe the high miles on the motor actually helped-usually doesn't work that way of course.

    I bought it about 1 year ago-$2950-195,000 miles.I mixed city driving,some city interstate-using motor on P&G I get 16.5 mpg.In pure city-no city interstate-I get 14 mpg using motor on P&G.I don't shut down at redlights-too old,too much wear to risk it.If I did shut off at stops, it would get about 17 mpg in pure city driving.

    Big surprise to get almost minivan mpg from a worn out huge guzzler.

    Any GM fans out there with an explanation for the too good mpg?It does have to taller rear end-3.42-but turns 1750 RPMs at 60 mph-no really tall gearing.My Pilot turned fewer RPMs at 60(1650).The Pilot got 22.25 mpg same trip.

  2. phoebeisis

    phoebeisis Well-Known Member

    Return trip 21.8 mpg with the CC set on 70 mph.Roundtrip(3021.7 miles) mpg was 21.3mpg-ac blasting the whole time.It used 141.7 gallons.

    Granted 21.3 mpg is nothing to brag about.The usual mpg folks report from Suburbans-2wd 5.7 Lt-of this vintage is about 17.5 hy.The current EPA on it is 12/17(old sticker 13/18).

    The tire pressure-43 psi-;Mobil I 5W30; using the CC instead of trying to hold steady speed with my foot; and keeping my speed at 68-70 mph instead of the 4-5 mph above that that most vehicles were doing helped the most I think.My tires are worn-not dangerous, but the thread was worn-certainly helped also.If it was a shorter trip,I would have DCed the CC going uphill and gotten another .25 mpg maybe.

    The current Suburbans are rated 3 mpg better HY-so they should get very close to 25 mpg highway if driven with mild hypermiling techniques(tire pressure,speed down,syn oil, no stuff in roof rack ,use a hitch carrier like we do).

    Folks who have huge SUVs will come to this forum.Hypermiling "lite" is worth 20% savings in city and HY.This is probably 200 gallons per year-$800!! Many folks have SUVs-some need them,some need them less and would like to bail out but they won't be able to because of their loan,and the dramatic drop in resale.Big SUV owners of modest means are STUCK! Their only option is to max out -mpg wise-what they have.
    They can probably save $70/$80 month with only modest changes in their techniques.If/when gas hits $5 it will be $100 month savings.

    Obviously what works for big SUVs will work just as well for pickups.

    My suspicion is that these techniques actually work better-% wise-on FSPs than on smaller more inherently FE vehicles with ATs.Not just in gallons saved, but in actual % improvement.Big FSPs usually have higher aero coef. than FE cars,and they usually have tires with poorer RR.A drop in speed improves their Hy mpg more,and higher tires pressure probably helps those poor RR tires more-tire wear helps more also.

    I've never gotten any direct negative comments on the forum despite owning a FSP;I appreciate that.No doubt others will appreciate it also.

  3. laurieaw

    laurieaw Sorceress of the North

    you make a good point, charlie. 20% increase is a large one, and even more so with a large vehicle. sure, i can pull high numbers with my small car, but i can't say that i can increase it by that percentage.

    good driving on your part :)
  4. SlowHands

    SlowHands Hypermiling Ironman

    good job Charlie... that's pretty close to what we do with the Guzzler, but I think you beat me because of running the AC. Nice work.
  5. phoebeisis

    phoebeisis Well-Known Member

    I wish I could "prove it" but I'm pretty sure big vehicles respond better(%wise) to hypermiling techniques than smaller more inherently FE vehicles that were designed with FE economy in mind. Older big vehicles just weren't designed with EPA numbers in mind. Why would they be, gasoline was ~$1-$1.25 gal until 2002-2003 when we got the first good hint that gas was really headed up in price?

    I remember I was commuting 90 miles a day in late 2002 early 2003,and the spike from about $1.20 to about $1.69 was a BIG deal to me with my 18 mpg 4.7 lt Tundra(driven carefully at 56 mph standard tire pressure no P&G).It cost about $9/day and that seemed like a lot!!Ha,ha little did I know what was in store for us!!

    Full sized pickups and SUVs-pre 2002 designs- were not maxed aero wise,so they respond more favorably to slight drops in top speed than an econo car with a .31 cd.

    They also had kinda aggressive tread patterns-not good for FE.Tire pressure,tire wear,and better tires can help their FE more than "better tires"help an econo car that started life with better RR tires.

    They use lots of fuel at idle- my 98 2wd Suburban uses .7 gal/hr with AC on vs my 2001 Prizm .2 gal hr with AC on.The Suburban is just 1.8 times heavier, but it uses 3.5 X more at idle.Turning it off at a redlight saves relatively more fuel.My departed Pilot-4400 lbs-used .4 gal hr at idle with AC on 2X the Prizm despite being just 1.57X heavier.

    Large and midsized SUVs/Pickups with inherently low FE will be around in large numbers for at least 7 more years despite gasline prices that could hit $10/gal.Most of the owners- middle class family income owners-are absolutely stuck with them.Affluent folks(family incomes $125,000 or more) really aren't being hit by fuel prices,and they can afford to take the trade in hit and buy more FE vehicles. Folks with $40,000-$75,000 family incomes can't trade in a car they financed because they owe much more than the vehicle is worth.Suburbans and other big and mid sized SUVs,and pickups have plummeted in resale and trade in value.

    In your neck of the woods-MINN- there must be lots of folks with middle class incomes who own biggish-maybe 4x4-vehicles.They are getting killed by fuel prices-especially if they are in rural areas and need to drive distances to larger cities to get essentials,go to work,school haul "stuff" for their animals etc.

    I'm lucky-my son drives the Prius-I'm stuck with the Suburban.The lucky parts is I substitute teach now,and there are at least 10 schools no more than 4.5 miles round trip from home. The poor FE is balanced by the very few miles I have to drive-maybe just 30/wk-2.5 gallons per week.

    I expect we'll see lots more BIG SUV and pickup owners here in the future.With the simpler Hypermiling techniques they can get at least "crossover" midsized mpg or even minivan mpg.Not spectacular mpg numbers, but HUGE savings in respect to gallons saved.If everyone did just basic hypermile "stuff" we could save 1,000,000 barrels a day(I think personal vehicles in the USA use 8,000,000 barrels/day of oil of our 20,000,000 day total and 11,000,000 day imported).

    Thanks,sorry to run on.
  6. laurieaw

    laurieaw Sorceress of the North

    charlie, you are right that MN has lots of pickups and i am sure a lot of middle income people are suffering with them. the area is live in is small enough that when i had my long commute on the 4 lane, i would frequently see the same people every day, and there were many in 4x4s and SUVs. the parking lot at work must be 65% trucks. however, there are still alot with fart pipes that i can hear leaving the lot for lunch or at the end of the day, who still have not made the correlation between slower speed and better FE. and some them still just don't care.
  7. phoebeisis

    phoebeisis Well-Known Member

    Vans of your vintage-1992-were pretty flat nosed and must have the CD of a brick. I've noticed than work type vans-conversion vans-have a bit more nose now,and seem to be a bit smoother.Nothing like the older flat nosed conversion vans of the 70's- like the A-Teams van "I pity the Fool." This mpg was a very pleasant surprise.My Scan Gauge when used in the city-shows 5.3 m/l at about 58-60 mph,so I expected maybe 17 mpg at the 70 mph I drove.I didn't take the SG with us because I would get us in a wreck while staring at it.Besides,I knew I was going to set the CC and drive-nothing tricky-too many miles for P&G.A very nice surprise for a very modest reduction in speed-69.5 mph-decent increase in tire pressure-43 psi- syn oil-worn tires-and well broken in engine(but I don't think the loose engine was a plus, but who knows??.

    Laurie-I spend some time on a GM forum,and I see lots of posts with folks claiming their low restriction exhausts (NOISY FART PIPES),and cold air intakes(a different sort of increased noise) improve their mpg!! They are dead wrong of course.Those mods can't possible increase part throttle FE-no reason they would.In fact,they frequently make the acceleration slower even if they increase peak hp.They probably adversely effect FE, but it is tough to convince folks that they just wasted $200 on the CAI,and $500 on the exhaust.When I was younger I probably believed it also(but with the old carbed 60's 70's vehicles it might actually have been true).With $5 gas on the horizon folks are seeing the light!!

    There will be a lot more folks visiting here to improve the mpg of their V-8s.They are essentially unsaleable,so they have to make the best of it.

  8. philmcneal

    philmcneal Has it been 10 years? Wow

    wow doesn't a surburban fit more people than a tahoe hybrid? well done!!
  9. phoebeisis

    phoebeisis Well-Known Member

    philmcneal, I wish I could take credit for it, but it is all GM.All I did was keep at 70 mph or less,use SYN oil Mobil I 5W30, use 42 psi in the tires(it calls for 33 front 41 psi rear),keep my somwehat worn tires,and use the CC-even on hills.DCing the CC on hills would have added about .25 mpg I think.At 70 mph with the CC and normal tire pressure it probably would get 20mpg-just my guess.For regular drivers the CC is 1-2 mpg better than their foot-even with occasional hills.

    The Tahoe-Hybrid or otherwise- is also 8 passengers, but the back row of 3 are kinda cramped.In the Suburban,the back-3rd row-isn't one bit cramped.The Suburban has about 14" more interior length that the Tahoe. Frankly,I figured if I'm going to take the huge mpg hit for a big body on frame SUV why not get the Suburban that can carry 4x8 building material with the hatch closed.The EPA mpg #s are always the same for Tahoe and Suburbans-get all the utility you can from that FSP-same mpg penality.The Suburban is like a long bed covered pickup-just cheaper-used-now,and more comfortable, more useful. We sleep at rest stops in ours-saves on motels,and loading/unloading at motels.

    Many-most- older Suburban Tahoe owners report 17-18 mpg in pure hy driving.My guess is that they are driving at 75 mph or so, have lower rear ends(I have the 3.42-the tallest), have stuff on the roof(absolute poison),and are running with 28 psi or less because they don't check their tires.

    The newest Suburban-2007 on-are rated 14/20.Mine is rated 12/17(new EPA numbers).Those Suburbans should get very close to 25 mpg on pure hy trips if they drive at 68 mph,and keep tire pressure up. Moving 5-8 people at 25 mpg isn't too bad.We had 2 adults,a big leggy dog and a huge amount of stuff-mtb,2 ice chests,lotta tools(202,000 miles),futon mattress, clothes,pillows etc.Weight doesn't matter much in interstate driving-just drag. I tried to take my wife into the Prius, but no go.She has to keep her legs/hips/arms straight(autoimmune problem with her arteries-PAN), so the Prius was a no go.

    "Someday",I plan to convert the Prius to a road trip vehicle by adding a 3 foot removeable box to the back of it.It will be contiguous with the interior,and supported by a hitch support. I'll probably use plywood,and maybe fiberglass.It will probably drop the mpg by 5, but still very good.The tricky part will be smoothing the roof to box junction- my well calibrated eye will just "guess" what looks aerodynamically "right."

    99% of the credit goes to GM- Owners should just drive a bit more slowly,pump up the tires,avoid 4x4 and AWD if possible(1 MPG penality),use syn oil,keep their tires a bit longer,and don't put stuff on the roof.I've noticed that my 98 has a factory chin spoiler-some newer ones don't-not sure why?

    Sorry to run on-not much credit due here-.They-body on frame SUVs-respond very well to city hypermiling techniques.New visitors-stuck with their SUVs- will be happy to learn that.

    Sorry to run on-thanks
    PS- I expected to get 17-18 mpg at 70 mph because on my short runs on city interstates my scangauge would show 4.5-4.8 mpl-not 5.7 mpl(21,5 mpg).I would only get 5.3 mpl(20 mpg) at 58-60 mph.My only explanation is that there are lots of overpasses on city expressways-and maybe my scangauge is off??I don't think it is because my city mpg-14 mpg-is roughly what my scangauge indicates?

    One of life's mysteries,I guess.GMs big SUVs are pretty efficient way to carry lots of people and "stuff."Unfortunately most of we owners have to use it as a second, or first vehicle,so we use it at 15% of capacity most of the time.I have some ideas to improve the city mpg-mainly weight loss-the spare,part of the second row etc..
    Last edited: May 27, 2008
  10. basjoos

    basjoos Well-Known Member

    If you shaped this box so it forms a boattail, it would improve your mileage by reducing the Cd. This is similar to what I did with my Civic. I removed the hatch and built a permanent boattail extension onto the back of my car, which added over 2 feet of additional cargo space to the existing cargo space behind the back seats.
    Last edited by a moderator: May 27, 2008
  11. phoebeisis

    phoebeisis Well-Known Member

    Good idea;I have eyeballed your Civic and wondered about it.Instead of a simple box something boat tailed tapered could make it a plus plus-more utility with as good a CD instead of the significant drop in FE I was planning to take.

    I wonder how a similar treatment would work on the flat backed Suburban?We use a hitch carrier on it now to carry stuff-ice chest,tool box,mtb- we don't carry inside because we need to inside room to sleep at rest stops. We will be stuck with it for a while-$$ problems-so improving its FE saves real $$.It is essentially a box on wheels with a bit of taper in the front, but none in the rear.

    How do you lay your fiberglass.I was thinking-for the Suburban-I would make a box wedge of thin plywood.It would have a flat floor, a sharply raked down top,and two sides raked inward-triangular shaped.All the pieces would be triangular.I could then "round it" with foam,and lay the fiberglass on the foam.

    I'm thinking of doing the Suburban first because it won't matter much if I screw it up(not like I can drop the resale much more)


Share This Page