21 MPG on the highway
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RWD 19xx-1984 DeVille and Fleetwood,
1985-1996 Fleetwood and Brougham Forum Discussion, 21 MPG on the highway in Past Cadillac Vehicle Discussion; Finally got to do some serious highway driving today, about 125 miles one way, and then 110 the other way. ...
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    carguy16's Avatar
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    21 MPG on the highway

    Finally got to do some serious highway driving today, about 125 miles one way, and then 110 the other way.

    Kept the cruise at 70-72 most of the interstate, which was 60 miles each way, then on the highway, this was highway 70 east near Arcadia, kept it around 50-65. Burried the gas pedal a couple times passing some clowns,

    I had the A/C on and off through out the entire trip.

    That damn low coolant light has returned, I get up to 79 not even flooring it, the lights on, have it in over drive doing 55 or 65 and its on. Does this mean my engine is being starved for coolant when it comes on, or is it just something else?

    I noticed that my MPG in the city bumped up from 14.2 mpg all the way up to 15, but let me tell you this. I went about 180 miles or so on this new oil change(10W30 Valvoline Durablend synthetic blend oil), and a new oil filter, have of that mileage was with the old oil and old air filter, and I was driving the car with a lead foot most of the way too. I am suprised to get 15 mpg, it should be around the block of 16 or 17 in the city next time, if I calm down on my driving habits. Moral: Oil and Air filter, very important.

    I even got smoked by a semi. Thats a story for the kills section.

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    Barry Clark's Avatar
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    Re: 21 MPG on the highway

    I get at and about 20mpg. Well, unless I bury the pedal alot then it is far less.

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    Re: 21 MPG on the highway

    Quote Originally Posted by carguy16
    That damn low coolant light has returned, I get up to 79 not even flooring it, the lights on, have it in over drive doing 55 or 65 and its on. Does this mean my engine is being starved for coolant when it comes on, or is it just something else?
    When was the last time you checked the coolant level?

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    Re: 21 MPG on the highway

    A little while ago, but the coolant is reading normal.

    I actually rounded this up a little while ago, its close to 22 MPG, that includes me burying the gas pedal a couple times, and easy acceleration up to 90 passing some clowns.

    I think that was pretty descent, anyone else think around 22 MPG is descent?
    All in all, thats pretty damn good!
    Quote Originally Posted by davesdeville
    When was the last time you checked the coolant level?

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    Re: 21 MPG on the highway

    I don't know if this will help you but it helps me keep my radiator full, park the car on a flat surface let it cool take off the radiator cap look in if it looks full have someone start the car while you watch the coolent level to make sure it doesn't surge up it should begain to drop as the thermostat opens then while its running fill it up plus check to see if you have a pinched hose running from the coolent resovor but be carful not to let any coolent spray out.

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    Re: 21 MPG on the highway

    Carguy, do you have to add any radiator fluid when the light comes on? If not, it may just be a sticky switch. I've never taken the switch out, so I haven't inspected its construction, but it is installed in the front bottom of the full reservoir. Most likely it gets a good coating of the slime and grime buildup over time, which may increase its faulty readings.

    If you are adding fluid you obviously may have a leak.

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    Re: 21 MPG on the highway

    Hate to tell you this but the mileage you are getting is not very good for a stock LT1-powered Fleetwood. You should be more like 23-25 at least. I'm getting 22mpg on my car with all my mods. Around town, you should see about 15-16.

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    Re: 21 MPG on the highway

    I'll be taking a trip from Northern Va. to CLeveland this weekend in my road warrior Fltwd. It's a business trip, so I'll be keeping the mileage. I'll try to get a good reading for the entire trip (720 mi round trip). I've had as much as 20 mpg combined mileage over a 250 mi trip.

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    Thumbs up Re: 21 MPG on the highway

    Quote Originally Posted by Katshot
    Hate to tell you this but the mileage you are getting is not very good for a stock LT1-powered Fleetwood. You should be more like 23-25 at least. I'm getting 22mpg on my car with all my mods. Around town, you should see about 15-16.
    I get about 23/18 (city/hwy) with 3.08's and an Eaton posi and a good tune. What you have to watch out for is speedo errors that will make it look like you're getting great mileage when actually you're just putting more miles on the car. I just caught someone's car, locally with that issue as they complained about my tune netting them less mpg.


    Brian 96 FWB / 96 Impala SS

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    Re: 21 MPG on the highway

    Man! This thread is gonna make me cry. I'm getting like 11-12 mpg. I gotta look into this. Even with my stock motor I got crappy mileage, but I saw a bigger decrease after the swap. Oh well I'm used to spending $50.00 a week in gas anyway.

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    Re: 21 MPG on the highway

    Quote Originally Posted by Mr Cowl Hood
    I get about 23/18 (city/hwy) with 3.08's and an Eaton posi and a good tune. What you have to watch out for is speedo errors that will make it look like you're getting great mileage when actually you're just putting more miles on the car. I just caught someone's car, locally with that issue as they complained about my tune netting them less mpg.


    Brian 96 FWB / 96 Impala SS
    Good advice but in my case, I reflashed the PCM myself with LT1-Edit, so I know it's accurate. Plus I checked it against a police radar gun! On top of that, I'm running 3.73's and a Detroit Locker. It's all in how you set it up!

    Kevin Donovan
    '95 Cadillac FTS
    (Click the link to see my car)
    http://members.cardomain.com/katshot

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    Re: 21 MPG on the highway

    Quote Originally Posted by FASSTWOOD
    Man! This thread is gonna make me cry. I'm getting like 11-12 mpg. I gotta look into this.
    hey man, even though i don't have the same engine, i have the olds 307. i'm only getting 10-11mpg. and the car runs perfectly. ahh. eh, well i may be switching to a 403 in the future. so i won't worry too much. i also think my odometer is part of the problem. it doesn't register the correct amount of miles, they go by too slowly.

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    Re: 21 MPG on the highway

    Quote Originally Posted by Katshot
    Good advice but in my case, I reflashed the PCM myself with LT1-Edit, so I know it's accurate. Plus I checked it against a police radar gun! On top of that, I'm running 3.73's and a Detroit Locker. It's all in how you set it up!

    Kevin Donovan
    '95 Cadillac FTS
    (Click the link to see my car)
    http://members.cardomain.com/katshot

    MMMm, no....and yes, I guess.

    With LT1 edit you could set the trans and speedo settings to reflect better mileage (my point earlier) but it won't be right. Even a 1% higher speedo reading will make it seem you're getting better mileage. Some people set that the other so they put less miles on the car! If you're referring to "that setup" then, well, I guess I agree.

    I don't know any full size GM cars getting your mileage with 3.73's, set up correctly.

    Is that through a whole tank of gas? Some people report great mileage of the run two exits down the HWY and fill up again. I'm talking about trips of 300 miles and more to get a good idea of your mileage.

    Perhaps the radar and mileage are off. Radar guns are notorious for that (a point I win in court).

    Did you measure the tires?

    Brian 96 FWB / 96 Impala SS (not starting a flame war, just speaking from lots of experience)

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    Re: 21 MPG on the highway

    Yes, you CAN setup the speedo cal any way you want but what would be the point? In my case, I don't care about cumulative mileage. I want to know that when I blow by a cop, I know EXACTLY how fast I'm going. Yeah, the radar gun "could" be off but the chances of it being off the exactly the same as the speedo are rather remote I would think.
    My tires are Goodyear Eagle RS-A's in 255/55-17 with an overall diameter of 28" as given by the manufacturer and crudely verified by myself.
    As for my MPG, I was on a trip from Phila, PA to a small town in Connecticut a couple years ago when I checked it so I felt it was a solid number, especially since I DID have a couple road races on the way!
    I don't routinely check my mileage since I don't pay for my fuel (company gas card) so I can't say for sure what my "current" mileage is. But since I've only made minor changes in my PCM flash over the last year and HAVE installed new plugs and wires this past summer, my mileage should be at least as good as I stated previously.
    As I said, it's all in the setup. Meaning how I have my car setup totally.

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    Re: 21 MPG on the highway

    Quote Originally Posted by Katshot
    Yes, you CAN setup the speedo cal any way you want but what would be the point? In my case, I don't care about cumulative mileage. I want to know that when I blow by a cop, I know EXACTLY how fast I'm going. Yeah, the radar gun "could" be off but the chances of it being off the exactly the same as the speedo are rather remote I would think.
    My tires are Goodyear Eagle RS-A's in 255/55-17 with an overall diameter of 28" as given by the manufacturer and crudely verified by myself.
    As for my MPG, I was on a trip from Phila, PA to a small town in Connecticut a couple years ago when I checked it so I felt it was a solid number, especially since I DID have a couple road races on the way!
    I don't routinely check my mileage since I don't pay for my fuel (company gas card) so I can't say for sure what my "current" mileage is. But since I've only made minor changes in my PCM flash over the last year and HAVE installed new plugs and wires this past summer, my mileage should be at least as good as I stated previously.
    As I said, it's all in the setup. Meaning how I have my car setup totally.

    Check it again. I'll bet you're not seeing 25 mpg.

    Also, typical 255/55/17's as in the Impala wheels/tires measure closer to 26" so that's a little off right there.

    My engine's tight and I've only seen 24 mpg, using cruise control set at 75 across OH (where it's really flat), with the wind on a course of ~300 miles.

    I'm from the mdwest so telling me you went from Philly to CT, doesn't mean much. Once you drive a good 300 miles calculate it back and you'll see that you're not getting that kind of mileage.

    FOr the others who get 11-13 in the city, I can get that low too if I get on it stoplight to stop light -- very easily. It's all how you drive it.

    Brian 96 FWB / 96 Impala SS

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