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Fuel mileage on L6's


olie05

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Hello,

I have recently been checking out the honda insight forum (insightcentral.net) and I started to wonder what kind of gas mileage I should expect when I start driving my car around (hopefuly by the end of april) i know the z's aren't designed to be economy cars, they are sports cars, but economy is important too. So I want to know what you all have experienced as far as gas mileage goes and what has caused this mileage (good or bad) such as modifications or driving techniques.

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Guest comeandzpa

1981 280ZX N/A, 3-speed auto...I believe the original EPA estimates were something like 18 city and 23 highway, but most members seem to get slightly higher on the highway. I, however, average 8-12 around town and 16-20 on they highway. I usually get 200-300 miles on a (20 gallon) tank. I'm not sure what exactly the problem is, though I'm sure there are many. First of all, the car idles around 1200 RPM which cant help. There are a few broken/cracked hoses and lines in the engine which I still need to fix. I drive like a bat out of hell 80% of the time.

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On the drive up from florida when I first got my car I got something well into the 30's mpg. On top of that my car runs really rich, to the point of sooty exhaust and poor idling. I have a 72 carbed, built, l24 with a 5spd.

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Guest bastaad525

I'm really starting to wonder about some of you guys and you're 25-30+mpg's that you say you're getting.

 

No one take offense at that... but you gotta figure, we're talking about a 2.8L six cylinder motor (for most of you) with what is well known to be a horribly inefficient head design, running 20+ year old EFI systems or for some of you, even older carbeurated setups (sorry zero but I have a REAL hard time believing you're getting 30+ running carbs and knowingly running rich). Even brand new cars these days, many of the smaller four bangers weighing it at around 1.6 to 1.8 liters, barely eek past 30mpg's in freeway driving, average V6 these days is in the mid 20's and high teens to low 20's on the streets, with MUCH more efficient fuel delivery that our old Z's ever had. Not to mention, most of the guys on this board are running slightly to extremely modified setups making more power than stock, and it's only common sense to say that obviously if you're making more power you're using more fuel (though this isn't always the case). Now, I know some guys on here run aftermarket programmable EFI so obviously this does not apply to them, and I can see how they might be getting such great mileage. Even with the later, smarter 300zx systems I could see that happening. Personally I dont see how anyone else is claiming to get mileage far beyond what even the EPA rated them at...

 

I wonder if some of you aren't taking into account if maybe you are running different tire sizes than stock (shorter tires spin more for a given distance, hence you're odometer will go up faster) or have changed differentials or transmissions and not changed the speedo gear. Anyone who's not sure about the accuracy of their speedo (and hence, their odometer) should head over to http://www.geocities.com/z_design_studio/

and plug in their numbers (tire size, diff, and tranny's year) and verify if your speedo is correct or not. When I was running my rebuilt, and built up, N/A motor with '75 5 spd and 3.9's, and 205 70 14 tires on ZX rims, my speedo was WAY off (I think it was the wrong gear) showing about 75mph when I was actually doing 60 or so. Obviously my odo would have been showing way more miles traveled for a given tank of gas... scary thing was, that even with going with those inflated numbers, I was only getting about 12mpg's. This was running SU's on a moderately built high CR N/A motor. And yes they were tuned right, I tuned them on a dyno for peak power and torque, and my A/F ratio was only a little rich.

 

Well, I'm sure this is gonna ruffle some feathers... but I just find it hard to believe. I've owned three Z/ZX's so far, running different setups at different times, from a bone stock '82, to my built N/A motor running stock EFI, then modified EFI, then rebuilt SU's, now to my Turbo motor running stock EFI. I've never seen more than 23 miles to the gallon out of one full tank, even with mostly highway driving, on the best of days. Even after dyno tuning a few of those setups I never got more than that. My current setup averages 17-20mpg's with 2/3rds of my driving being on the freeway. I've also had this one on the dyno and though it does run rich under boost, at light cruise it wasn't running rich at all, the dyno guy said it was running just about perfect, I forget the A/F ratio. My spark plugs always look nice and tan when I pull them. I actually just posted a thread about this last week on the turbo/supercharger section, where I did an experiment, driving a full tank w/o getting on it at all, just driving like a 'normal' person would drive... didn't floor it once, and managed 19mpg's. I will say this, I am currently running with my O2 sensor disconnected, BUT, this is a relatively recent thing, I did used to have it hooked up and was still getting about the same mileage, even after installing a new sensor... When I dynoed with it connected and disconnected it got higher torque in the midrange with it disconnected so I left it that way (though I've heard that under WOT like on a dyno pull it doesn't do anything anyways).

 

Maybe I'm wrong, sorry if I am... but again, after three different Z's and multiple setups and for the most part, those setups all ran great, and always kept in a good state of tune, and NEVER getting those kinds of mpg's... I'm sorry if I"m a bit skeptical.

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I wonder if some of you aren't taking into account if maybe you are running different tire sizes than stock (shorter tires spin more for a given distance, hence you're odometer will go up faster)

 

When I read this I felt a little :oops: . You are right (in my case at least). My odometer reads more than 10% to high. (195/65-14 + inaccuracy of the meter itself). I guess I never wanted to think about that when I calculated my milage. But I still get more than 23 miles per gallon :2thumbs:

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sadly' date=' i've been getting approximately 14-16mpg i'd say..

full tank of (93 octane) premium @ high 200s to 300 miles or so..

i run very rich though haha :)[/quote']

with a 17 gal tank (280Z), 300 miles per tank would be 18 mpg.

 

I'm getting about 20 - 25 (I think). Running real rich. :(

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Guest bastaad525

How large is a '72 240's gas tank? I'm estimating 12 gallons as when gets down to about 1/4 I can always put about 9 gals in, but I dont know how accurate my gauge is.

 

I dont doubt about the LS1 getting such high mpg's... again... we're talking about a much newer, smarter EFI setup, designed to get the best mileage possible and run as clean as possible for stricter emissions standards. Not to mention if you have the six speed you can cruise it at such a low RPM it uses even less fuel. Stick that in a Z, much lighter than whatever car it came out of I'm sure, and 30mpg's seems very possible if driven sanely. I dont remember the exact numbers but I know I've been impressed when I read the stock Vette and Camaro's EPA numbers given the amount of power and size of the engines.

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I guess I get around 18mpg out of my custom L28ET setup (automatic, 3.90:1, 215/60/15). But once I'm finished, I'll be measuring things like gallons per hour too :D

 

For comparison: My stock DR30 Skyline (~1200kg, ~200HP, FJ20ET) gets around 24mpg city or country.

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cool, thanks for the responses. Once I get my z together (and before I start moding it any further) I will post the mpg since I have a completely stock set up as a far as drivetrain goes and see if I can maybe set a record! :lol: we'll see, one step at a time: Install engine, then drive.

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sadly' date=' i've been getting approximately 14-16mpg i'd say..

full tank of (93 octane) premium @ high 200s to 300 miles or so..

i run very rich though haha :)[/quote']

with a 17 gal tank (280Z), 300 miles per tank would be 18 mpg.

 

I'm getting about 20 - 25 (I think). Running real rich. :(

 

ahh sorry, saw someone comment on it being a 20gal tank (however i filled it up and it took like 18.5 gallons.. lol?)

 

but yes, i suppose its around 18mpg (havnt used the WHOLE tank, down to an 1/8th)

 

makes me feel a bit better though hehe

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Guest comeandzpa
sadly' date=' i've been getting approximately 14-16mpg i'd say..

full tank of (93 octane) premium @ high 200s to 300 miles or so..

i run very rich though haha :)[/quote']

with a 17 gal tank (280Z), 300 miles per tank would be 18 mpg.

 

I'm getting about 20 - 25 (I think). Running real rich. :(

 

ahh sorry, saw someone comment on it being a 20gal tank (however i filled it up and it took like 18.5 gallons.. lol?)

 

that was me...I'm almost positive the 280ZX's have a 20 gallon tank. Maybe I'm losing my mind?

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Guest bastaad525

Haynes manual says the 280zx has a 21 gallon tank, if I remember correctly. However, I was never able to actually fit that much in there :) After draining the tank once to clean it in my '80, I dumped a full five gallon fuel can in there, then drove directly to the gas station and filled it up, and was only able to put about 13 more in there. Started her up and the fuel gauge indeed read full tank. Also, I'd always try to fill it up at about exactly 1/4 left (when the cool little secondary fuel gauge that ZX's have would start to go down) and was always only to squeez about 13-14 gallons in there, leading me to believe that somehow, someway, the haynes was incorrect and that I only had 18 gallon capacity in my fuel tank. From that point on I always based my mpg calculations on that and came up with around 20-23 mpgs.

 

 

Now come on guys... surely at least one of you knows how big the 240 tank is... I've been asking this one for months and can't seem to get an answer!

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well, my mileage was definately a guesstimate and very well could be WAY off. I do know that we went from jacksonville fl to newark nj, nearly 1000 miles, on 4 tanks of gas comfortably and never going close to empty.

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Guest bastaad525

okay, you guys are giving me a reason to make an excuse for a long road trip! l've noticed one common theme here and that is that you guys getting the REALLY good mileage are noting that on long azz highway trips. Maybe this weekend I'll top her up and go for a drive up Pacific Coast Hiway... up to santa barbara and back should be at least a good 120 miles, usually relatively traffic free, can maintain a good 80mph most of the way. And I'll see what kind of mileage I get then.

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