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I think the best I ever got was 26.5 hwy in the 86 after I put on headers and before I messed with the intake or engine while I was driving from SC to La. 900 mile trip.
Now I can get 23-24 with the 398 if I just keep it right about 75mph and cruise and 19 overall. Maybe if I had a 6-speed I could do better.
I seem to be hanging around the low 20's on the highway with the '96 LT1 and 3.07 gears as long as I don't enjoy the on ramps.
It sure would be nice to get the mileage that the turbo diesel in my wife's Equinox gets. Best so faris 50.6 mpg. It averages 35 to 40 combined and has good get up and go.
Regardless C4 mileage is lots better than the '69 350/300 we picked up for my friend on Saturday. We had a little over a 100 miles to drive and it went through 1 tank of gas and most of another.
I am one of the ones that has stated 30 mpg in a few threads.
My daily driver '94 6M (all stock) regularly got 30 mpg or a little over when cruising on the highway. My daily commute was 100 mile/day on a US highway (55 limit) and Interstate (65 limit) I usually set the cruise at about 7 over the limits. This was in central Illinois and things are pretty flat, so not a lot of up or downhills to lower mileage either.
I also had an '87 4+3 (with the 0.6 OD ratio) that also got over 30mpg on the highway cruise. On a 1000 mile trip from Colorado to Illinois I got 30.2 mpg for the highway trip.
My current stock '95 6M is very close to the '94. My '96 6M is maybe a mpg lower.
All of mine were based off the number of miles divided by the number of gallons when I filled up. The dash display on some of my cars has been incorrect ( a few were close). On the inaccurate display ones, the '96 reads lower mpg than actual, but the others are optimistic and read higher than actual.
On any of mine, driving in town reduced mileage quite a bit. I guess I like to use the throttle, but on the highway I just cruise.
I am surprised and happy that they get so good mpg's. That was used in a discussion with my wife that I should get a commuter car that gets good mileage. That helped to "sell" the idea to her. It worked for my '88 and '94 arguments anyway.
St. Jude Donor '05-'06,'07-'08-'09-'10-'11-'12-'13-'14-'15-'16-'17-'18-'19-'20-'21-'22-'23
Display shows about 25MPG avg on the highway, driving between Ventura to Thousand Oaks CA a couple of times a week. Is it accurate? Ehhh perhaps I'm really getting 40mpg or 10mpg, don't cross my mind often lol In my truck though, I always seem to try to squeeze every foot to the drop of gas
St. Jude Donor '10-'11-'12-'13-'14-'15-'16-'17-'19-'20-'21-'22-'23
I am another one that posted a few over 30mpg trips.
It takes the right set of conditions, and it does not happen often.
This picture shows my 2009 Power Tour trip.
Leaving my house in Rolla, MO and driving to the starting point of Madison, WI.
That first fuel stop I drove 295 miles and used 9.277 gls. That works out to 31.799 mpg.
I calculated mpg for every fuel stop along that trip. All of the numbers below are calculated and not from the display.
Notice that on the long Interstate runs, the mpg goes up, but there are also a lot of local and two lane "spirited" driving legs also.
Also notice that on the next to last leg (driving back home from Bristol, TN), I drove 453 miles before stopping for fuel.
In the end, I drove a total of 2,522 miles and used 105.04 gls. of fuel for an average of 24.010 mpg.
Not bad really.
Just returned from Gatlinburg, Tn back to Louisville, Ky. 1985 Base Coupe, Auto, 2.73 ration, 50k miles. 23.5 mpg. Think the indicator was showing around 24 MPH. Around 18 in town. True mileage not the dash reading.
I'm sure it's not as aero dynamic as a coupe. And you've lowered compression. And added the drag of a supercharger. And, and, and....
HERE is ONE trip where I got 30+ and posted about it. From that thread: "Got up the next morning at 5, packed the car and hit the road. I stopped in Montrose to top off the gas and then set the cruise at 84 and drove w/o stopping to St George where I got lunch, and decided to top off the gas. 431 miles and 13.8 gallons = 31.23 mpg. Drove the rest of the way to: LVMS Drag TrackLas Vegas and straight to the track"
I'd guess that another reason you aren't getting better numbers is, how often are you getting on the highway, and driving, uninterrupted for 4, 5, 6 hours straight...get off and pull right into a gas station and do your calc's? That is the driving "mode" where you're going to get the peak numbers that the car is capable of generating. Constant speed, zero interruptions (idle time really kills the mpg), warm oils, etc.
On 7 1/2hr trips between SE NY and NE OH my prior 35th Anni. pulled ~30mpg while turning low 13s at the strip.
The current 35th Anni gets 15s around town; while the "new" '89 MN6 vert gets close to 20mpg.
Looks like the MN6 would pull >30mpg on that trip but not likely to find out.
Your lowered compression and that largish cam will really hurt your highway mileage. Your car (and mine) turning something like 1700rpm at 70mph. and there's probably a fair amount of reversion happening at that low an engine speed. How much overlap you have (LSA) makes a big difference too. FWIW, my car with the same gears and ZF6, with 396 but smaller cam (but 108-deg LSA, so maybe not a lot less overlap?) and close to 11:1 CR, got 26mpg on a couple all-highway trips, even with 315 tires at all four corners. This was when I really stuck to 70mph speed limits. I was really surprised, but I think if I bumped the cam up to any longer duration it would be a very different story.
Last edited by MatthewMiller; 05-07-2019 at 08:31 AM.
Yeah you used to see these posts all the time whenever gas prices get high
maybe theres something to my vert where its not as aerodynamic as the coupes. Who knows
I have to wonder too, how L98s with their 8.5-9.5 compression (depending on year) fair compared to the LT1/4s with their 10.5-10.8 compression w/ stock gears and ZF6 trans. My black 95 coupe w/ 73 mph w/ cruise control on, actual GALLONS PUMPED/MILES between IL and Denver Co and back = 31.1 mpg.
I don't think there's any argument that higher compression ratios produce better efficiency = more mpg, in most cases. Of course O2s have to be working properly too.
When I first got my 92 (in 94), I was doing 4 hour trips to see my then gf almost every other weekend for quite sometime.. On full tank, and on cruise control, I could drive from my house to hers, park the car for the weekend (we would take hers around the weekend), and drive back again on a single tank of gas - about 550 miles roundtrip, and still have enough gas to do some local driving. I was thoroughly impressed, typically around 600 miles a tank - in a corvette...in 1995.
But its not 1995 anymore...and I ain't getting 600 miles per tank anymore and lucky if I can get 450 miles a tank now...that's a 25% reduction in performance.
Aside from a standard tuneup, what are somethings I can do or be looking at to get that kind of performance back? My 92 has 112,000 miles on her.
Anyone else here never figure their mpg?When its empty fill it. Ignorance is bliss!
I dont think anyones getting 5 mpg better than advertised, if it feels good to say it more power to ya.
No offense but you can "think" all you want, however the facts don't lie. My post below, like many others, is true. Round trip was 550 miles...did that trip on a single tank of gas every other weekend for nearly 1.5 years..and still had 2 gallons by the time I got home. A refill was usually right around 18 gallons. The math doesn't lie...600 miles / 20 gallon tank = 30 mpg. Unless you are aware of some hidden reserve tank the rest of us don't know about (just ribbin' ya).
05-06-2019, 12:21 PM
all the ‘my c4 gets 30+ mpg’ comments
