Hyundai Genesis Forum banner

1 - 15 of 15 Posts

·
Registered
Joined
·
2,795 Posts
Discussion Starter · #1 ·
I first drove about 180 miles one way to Houston last night for the Houston Supercross. I spent about 30 minutes of the trip right around 90mph following a 350Z and the rest around 80mph. I averaged 25.7 mpg which I thought was good considering the speed I was traveling including 3 people and luggage.

After the race was over I headed back. It was night time on the way back which meant I can't watch for coppers so I put the cruise control on 70 mph and fully expected 28 mpg or better. Nope, 25.9 mpg.

Both trips had a couple of hard accelerations in there and about a dozen redlights in some traffic.

I've read in the past that an engine is the most fuel efficient at it's peak torque. Can anyone elaborate on this?

Does anyone feel that their 3.8 6 speed GC gets better mpg at slightly elevated speeds? Like around 75 mph? Any proof?

Thanks.
 

·
War Eagle!
Joined
·
705 Posts
mine seems to be just the opposite. I find that the best mpg is at 55 in 6th gear > 2k rpm. I've got above 30 mpg at this speed. Also, it seems like 2k rpm is the sweet spot, no matter what speed. Just put it in the right gear to cruise at that engine speed.

55> 6th gear
45> 5th gear
35> 4th gear
etc....

That is pretty good mpg at the speeds you mentioned though, but were you going off the trip computer or did you calculate it yourself? I get awful mpg when I go that fast lol
 

·
Registered
Joined
·
2,795 Posts
Discussion Starter · #3 ·
If I go really slow, like 55mph, then yes I know that will optimize my mpg even if it's below ideal engine efficiency just because I'm pushing less fuel sucking wind. I don't feel it's safe to drive that slow under most conditions.

I used the trip computer but only because the couple of times I checked it, it was dead accurate.
 

·
War Eagle!
Joined
·
705 Posts
Its definitely not safe to go that slow on the interstate. If you're on some other highway with a speed limit of 45 or 55 though....

In my experience, this speed is where you will hit and exceed the highway mpg rating for this car. I have yet to get above 26 on the interstate doing 75-80.
 

·
Car newb
Joined
·
2,239 Posts
I average about 28 going mostly 75 with a few towns along the way going straight down 36 from Caldwell to Freeport. At work when I drive around Freeport and Lake Jackson when I'm at the dock, it'll decrease to about 24-25 average on the same tank of gas (mpg gauge, not calculating).
 

·
Registered
Joined
·
2,795 Posts
Discussion Starter · #6 ·
I'm thinking that 75mph on the 6 speed puts the engine at peak torque which is where optimum engine efficiency is at.

Need to track down more info on MBT (Maximum Braking Torque) and find some dyno charts for the 3.8.
 

·
Registered
Joined
·
176 Posts
The highest I've ever seen during all highway driving from little rock or Shreveport to Houston is 31.5 mpg. Average speed 60-65. I think somewhere between 55-65 is the sweet spot.

However, when I calculate it I always come out about 1.5mpg lower than its indicating... I think the highest I've ever actually seen was 29.5 calculated. that was on the same trip it was indicating 31.5 but after i got home and drove around town a little, so it went down some... I get the same thing in the wifes sonata, the computer is always about 1.5 mpg optimistic.
 

·
3.5L TT but wrong body
Joined
·
12,003 Posts
I think you should watch throttle position and engine load over RPM and speed. Scan gauge comes in handy for that.
 

·
Knows some stuff...
Joined
·
6,852 Posts
The highest I've ever seen during all highway driving from little rock or Shreveport to Houston is 31.5 mpg. Average speed 60-65. I think somewhere between 55-65 is the sweet spot.

However, when I calculate it I always come out about 1.5mpg lower than its indicating... I think the highest I've ever actually seen was 29.5 calculated. that was on the same trip it was indicating 31.5 but after i got home and drove around town a little, so it went down some... I get the same thing in the wifes sonata, the computer is always about 1.5 mpg optimistic.
I agree with ernib and the rest of most of you, but QM said he got the best at 85mph.
I found my setup hit it's MPG sweet spot a little above 85ish.
I can only assume it has to do with the dryer less dense air in CA and highly inflated tires? Since hwy fuel efficiency is mainly based off of drag and tire friction. Though I am assuming his later post possibly was retracting that when he said
BTW just calc'd a few tanks in the 3.8L, the mileage calculator is WRONG. It's trying hard, but it aint right.


Here is a thread on the same topic basically. http://www.gencoupe.com/2-0t/52140-horribleee-gas-mileage-suggestions-please.html#post554900


Though I am glad to see five or six people agreeing with me in my "own crazy physics world" lol.
 

·
Registered
Joined
·
2,795 Posts
Discussion Starter · #10 ·
All highway and only 30 miles so not a real test at all. It's warmer which means the air is thinner and there is less wind resistance; no AC and the sunroof was open. Cruise control on 65mph; I got 37.X mpg.

Scratch all that crap about sweet spots as that is just wishful thinking. Whatever the lowest speed you can maintain w/o bogging your engine in top gear will result in the best mpg imo.

I think if the 55mph national speed limit was still in effect and I maintained that low of a speed my 300hp GC would be a 40mpg car all day long.
 

·
GenSport Carolinas
Joined
·
3,001 Posts
I think you should watch throttle position and engine load over RPM and speed. Scan gauge comes in handy for that.
Exactly. Gas consumption is related to the throttle and not directly to rpms. You can be doing 3k rpm and have different results, depending on how much throttle you've got to give to get there. If you're cruising down a mountain at 3k, your gas mileage could out the roof since your not using the gas to get you there.

In 6th, on a flat hwy, I do find that I'm fairly efficient at 80 and 3k rpms.
 

·
I'd rather do it myself
Joined
·
19,307 Posts
It's more the change in throttle position that sucks up the gas. Stops and starts, up hill and such. You can easily see this with a boost gage.
 

·
Registered
Joined
·
120 Posts
managed 36 mpg @ 60mph...winter tires and rain/icy roads! i recently switched to premium around 4 tanks ago...been noticing a 10% improvement in mpg.
 

·
Knows some stuff...
Joined
·
6,852 Posts
It's more the change in throttle position that sucks up the gas. Stops and starts, up hill and such. You can easily see this with a boost gage.
exactly, I have said it before. It is a mix of load (MAP basically), rpm, and AFR. Though since we have VVT, a MAF reading would be more accurate, but MAP is close enough. Any of those are pretty key.

Going form 14.7 to 12 is going to result in ~18% loss in mpg. Though 15.6 is the most efficient AFR, it pollutes more and you won't see any facotry car at that.

Going from 2000rpm to 4000rpm will result in 50% loss in fuel consumption, but your speed also goes up ~%50 (acording to gear calculators). Your fuel consumtion that goes up is dierctly proportional with your speed and your mpg's stay the same.* The "*" is because this is only theoretically true. As your rpm goes up, your speed goes up, and your drag goes up and your friction and you load goes up. So there is a certain point that rpm increase makes your mpg go down.

And load, if your AFR and rpm is the same, but you go from 10ingh, to 0, your enigne is getting much more air in per stroke (and if your AFR doens't change, more fuel) so agian your consumption goes up, but so does your speed so it is hard to say how much your mpg's are affected, I don't have any real data to calculate. But on like a hill where your load goes up and your speed doesn't, your mpg's go down.
 
1 - 15 of 15 Posts
Top