Mileage with custom tunes

[MoparMap]

Just curious if there are any gains to be had in the mileage department with custom tunes. I'm going to a dyno day in the coming weeks, so I'll have some data to pass on to whatever tuner I choose and was thinking about getting an update on my current setup. The car came with an SCT and some tunes, but they were from a local shop where I bought the car from that I don't think has quite as much experience with Vipers. I've heard of some impressive mileage numbers from people ranging from mid 20's to even a rumor of over 30 on a long highway cruise. I've yet to crack 20 myself and was wondering if custom tunes made any difference in this department or if it was more about the top end power and smoothness.

And yes, I know, I didn't buy my car to be fuel efficient. However, I do daily drive it 25 miles to work and back on the highway every day and do lots of weekend cruises with it with different groups, so if I can get better mileage out of it it would just be extra money in my pocket, which never hurts. To that end, are any of the tuners better known for this? From what I've seen and read it seems like you really can't go wrong no matter who you pick, but just interested to hear if anyone has noticed better gains with any one in particular.

 

[dester243]

When I drove back from houston I got about 24 mpg ac on. But I was also cruising 80 to 100 mosy of the time.....

 

[MoparMap]

Geez, I'd love to see that. When I drove the car home from Mississippi I pretty much got 16 every single tank, even just cruising 70-75 down the highway in 6th mindlessly. Swapped to the hot tune (got a hot and cold with the car) when the weather turned and have gotten more like 17-18 on the commute. I realize at cruise that it should be shooting for 14.7 anyway if it's going closed loop (that is assuming it actually does still go closed loop), but I didn't know if there were other factors that might help with the mileage like timing. I suppose my fuel map could be so far off that it can't auto-adjust it back, but I find that highly unlikely since the pipes don't show overly rich or anything.

 

[STUGOTS]

just cruising in high gear you can get some decent MPG actually

 

[345s-bspinnin]

On the highway, stock 25mpg, headers/catless/tune 26mpg, and 24mpg with heads/cam; all while averaging about 85mph.

 

[MoparMap]

Dang, I'm not sure what else could be it other than the tune then. Car came with a K&N and a Borla catback, otherwise stock to my knowledge. Surely the speed wouldn't make that much difference on mileage? I've spent most of a tank on the highway doing 70-75 in high gear and still haven't gotten to 20.

 

[Sonoman]

A lot of things can affect mileage... draggy brake caliper, bad wheel alignment, dirty injectors, clogging cats, etc. Have you checked the rear end ratio (either measure driveshaft revolutions per wheel revolution or compare engine RPM Vs. speed as charted for this car)?

 

[Stonep185]

Rear end ratio was my first thought.. My stock gts with catbacks gets 27mpg averaging 80-90mph

 

[dester243]

That's weird that your straight highway only got about 16 mpg. Your o2 sensors can be "dying" saying that the car is running lean and the ecu dumps in more fuel. You can either throw in a wideband to see what it's doing then go into a GOOD scan tool that can give you short and long term fuel trims to see what the o2s are doing. Live LTFT and STFT can sometimes help diag lazy o2 sensors which are not out of range yet but makes the car run very rich.

I know in different tuning software/hardware packages, you can also disable the o2 closed loop system. Not sure if the SCT scanner can do that or if the tuner did that or not.

I get 24 mpg if I do straight highway driving. City/highway mix is about 15 mpg. I don't look at it as Miles per gallon. I look at is as SPG. SMILES per gallon haha that was cheesy.

 

[TheOldViperMan]

I notice the people getting better milage are cruising at 80-85.
Going 70-75 in 6th means less rpm's, but IMO going anywhere near 65-70 in 6th gear is a no-no.
Going that slow in 6th, I'm guessing the engine is laboring at that speed and causing the low mpg.
Why not try driving at least 80 on the highway on a tank.
But the tickets will offset any savings - LOL.
BillyC

 

[MoparMap]

A lot of things can affect mileage... draggy brake caliper, bad wheel alignment, dirty injectors, clogging cats, etc. Have you checked the rear end ratio (either measure driveshaft revolutions per wheel revolution or compare engine RPM Vs. speed as charted for this car)?

Car was recently aligned, so I think that's taken care of. Have replaced all the rotors and checked out the brake system, but no caliper rebuilds, so might be something to look at. It seems like it rolls pretty easy though for what it's worth. Have run a bottle or two of injector cleaner through it since I've owned it, but obviously not as good as a real professional clean, so might be another thing I look into if I can't bring the numbers up. I think the cats are doing okay since I haven't noticed any extra heat that I can tell. Any easy way to check how clogged they might be? I know the rearend is stock gearing, just replaced it with a gen 4 takeout and I only turn 1250 rpm or so cruising 70.

Pretty sure the rear sensors have probably been disabled given the custom tune, even though I still have all the cats in the system. Guess I could try unplugging a rear one as an easy check. I suppose they might be getting toward their normal service life. Almost 60k on the car now, and from what I've heard O2's are generally either 60 or 100k service.

 

[dester243]

I think the rear ones are to just monitor the cat. The fronts are the ones that would do the fuel trimming.

 

[MoparMap]

I notice the people getting better milage are cruising at 80-85.
Going 70-75 in 6th means less rpm's, but IMO going anywhere near 65-70 in 6th gear is a no-no.
Going that slow in 6th, I'm guessing the engine is laboring at that speed and causing the low mpg.
Why not try driving at least 80 on the highway on a tank.
But the tickets will offset any savings - LOL.
BillyC

I've actually tried driving a whole tank not using 6th for my commute a while back and didn't see any kind of noticeable difference. Less than 1 mpg difference as I recall. The commute is roughly 20 miles on the highway, probably 15-17 at 70, the other 3-5 at 55. Have maybe 5 miles of town driving in there as well, but it's early morning so there's not much traffic (no little to no stop and go, though the way home is a little different). Will be doing a long highway trip here before too long where I can try to be really consistent for a whole tank a few different times. Might try one tank at 80 mph, one at 70, maybe one in just fifth gear even.

 

[MoparMap]

Well, interesting update after my weekend trip. Didn't really hold to my ideas of different cruise speeds, gears, etc. as I really just wanted to get where I was going and followed along with traffic. First tank was pretty much non-stop down the highway, probably averaging around 85 mph and got me around 21 mpg. Second tank was more of the same and got me 22.7. The really interesting one was the first tank used on the way back home. Even getting held up by some construction I pulled off 26 mpg. What I noticed on that tank though was that I was burning 93 octane at the time as opposed to the 91 that I normally use (as that's about all I can get back home). Wondering if maybe that has something to do with it and perhaps my current tune is being a little too aggressive for the fuel I have available.