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Hi - Anyone any idea why I seem to get about 2 mpg better fuel consumption at 70 than I do at 55mph? Some aerodynamic effect. Running a 3.0CRD with Supachips remap, Quadrdrive II transmission. Is in 5th gear at 55 and 70, so that's not the answer. I'm puzzled.
Ima.
 

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I would be surpiised if you are in 5th at 55. I think that standard upshift point is just above 60 and superchips move it up a little more if I am not mistaken. I have that tuner and the transmission app. I have the same issue on my 5.7. Unless I conciously drive it fast enough for it to drop into 5th it stays in 4th. I can then slow down again to about 60 and it stays in 5th.
 

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Assuming you have stock sized tires and wheels:

Being in 4th gear at 55mph and 5th gear at 70mph is the most likely to account for the difference, you can verify it by engine rpm at those two different speeds.

If you're in 5th at both speeds, one possible explanation:

Engine power and efficiency varies with the engine rpm, as well as rolling resistance and aerodynamic drag varies with speed. You only have 5 ratios to match up those desired sweet spots of engine rpm to vehicle speed. Thus, one cruise speed may match up to an efficiency sweet spot, while another does NOT.

Bigger tires will change rolling resistance/aerodynamic drag vs engine rpm.
Retuning your motor with the super chips will change the efficiency vs engine rpm.

Yes, usually the lower the engine rpm the better the efficiency. BUT, it is possible to get the engine rpm too low, especially if the engine rpm is too low for the load its under, and efficiency will actually suffer.

A vacuum gauge is a good way to measure this, if you're in the same gear for 55mph and 70mph, and you get better mileage at 70mph, I'm willing to bet you get more engine vacuum at 70mph. To high of a load for the motor at the low rpm, will require you to open the throttle plate more to produce the power to match the load, that will drop the engine vacuum. Getting rpm higher, where the engine makes more power, will result in having the throttle plate closed more and having more engine vacuum. i.e. there is a sweet spot to match the vehicles load from rolling resistance and aerodynamic drag to the amount of power the motor can make while burning the least amount of fuel, if the gear ratios are right you match up at the right point and get the best mileage. Yea, the sweet spot is fairly low rpm, and generally, the lowest rpm you can get while cruising gets the best mileage, BUT, its possible (especially after modding the vehicle) to get engine rpm too low for the speed your cruising and the motor actually has to burn more gas to match the load at the slower speed. If mileage jumps up for 60mph or a little faster, that would confirm my theory here.
 

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Discussion Starter #4
Thanks

Thanks Mongo, I'm definately in 5th as the revs rise if I knock the shifter down to '4'. Your sweet spot theory may well fit the bill though.
 

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Isn't that the mds kicking in? The hemi goes from 8 to 4 cylinders when in a constant speed, not sure what that speed is though.


Sent from my Autoguide iPhone app
 
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