MPG and vehicle speed

[johnj92131]

For the last 3 months, the Comanche has been my daily driver while my 96 Passat is in the paint shop for a full repaint.  It has been quite a difference in fuel costs, going from the 40 to 45 mpg with the TDI to the 15-18 mpg with the Comanche.  Because  I have been making a weekly freeway trip of about 150 miles on each tank full for the last few weeks I thought it would be of interest to see what happens to my MPG if I slowed down from my normal 75+ to a more reasonable 60+.

 

With the last 75+ tank, I traveled 395 miles on 22.25 gallons for an average of 17.75 mpg

 

This tank at 60+ I traveled 434 miles on just about the same 22.34 gallons for an average of 19.42 mpg

 

So by just slowing down my max speed, the mpg went up by 9.44%

 

My truck is a 91 long bed with a 4.0 and AW4 transmission 2 wheel drive.  Tires are Kumho 225/75 x 15.

 

 

[HOrnbrod]

I can relate, although it's not MPH that causes such a wide MPG disparity for me, it's city vs. highway. Quick specs: 91 LWB, bored / stroked 4.6L with modded and and balanced intake and exhaust systems, 31's, 2WD w. 4.10 rear, and stock AW4, mid-grade fuel, 23.5 fuel tank.

 

For a tankful of 100% highway driving I have averaged at least 21 MPG consistently for years. I normally cruise at 70-75 MPH.

 

For a tankful of 100% stop and go "running around town" type driving I'm lucky to get 14, usually 13.x MPG. Consistently.

 

I have no idea why there is such a difference. Even driving super conservatively around town, the best I can get is 14MPG. Any ideas outside of always driving at speed on the interstate?

[gogmorgo]

Simple explanation, you use the most fuel when you're accelerating.

On the highway, you accelerate once and sit at a reasonably constant speed. You require minimum power, you're at minimum throttle, and minimum fuel. Around town, you accelerate, decelerate, sit at idle for a bit, rinse, and repeat. You require lots of power to get all that mass moving, using far more throttle, and far more fuel. And then you hit the brakes, scrub off all your momentum, and sit for a bit burning fuel and going nowhere.

[HOrnbrod]

7+ MPG difference? Thanks for the simple explanation gog, but no way man, that's not close to normal for for any Jeep I6 I've ever had, or any other vehicle. It's not a big deal - I'm just curious if anyone else has run into this. 

[gogmorgo]

I pulled it from my sig cause it's not up to date at all, but in mostly city driving I was averaging right around that 13 mpg mark. On the highway I was getting 16-18, but that was back when I was still trying to drive everywhere at 80 mph, which I eventually realised was pointless on two-lane highways when traffic's doing 60. The swings are especially obvious at first as I was going back and forth from school (300 miles on the highway, the range of the tank so I filled immediately before and after) and my driving while at school (in the city) was minimal.

I've now adopted a more relaxed driving style, adhering to speed limits and so on. Purely highway I'm getting around 20 mpg at ~60mph, and the odd time I run into the city for more than a couple hours, I'm back down towards that 13 mpg mark.

[johnj92131]

My so called city driving still includes a lot of freeway driving - even if in a few miles of stop and go morning rush hour traffic.  If I were doing a more city street stop and go driving I could easily see 14 mpg as a regular item.  Also, short trip driving doesn't give the car time to warm up.  Don - aren't you doing a lot of short trip driving these days? That also kills the mpg.

[HOrnbrod]

Yes, mostly city driving nowadays. I suppose I should be thankful for the decent mileage I do get on the highway.  :thumbsup:

[64 Cheyenne]

Don regardless of what you think you doing, stroker, more pedal... even if you do/could think conservative...around town....yeah.

[johnj92131]

Don,

 

Any idea where your timing curve is when your are doing city driving?  I "assume" for highway driving you timing is well advanced.  Not sure it is the same with city driving.  Know the total spark advance can be a big help for highway mpg.

[HOrnbrod]

Yes, very true John. No idea on the RPM timing curve, but I do look at timing with the light occasionally and it's jumps around quite a bit on the lower RPMs but settles down pretty steady at around 3K RPM @ ~18*-22* BTDC or so, and 3K RPM is about what I cruise at on the freeway. Non-freeway mileage was actually worse (around 12) until I swapped in a throttle body MAP from a 96 last year. I do have an adjustable MAP encapsulated circuit box I've never used and forgotten about until now. A got it years ago from a guy on the strokers forum who used to build them. I suppose I could wire it in and play with different MAP input voltages to see if it helps the around-town mileage. But the adj. MAP voltage only alters the A/F ratio at throttle openings over 80% and before the engine is up to temperature in open loop. The ECU regulates the A/F mix all other times, so it probably wouldn't help much.

[johnj92131]

Don,

 

Have you given any thought to changing over to OBD2 engine management?  Thinking possibly that would allow different/"better" tuning for city mpg and for more power if requested.