So on my 2000 ram with the 360, I was getting 8mpg in town and 12 on the highway doing 70. I installed a si/do super 44 flowmaster muffler, mopar round air cleaner, autolite plugs, and changed the front o2 sensor. My highway has gone up maybe 1 or 2 mpg. In town it is still the same. IfI am driving 35-40 and it is on OD then I am getting 15 or higher at that time but the average is still at 8. I know it takes some power to get this thing to move at stop lights but I was hoping for 12mpg average for just driving around town. I tried doing that test for a plenum leak and I didn't notice anything and the intake is just barely wet inside and I am not loosing any oil. So I don't know what to do. Any advice???? Oh ya and my tires are 285/?/16(can't remeber exactly) and the gears are 3.55 I believe. Factory was upgraded from 245s to 265s. I still have original sticker. Thanks

I have read in one of the DIY posts that SI/DO doesn't give the best performance, a SI/SI acutally does. something to do with exhaust pulse. I am not that in to the science of it, I too and looking to get better MPG. I get 11 in the city. I rarely hit the highway so i can't offer that MPG. I am getting a tranny rebuild soon so I am hoping i gain a little from that. i know my clutches were done. So I know that I am getting unanswered revs in there.

Smaller tires help, but there are other larger wider tires that have less rolling resistance.

You eliminated all the items i was hoping to replace except for getting a hiflow cat. I think mine is starting to make that rattling sound, but can't tell if its coming from the dying tranny.
Maybe you can try going to synthetic?
Maybe your rear end is slowing you down. I think someone in here had a bad rear end that was causing sluggishness.

does the motor/tranny feel strong?

its the tire size. I had 245/75R16 with no lift when i bought my truck. I got 17 mpg on the interstate with a 4x4 5.9L QC. I put a 3 inch lift on my truck and upgraded to 285/75R16 and now i am only getting 11 mpg. I knew it would drop. You want gas mileage, go back to 245/75R16. Otherwise than that getting 12 mpg is better than what i am getting.

ok, math lesson.

you've got bigger than stock tires, so it affects your speedo and your odometer by as much as 20%.

did you correct your speedo and odo, or are you calculating
mpg = under-registered miles / actual gallons.

Aside from checking the O2 sensor (was it good or bad?)
you are heading in the wrong directions
if you want better MPG.

Your tires are probably hurting you
with high rolling resistance.

When the engine is operating with the throttle blades partly closed
like yours is 95% of the time
all MPG improvement from exhaust system mods
are entirely due to any reduction in backpressure
and SI/DO and such
means nothing.

More info in this LONG read
that has been proof read by current or retired
Dodge, GM and Toyota engineers:

https://dodgeforum.com/m_447500/tm.htm

For short trips and city driving
consider getting an electric block heater to shorten the bad MPG
during the warm-up of the engine:

Better breathing means more air, more air means more fuel, more fuel means less gas mileage.

So, my inductive illogicalness deduces that any change from stock reduces fuel mileage.

How wrong am I? [sm=boink3.gif]

More Air does not necessarily mean more fuel. Look at A/F ratios for example. The ideal is 14.7:1 meaning 14.7 parts air, to 1 part fuel. But a vehicle can run on a ratio ranging from 8:1 to 18:1. The best economy on most engines is reached at approx 15-16:1, where as best performance is approx at 12.5-13.5:1. So, basically, your pcm controls the amount of fuel given, based on the amount of air coming in to the cylinders. If it can get more air while still giving the same amount of fuel, the pcm is going to do that. The PCM is constantly trying to achieve the ideal ratio, or slightly higher, during cruising speeds, and light load conditions. When your towing, or at WOT, your the pcm will actually richen the A/F mixture for more power, which is why the gas mpg decreases.

Basically, if you can get the engine to breathe better, meaning less restrictive in and out, the engine should use more air with the same amount of fuel.

Remember though, this is just a basic concept. it doesn't always ring true.

Hope this helps.

Also, more power can = less throttle applied = more mpg. As has seemed to be the case with my truck.

Blknt18: One yearaverage = 16.4 mpg 95 ext cab 2wd. Highest was 18 worst 14. After you wrote the numbers 360 you should have stopped! Think about that: a 6000lb truck with a 360!!?? Youcan put junk on your truck but you will never get more that 19-20 if you have a 2wd. Ihave been keeping a fuel log on gasbuddy.com for more that two years. I plan a new exhaust and tonneau cover soon astaxes are in. I drive almost all highway at 55. Hope I get 2 mpg.

The payback starts to play a factor too. Cost may outweigh return (the gas you could have purchased).

What would you rather step out of a ricer or a Ram?

I know I can't get great milage. I was trying to get it up a bit. The main problem is just I drive really short distances with lots of stop lights and such. My cel wasn't on, but I changed the o2 sensor anyways and I think it helped highway. I will probably be putting on the 265s(since thats what mine came with stock and thats the ones i have) so hopefully that will help. These tires now may be really heavy. I love this truck. I just don't have a lot of money to be spending on gas right now.