ve is a 99 nissan altima. It's a 4 speed automatic and the overdrive gear activates at 37mph and goes all the way to 100+ I can't seem to break 26mpg. Here are some things I changed. ...
 

The car I have is a 99 nissan altima. It's a 4 speed automatic and the overdrive gear activates at 37mph and goes all the way to 100+

I can't seem to break 26mpg. Here are some things I changed.

1) Kumho tires at 35 front, 33 rear
2)Transmission cooler( it works because it gets hot after driving for some time)
3) Higher bar radiator cap which means the coolant can obtain a higher temperature without over heating the car.
4) Spare and jack removed.
5) Royal purple synthetic engine oil
6) Use NGK V power copper spark plugs
7) Use AC only when it's really hot outside, besides that it's never used.

I drive 55mph on the freeway and my consumption went down, I try to coast as much as I can on the freeway and on the streets. I use cruise control all the time. My driving is usually 55% freeway and 45% streets.

Now before I started doing all this my mpg for the whole tank was 24mpg. Now it's 26mpg, but I can't break the 26mpg barrier. Any advice???

Also nothing is broken on the car, everything works.

I used to have a complete MSD system: Box, coil, tach adapter. But the mileage is the same between stock and msd system.

I'm doing everything I can but I want to go over 26mpg and even want to hit 28mpg if possible.

I've heard that msd
 

I've heard that msd ignitions don't do crap, if you look for my post about engine management it'll say why.

In any case, try some acetone or something, there's lots of small stuff, you tires aren't pumped up very high, and most performance tires can usually be pumped up to 51 psi, but I dunno about yours.

I'll think about this some more now.

Re: I've heard that msd
 

For the freeway I see some use for pumped up tires, but for the street would more psi hurt because the car accelarates and there is more weight in the tires?

More air in the tires for
 

More air in the tires for acceleration will be a miniscule difference in inertia, even under that most stop and go you'd still be better off pumped up.

PS: Nice to see you here, I'm working on an ecu now, but later I'll get around with some nicer tips.

Re: More air in the tires for
 

Ok I'll wait on those tips.

26 mpg seems pretty normal
 

26 mpg seems pretty normal for your car, but who the hell wants normal!? 40 mpg would be sweet, but hard to achieve, but possibly possible, I think.

Things to try:
acetone 1-3 ozs (testing)
modifying your spark plugs (look for the thread I made on it, more power without fuel consumption increase means less need to punch the gas all the time)
any of the random many fuel additive out there (poke about for some)
fuel vaporizations device (not as good as on carbed cars though)
mothballs in the tank (crushed)
45 psi in your tires
Check your 02 sensor to see if it's not looking good, might need replacing

More to come...must shower now (still soldering anyway).

The first thing to do is to
 

The first thing to do is to avoid most of the gadgets out there. The purpose of this site is not only to discuss possible solutions to increase gas mileage, but to back it with data.

Since your car is a 99 I doubt your O2 sensor needs replacing. I just replaced mine in my 89 Civic and the original one was still there. Hell, it didn't even need to be replaced. I just did it to do it.

Another easy thing to do is to make sure your car is clean. Make sure the cap/rotor on your distributor is clean and that your wires and plugs are in good shape. You can always test your wires with a multi-meter.

have you replaced your fuel filter recently? It's never a bad idea to do so. There are a few articles on this website that have simple tips for making your car more efficient.

I owned a 2001 Sentra (manual transmission) for a few years until I was on the receiving end of a hit and run last year. it got around 26mpg. The Altima is a much bigger car than the Sentra, and I believe the engine is larger too. You're still getting better mileage than I was. I'd be curious to know what the EPA rating is for your car.

Lastly, welcome to the board!

The fuel filter is clean.
 

The fuel filter is clean. The EPA rating is 22 city/ 30 hwy.

The motor is 2.4 Liter it goes dead rich at 3000rpm.

At 55mph I'm at 1800rpm
At 60mph I'm at 2000rpm
At 65mph I'm at 2125rpm

My best results so far is 55mph.

That's pretty tall gearing
 

That's pretty tall gearing for an auto, mine is 3k rpm at 65 mph.

Hmm, 2.4 liter is a lot of displacement, I'm right to assume 4 cylinder though?

You could try a warm air intake.

Re: That's pretty tall gearing
 

yes 4 cylender. I actually did buy an Injen WAI( warm air intake) but all it did was make loud noise and killed my low end. I took it off and put the stock intake back.