So recently I bought a 2004 a4 GTO, love the car but I ol do A LOT of driving and am trying to get the best mpg possible (mind you I'm not complaining I know it's no economy car).
I drove my same 120 miles of highway stretch to and from work the last week resetting and keeping track of my mpg, it was very steadily 22.5 ( windows down no a/c).
Over the weekend I changed out the current acdelco iridium plugs and 7mm wires (looked ok) for msd 8.5s and tr55s gapped at .050. Did the same logging and my average is 21.5.
No variables have changed other than those 2 things and honestly I was hoping for better mpg. Most people will shrug off 1mpg but it matters to me especially since I just spent $100 on parts that are LOSING me money.
Any ideas advice is appreciated.

How are you tracking the MPG? You mentioned "logging", so I'm wondering if you're doing this with a device or just basic math after a full fill-up. If the latter, and you used two different gas stations, then one of their pumps might have not been accurate. If the former, then the device might not be 100% accurate (the on-board calculator for my '10 Malibu is always off by about 1-3mpg based on the hard math that I do after a fill-up.)

I would try to look at averages over several tanks of fuel vs. just one. Perhaps that 22.5 was a fluke, and previous tanks were only 21-21.5 even with the old plugs/wires (or have you been tracking this for a long time?) Or, perhaps this current 21.5 was just a fluke and the next tank will be 22-23.

Even the weather can be considered a variable, if there were significant temp changes between the two tanks in question. Lots of things can account for a small change in MPG, so again I'd be less concerned with minor changes from one tank to the next, and more concerned with the average over several tanks (before and after any sort of maintenance/modifications.)

The gto has a mpg calculator and its pretty darn consistent. Was the same everyday over a 1,600 mile period. Actually the weather is cooler this week so it should be even better conditions. I did use a different gas station, same company but I know the two stations get different gas brands, maybe that's it.

I wouldn't be concerned about that small of a change. My truck(2014 Silverado 5.3L) can gain/lose as much as 1-1.5mpg depending on which gas station I go to and what grade of fuel I get. I rarely drive my truck anywhere but to work so its gets driven the same way pretty much every tank. I have experimented with a few different gas stations and grades of fuel to see what would get me the best mpg. It gets the best mpg on Shell premium(also the most expensive of course). Fuel mileage will also go down in cooler temperatures too as RPM_WS6 mentioned.

You changed plugs and gap....its going to effect mileage. Delco plugs are dam good, why everyone goes to tr55 is ???????????? Toss the delcos back in and for best mileage I would run at least .055 gap.

If you are really looking for a small increase in fuel mileage I would go to a multiple electrode plug like Bosch plus 2 etc. I used them a few times and noticed a 1-2 mpg increase every time.

I would still double check using standard math, it just makes sense to do so if you're really concerned about any possible small change. You also have the variable of different gas stations to factor in, as well as the change in weather (cooler weather tends to reduce MPG.) Small tank-to-tank changes are pretty hard to attribute to any one thing.

I've never noticed any significant change in average mileage (meaning ~1mpg or more) using TR55 (gapped at .050-.060") vs. Delco Iridium (gapped at .040") vs. stock Delco Platinum (gapped at .060"). When new/fresh, all were pretty much the same in terms of MPG in my experience.

I didn't expect cooler weather to reduce gas mileage, you would think it'd increase.

I've never owned any vehicle that got better mileage in colder weather. There are several factors, ranging from longer engine warm up times (the engine is less efficient during this period, and also running on a richer mixture), to greater friction throughout the drivetrain as various fluids take longer to warm up, to reduced tire pressure (unless you have reset them for the colder temps) causing increased drag, to any fueling adjustments made by the PCM to compensate for the cooler, denser air with more fuel. Grouped together, these things can make quite a difference depending on just how much cooler the average ambient temp is from tank to tank.

Also, in the colder months a lot of places switch fuels to a winter blend I believe. I'm pretty sure this can contribute to the inefficiency as well.

Correct and correct, but that wouldn't explain the OP's concern as it's too early in the year for the winter blends.

You also have good old wind resistance. Biking in 50 deg weather vs. 75 deg weather I can feel the additional drag on my body and bike. It's no different for a car. I would suspect that once warmed up for highway speeds in either warm or cold weather, it's the external drag that is the biggest factor. I've had a couple dozen daily drivers over the past 40 years and none ever got better mileage in the winter. And 20 years of that driving were 50 mile (1 way) highway commutes to work....3 of the cars were 1993-1998 Camaro/Firebird V8's.

"A vehicle’s aerodynamic drag is proportional to air density, and the density increases as temperature drops. For every 10° drop in temperature, aerodynamic drag increases by 2%. This is mostly at highway speeds and long trips on the freeways."

Why would you expect it to increase? Cold air means denser air meaning more oxygen molecules enter the engine. More oxygen requires more fuel to maintain the same 14.7 ratio. So it makes sense that MPGs go down with the colder weather.

Although I still don't think 1mpg variance is enough to say it was the fault of the new mods. A simple headwind that you didn't have before could easily make that kind of difference. Plenty of other factors besides the new parts could account for 1mpg. Now if you lost 3 or more yeah, but 1...