Gear Swap vs. Fuel Economy
 

Just curious about how much one can expect fuel economy to suffer. I've got 3:15s. Any ideas how much of a change it would be going to 3:55s or 3:73s?

-Alex

RPMs will go up obviously.

These cars have a sweet spot for hwy MPG around 1800-2000 RPM.

For example:

285/35/19 rear tire on your car currently turns 1904 RPM @ 70 MPH in 6th gear on the 6R80.

If you upped your rear gears to 3.55, you would turn 2146 RPM, theorectically, at the same MPH.

3.73s would be a big jump and your hwy MPG would suffer, as you are now over 2255 RPM. You would probably lose 1-1.5 MPG on the highway, but city may go up a little bit.

Thanks Derek.

-Alex

I have 3.73's and get around 20mpg in town - if I can keep my foot out of it and 22-23 on the hwy at 75-80mph.

How the heck do you guys get that kind of mileage in city? I have the auto and only get about 14mpg

Part of my commute is feeder roads - higher speed. If I'm driving inner city, I'm probably closer to 18mpg.

Try changing your info display to show fuel economy. You can learn what you're doing that eats gas. As mentioned above, sweet spot for mileage is 1800-2000rpm, so I shift 2500 max when I'm trying to be efficient. When I coast up to stop lights, I put manual trans in neutral - not as convenient to do with auto.

I just can't lay off the throttle!

Hey D what app or spreadsheet did you use for those calcs? Thanks

Spreadsheet I made on my own. I'll PM you a link to it tomorrow

Gears
 

I have 3:55 gears in mine and on the highway if I drive it right I can get 24-25 MPG on my weekly trip to Dallas from Winnsboro out of my boss intake, JPC tuned, stalled auto.

Perry

Not sure how the fuck y'all with 3.73 get 20+? I stay around 16city without resetting every time I fill up, so the avg has more miles/some negative to work against.

I was just thinking about this today. I took the kids to the Fort Worth Zoo today from Waco, I never got above 20.8 mpg. I was traveling, on average, at 75-80 mph with an A6 an stock 5.0.

Mine has 3:31 Gears or what Ford calls "Rear Axle Promo Pack"

With Bama performance tune on the Highway I get 27-28 MPG...when I go the speed limit and I set Cruise Control. Above 2K RPM and the MPG drops like a ton of bricks.

Guess I'll test the under 2k theory out. I have a 80 mile commute to work. I always fill up befor going, so fuel weight or amount shouldn't be a factor. I'll see what I come up with when I set my cc at 2k instead of 75 mph.

I have about the same commute. 65-70mph gets me the best MPG. Much below 60 and the engine has to work harder in 6th gear. I usually reset my MPG each time I leave the house to make the 60 mile drive to work and IF I don't stomp on it much I get around 27-28. I got 29 on one trip but I drove 65 and slower the whole way. It's really hard to do. lol.

Not sure how accurate the MPG gauge is in the Premium but since I started driving slow I can make 2 1/2 trips to work and back before I fill it up again. Still too many miles and I'm going to buy a beater for DD asap.

I wasa driving a 4 door Jeep on 35" tires and getting close to 15 mpg. So, the 20 mpg I'm getting out of the Mustang has been great. I'll slow try to slow down next time, lol. My commute is only every third day. There one morning, back home the next.

The auto has a shorter OD ratio in 6th gear than the manuals do. 75-80 MPH in an auto is like turning 2400 RPM.

I have gotten 25-25.5 multiple times on trips home, south of Houston, doing 80-85 on I45 almost the entire time. Surprisingly the over 2k theory isn't as bad on these cars as I've seen in the past. I've actually found that there is a second sweet spot around 80 MPH that if you set cruise at, you can still get good MPG for the speed and RPM you are turning.

The in dash MPG calculator is really accurate. I've hardly ever been more than +/- 0.5 MPG off hand calculations with it, and over the life of the car it's been a running average of +/- 0.3 MPG difference

Everyone, here is a copy of the spreadsheet I use to figure out RPM vs speed, gear, tire OD, etc. Make sure to download it and save if to your computer.

https://www.dropbox.com/s/4weuy6umya...lculators.xlsx

It's a hot rod and a part time toy for me, who cares about mileage. lol

Not everyone has the luxury to just throw money at the gas station.

You mean the one or maybe two tanks (two tanks is rare) of gas a month? lol

In my opinion, if gas mileage is that great of concern for someone with one of these cars then you most likely can't really afford the car in the first place.

I personally wouldn't go past a 3.55. On old less hp mustangs I would always go right to 4.10s but anything close to 2500-3k rpms cruising on the highway is going to kill your mpg by 1-3 I would say. Every car is different so you won't really know what is going to happen until you do the swap.

I wasn't directing that comment at you.

I call bullshit on this. Some people actually try to budget themselves. Doesn't mean they can't afford it. Just means you can't do a whole bunch of random running the roads just burning gas to burn it. I can afford my car, but I can't afford to be careless about how much gas I use. There is a difference. Shit adds up quick.

You're in denial.

I kinda agree with you.

How does that make me in denial? I'm just curious to hear your reasoning, seriously. I can't agree with you or disagree unless I know how you came to that conclusion.

To me, if you're that financially stretched if you're having to budget the gas for it you can't afford the car. When you buy a car it's given its going to use so much fuel for regular use as a commuter. There was obviously much better choices for a commuter for fuel economy that would have been better suited for your budget. Being things are that tight, if something happens the car needs repairs, you're at the dealerships mercy depending on them to honor the warranty in order to fix it, if they don't, you're going to be in a bad spot. Side note to it, if you're stuck with a near stock car because of needing the warranty, you're not getting a full enjoyment from the car not being able to mod it as you want and that's if you can afford to mod it at all. /shrug. JMO's.

I can agree with all of that. But I'm not setting a fixed amount of money to be spent on gas. Just trying to cut down how much gas I use and trying to cut down the miles I put on my car as well, in the process.

However, I am driving my car more than the typical daily commuter, roughly 2k miles a month. I knew what I was getting into when I bought the car, and I knew gas was going to the be the most expensive part. As for dealership work, luckily the blow struts were the only issue I've had, and it was my own doing that caused that. I've also not be hard enough on myself in terms of my other spending habits besides car parts. Plus I never intended on having a dog as soon as I did, but oh well. Not sure why I felt to give an explanation for myself, but I did.

No worries. I just explained because you asked me to. To each to their own.


Btw, I completely understand being thrifty or just outright cheap for whatever reason. You have to be in todays economy in my opinion regardless of how much money you have because it's so unstable.

I just don't understand people who live beyond their means spending more than they make living off of credit cards or stretching themselves so thin if anything goes wrong they are screwed.

That's the new American dream. Media, politicians, everyone deems it as an okay lifestyle, so naturally everyone believes them. Unfortunately people don't realize what they are doing, and that by doing so, it only drives the economy down even more than it already is.

The moment IRS and CC debt settlements started to be paid off at a fraction of the value was the moment all of it was doomed. IMO, that should've never been allowed. You rack up $20k in CC debt, you will work and pay that debt off to $0, not pay $4k and have the rest wiped clean. Because that just shows they can go out and spend that much again and still get off for paying $4k again....:mad: /rant

Fucking A.

More like the American Nightmare. Our society has become one of instant gratification to buy now, pay later without any thought of long term thought of the consequences or costs. Most people we all know, will not be able to retire at all having to work much later into life.



Patience is a lost virtue.

Yup.

We have a great 401k here at the casino. They match up 7% of whatever you put in. So when I started I put in 7%. Most people I know up here deduct nothing from there checks and upon Tax returns blow it on stupid shit. I now deduct 11% from my check to my 401k. I make the same because every year we get a raise based on our performance. Doesnt bother me one bit because in the long run, I should be alright. These people are the same that say minimum wage should be raised and its the companies fault for not paying them enough to keep the 150k home they obviously cant afford even with double the pay.

I agree with Steve. If you are that worried about the mpg, you cant afford the car. In my case, I could make do buying the car, but I wouldnt save shit on mpg, Id just be better off driving my fox and saving the entire payment.. whole reason for me to not have a new Mustang or drive my old one is to save at the pump.

Change the gears as you see fit and dont worry about the 100 to 200 bucks more a year it will cost ya. Get a better air box or tune and make the mpg up. I picked up 2 mpg on my wife's c5 by swapping intake manifolds, and gained 11 rwhp too. Ha

Many thanks! Nice to be able bench race the different rear gears now! Also tires!!

Back to the op's question. . 3.73 fords racing gear!! With the m6 in town I get 18-19 highway. Low 20's not that mpg matters but it's nice to say I get that high with this performance,

My car was originally a 3.31 gear m6 and I got about 18mpg in the city and 22-23 on the highway at mostly 80mph. I got my 3.73's put in today so I'll see what it does to my mpg, I've also got 20 inch wheels and I think my total tire height ends up being around 26 inches.

For some odd reason I pisked up 2mpg on the hwy after lowering my car. I know it lowers the drag but has anyone else had this?

22-23 @ 80 with 3.31s? Damn, do you stay in 5th gear at 80 MPH or something? I can get 25.5-26 @ 80 with 3.55s.