Five Oh Brian

4.10's don't worsen your fuel economy (thanks to the generous overdrives in our transmissions). My 07 GT Automatic gets 26-27 mpg rolling down the freeway with 4.10 gears, which is better than the EPA ratings for the stock 3.31's according to my window sticker.

Also, for those with a manual trans, you won't be shifting more with 4.10's, just sooner.

Steeper drag gears will also help overcome top-end aero drag, as gears are a torque multiplier. Even with 4.10 gears, you are not redline limited for top-end (165 mph is theoretically possible at 6,000 rpm's for my car).

metroplex

Based on my calculations, a S197 GT with 235/55R17s and 4.10s with the 5R55S will cruise at 2800 RPM in 5th gear with 4.10s at 75 mph versus ~2200 with 3.31s

I know in my 07 GT with the 3650 and stock 17s, I cruise at slightly over 2000 RPM at 75 mph,

shaun_beauchamp

3.55 gears are really a great middle of the road option. Obviously 4.10 will help launch, but will run higher rpms on the highway. It is disputed that if you do a significant amount of city that your overall will be comparable, but if you simply run hwy with 4.10s you will lose up to 2 MPG over 3.55s. 3.31 are fairly weak on launch, and when you drop down to third on the hwy it won't pull as hard as the 3.55s. I have an auto vert with 3.31s and a 5-spd coupe with 3.55s. I wouldn't consider 3.31s in my coupe!

Five Oh Brian

My 07 GT automatic has the factory stock 18" wheels and I have calibrated my speedo to be 100% accurate using my DiabloSport tuner against our freeway's mile markers (did a 20 mile check to get it perfect). I turn just 2,800 rpm's at 80 mph and just 2,100 rpm's at 60 mph with the 4.10 gears. I get the best fuel economy at about 60-65 mph (26-27 mpg). At 75-80 mph, fuel economy drops down to 23-24 mpg, which is what my factory window sticker says a 3.31 geared GT automatic should get.

In summary, there is minimal (if any) fuel economy drop by switching to 4.10's. However, there is a drastic and very noticable improvement in acceleration and drivability by switching to 4.10's.

metroplex

60-65 mph is well below the posted highway speed limits here in SE MI (70 mph). The bulk of the traffic drives much faster than this.

However I too have found that 2000-2200 RPM is the "sweet spot" for getting the best fuel economy. I confirmed my spreadsheet figures with my 07 GT w/ stock 17" and the 3650 5-speed manual. At about 75 mph, my engine is turning 2100 RPM w/ the stock 3.55s. I get about 23-24 mpg in combined driving (50/50 city and highway). I have not done a 100% highway trip and I don't have a trip computer so I can't get an instantaneous reading for highway cruises. However, I generally get about 3-4 mpg more on 100% highway driving, so this puts it well above the EPA rating and coincides with your figures for 60-65 mph cruising. It all depends on how you're going to drive it and where you plan to drive it.

Bottom line is that if I had to do it again, I would have stuck with the stock 3.31s for my TR-3650. The extra "gain" in acceleration w/ the 3.55s isn't worth it compared to the higher fuel economy Imagine the looks on people's faces when I can tell them my 300 hp muscle car gets comparable gas mileage to a Pontiac G6 or a Honda Accord.

Mark S.

I was always under the impression that the higher the gear ratio (i.e. going from 3:31 to 3:55 or 3:73) will lower your top end (by 5-7MPH) since you will rev higher at a quicker pace...

Higher gear ratios are great for short quick sprints (like the quarter-mile, etc.)...

I am perplexed with Brian's figures at the varying speeds, I would think that his gas mileage would be worse all around with 4:10 gears...

metroplex

His gas mileage did drop. He just drives at a slower speed so he doesn't notice it as much. I can't cruise at 60-65 mph on the highway... that's well below the posted speed limit. If my Stang had 4.10s I'd see a sizable hit on gas mileage.

On my GT, if I went with 3.31s, the engine is turning 2000 RPM at around 75 mph versus 2500 RPM with 4.10s. With my 3.55s, it's not so bad at 2100+ RPM at the same vehicle speed but I'd like to squeeze every MPG out of the car.

theedge67

Yet you are adamantly opposed to a CAI and Tune, which would net you more HP, better throttle response, improved throttle hang, and better gas mileage.

metroplex

I've been there and done that... that is not 100% true for every case with regards to fuel economy. I am not interested in more hp. Paying for the hot air intake would negate any benefit in fuel economy since I log less than 5000 miles per year on the Stang.

Five Oh Brian

Your top end is not reduced with steeper gears as long as you have an overdrive transmission. In the old days without overdrives, cars topped out because they hit the redline in top gear. Today's overdrive cars top out due to aero drag long before they hit redline in top (overdrive) gear.

For example, my 07 GT automatic would be going 205 mph at 6,000 rpm's in 5th gear with the factory 3.31 gears if there were 0 aero drag (like in a vaccuum). Common sense (and experience) tells us that we'd run out of horsepower and aero ability long before 205 mph (let's try more like 140 to 150?).

At 6,000 rpm's in 5th gear with the 4.10's, my car could theoretically go 165 mph with 0 aero drag (vaccuum again). But, by switching to a steeper gear (more mechanical leverage), my 07 GT automatic with 4.10's can push harder against the aero drag at high speed and is likely capable of about 150-160, which is still below the redline of 165 mph.

Instead of educated guesses, allow me to share two very real world examples of gearing and top speed runs I've done.

1989 5.0L LX Mustang with 5-speed manual trans (and overdrive, of course). 2.73 gears top speed = 135 mph (aero/hp limited). 3.73 gears top speed = 145 mph (aero/hp limited). Steeper gears improved top end speed and acceleration.

1968 Pontiac GTO with 3-speed automatic (no overdrive, but plenty of HP and torque). 2.56 gears top speed = 160 mph (aero/hp limited). 3.90 gears top speed = 106 mph (redline limited).

Five Oh Brian

Most cars get their absolute best mileage cruising in the 45-65 mph range. Proven fact! Aero drag really starts playing havoc on fuel economy above 65 mph.

I don't drive at 60-65 mph on the freeway, I just wanted to see what kind of mileage I'd get at that speed. I'm typically in the 75-80 mph range, like the rest of you. 90% of my driving is around town with very little interstate driving, and I only get about 14-15 mpg in town.

Rebel73

3:73s are the best compromise for performance, driveability, and mileage. Those are probably in my future. I normally bike to work and if I do drive, it's only about 7 miles so the fuel mileage isn't that big an issue to me.

351pony

I love my 4.10's...as everyone else has stated your actual gas mileage will vary depending on how you drive the car. If you have a heavy foot (like most of us) your gas mileage will probably suffer a bit but the improved fun factor makes up for it.

Gearhead

Stock wheel size: 235mm 55% 17"
235mm / 25.4 inches/mm = 9.252"
Diameter = 9.252" * 55% * 2 + 17" = 27.177"
Distance traveled per revolution = 3.14159 * Diameter
= 3.14159 * 27.177 inches/rev / 12 inches/foot / 5,280 feet/mile = 0.0013475 miles/rev
Shift point at 6,000 rev/min * 60 min/hour = 360,000 Rev/hour
Best Cruise Speed is very roughly about 45% of peak torque speed for this engine/load, which is 4,500 rpm * 45% = 2,025 rpm
2,025 rev/min * 60 min/hour = 121,500 rev/hour
Speeds with 4.10:1 rear axle:
1st Gear: 360,000 rev/hour * 0.0013475 miles/rev / 4.10 / 3.38 = 35 MPH
2nd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 4.10 / 2.00 = 59 MPH
3rd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 4.10 / 1.32 = 90 MPH
4th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 4.10 / 1.00 = 118 MPH
5th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 4.10 / 0.675 = 175 MPH
Cruise speed @ 2,025 rpm = 121,500 rev/hour * 0.0013475 miles/rev / 4.10 / 0.675 = 59 MPH
Guess efficiency = 24.0 miles/gallon @ 60 MPH
Speeds with 3.73:1 rear axle:
1st Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.73 / 3.38 = 38 MPH
2nd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.73 / 2.00 = 65 MPH
3rd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.73 / 1.32 = 99 MPH
4th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.73 / 1.00 = 130 MPH
5th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.73 / 0.675 = 193 MPH
Cruise speed @ 2,025 rpm = 121,500 rev/hour * 0.0013475 miles/rev / 3.73 / 0.675 = 65 MPH
Guess efficiency = 24.5 miles/gallon @ 60 MPH
Speeds with 3.55:1 rear axle:
1st Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.55 / 3.38 = 40 MPH
2nd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.55 / 2.00 = 68 MPH
3rd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.55 / 1.32 = 104 MPH
4th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.55 / 1.00 = 137 MPH
5th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.55 / 0.675 = 202 MPH
Cruise speed @ 2,025 rpm = 121,500 rev/hour * 0.0013475 miles/rev / 3.55 / 0.675 = 68 MPH
Guess efficiency = 24.9 miles/gallon @ 60 MPH
Speeds with 3.31:1 rear axle:
1st Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.31 / 3.38 = 43 MPH
2nd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.31 / 2.00 = 73 MPH
3rd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.31 / 1.32 = 111 MPH
4th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.31 / 1.00 = 147 MPH
5th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.31 / 0.675 = 217 MPH
Cruise speed @ 2,025 rpm = 121,500 rev/hour * 0.0013475 miles/rev / 3.31 / 0.675 = 73 MPH
Guess efficiency = 25.0 miles/gallon @ 60 MPH
Specs on the Transmission Ratios are found here, on page 8:
http://www.fileden.com/files/2008/3/...ng%20Specs.pdf

shaun_beauchamp

5-spd manual with 3:55s. I net 20 to 22 mpg mix. I get 19 MPG city and 26 MPG hwy. I usually do city driving during the week and a bit of highway on the weekends. I also have a C&L Racer intake with DiabloSport tune (93 Octane Performance) and UDPs. I drive my car about 10K miles a year. I find the mods worth it in both fuel economy and power adding.