99shadowGT

i'm putting 4.10's in my 5 speed within the next month, but i'm starting to question the loss of top end, as well as any possible damage at high speeds. i read someone saying they bent rods at around 140 with 4.10s, but i will never go over 120, and even at that VERY rarely, if not never. am i still going to be able to go 85-100 pretty safely? thanks

01GT4.6

no loss, should see a gain of ~5mph. I ran up to 150 mph with my 4.10's (5 speed), where the stock gears would only go to ~145mph. As for 85-100, your motor will be fine (you may get a ticket though). BTW I've gotten as high as 24.3 mpg (once) with 4.10's

Mike

99shadowGT

i know it's not the gear that drops mpg, it's the DRIVER...once you get the gears you go nutso, so i'm sure i'll go wild for a few days, and then calm down to regular driving. i can deal with a small mpg drop, every gain has it's drawback somehow

POISND U

Wtf do you need to go any faster than 90 anyway? Do you take the Autobahn to work everyday or something?

cliffyk

Lower gears (numerically higher) will cost you fuel economy; at normal highway speeds (80-85 on I-95 around Daytona) 4.10s will cost you 2.0 to 2.5 mpg--faster than that and you'll lose 3.0 to 4+ mpg. I installed 3.73s and lost 1.0 to 1.5 mpg on my normal 50 miles (one way) commute which is 15miles at 55-65, 30 miles on I-95, and 5 miles at 45-50.[/align][/align]I get a bit better than 22.5 on average, vs. a solid 24.0 before the 3.73s; if I push it hard (90-110 on the highway, and leaving everyone else in the dust at stops) I can easily knock that down to 20 mpg or less...[/align][/align]As for top speed, you will lose none, and perhaps gain a bit as suggested above, due to the increased torque delivered to the wheels. [/align][/align]Also, whoever cautioned you about "bent rods" didn't know what they were talking about. It takes "X" amount of thrust to keep the car going 140 mph and with lower gears the engine doesn't have toproduce as much torque to end up with X amount of thrust. [/align][ul][*]HP =torque * rpm / 5252[*]torque = HP * 5252 / rpm[/ul]
Lets say for sake of argument that it takes 200 HP to keep the car going 140 mph, and use the standard 811 revolutions per mile (25.7" diameter) 'Stang tires as well. This means that at 140 mph the wheels are turning (140 * 811) = 113,540 rph = 1892 rpm. So the torque required is 200 * 5252 / 1892 = 555 lb.ft.[/align][/align]Therefore with 3.27 gears, in 5th (0.62:1 with a 3650), the engine needs to produce (555 / 3.27 / 0.62) = 273.8 lb.ft. of torque (at 3835 rpm); with 4.10s it has to produce only (555 / 4.10 / 0.62) = 218.3 lb.ft. (at 4810 rpm). These are both the same horsepower...[/align][/align]273.8 lb.ft. * 3835 / 5252 = 199.9 HP; and 218.3 lb.ft. * 4810 / 5252 = 199.9 HP [/align][/align]This is where the big HP versus torque confusion comes in to play--torque makes you go at any instant, HP is torque applied over time (rpms)...[/align]

BleedinBlue

No, you will not lose any top end speed.

And can you go 85-100 mph safely? It depends on the roadway, other traffic, and presence of law enforcement [8D]

cliffyk

Another comment: This "loss of top speed" issue arises because on some cars top speed in rpm limited--I.e. you hit redline in high gear before you run out of horsepower. [/align][/align]My s/c'd '92 Miata is like this, 128.3 mph max just before the rev limiter kicks in. On our Mustangs top speed is horsepower limited, which is good because if you had the ponies top speed with the stock 3.27 gears would be around 220 mph at the rev limiter.[/align][/align]FWIW, here's a theoretical top speed list for 5th gear and popular rear-end gears:[/align][/align][align=left][/align][align=left][/align][align=left]As you can see, even 4.30s will still leave plenty of breathing room...[/align][/align]