vague315

so i went to the shop to get my FRPP 4.10s installed and when i get there, the guy that runs the shop starts explaining quite a few things to me about what else i should do such as build up the rear suspension a bit. anyways he ended up talking me into gettin 3.90s instead (though nothing has been installed just yet) because of the rpm jump im "supposed" to see. however im pretty sure the way he figured the rpm's is wrong, cause he claimed i'd be at 3000rpm at 70mph. though from what i read around here, everyone is seein about a 300rpm increase putting me at 2300rpm at 70mph. he also mentioned alot of engines overheating because of the 4.10s as well as guys breaking their lower control arms launching . im wondering, are his rpm calculations wrong and has anyone heard of the other problems.

i use my car solely for street driving and only hit the highway a couple times a week. just in case the makes any difference

03FstStngKB

I have 4.30 gears and I am not even at 3000 rpms in 4th gear...I have an auto...but if you have a manual your 5th gear ratio is pretty close to the Autos 4th gear ratio I assume. I just tested this tonight as I drove my stang to work half way thru the city and then halfway on the highway. With 4.30 gears I was turning 2800 rpmsin 4th gear at 70 mphwhichis OD for the auto. I had my gears installed about 2 weeks ago and had not tested this theory of where my rpms would be at at 70 which is usually my crusing speed on the highway or interstate. With 4.10's and stock tire height you should be turning 2500 rpms or about there. Your kind of in the situation I'm in just mainly use my car for street driving with occasional highway driving. You would be fine with 4.10's and that is the least you want to go if your an auto.

As far as overheating with 4.10's???...I think your stock cooling system should be adequate enough to keep your engine from overheating. I could only think of extreme conditions of like maybe doing burnouts in 115 degree Arizona heat that might put a test to your cooling system because you are not moving down the road but even then like I said your fan should be able to cool down your car in most major heat situations.

The only time I would think you would be breaking anything suspension wise or axle wise is if you had a sticky tire on and or supporting more horsepower w/stickies on. With the extra leverage you are creating by up'ing your gear ratio thatwould cause you to spin your tires much easier thus taking away some of the stress that would be caused thanif you were able to hook at higher rpms...that isunless you are dumping the clutch at 4 grand plus...then you might be asking for trouble with out modification to any other part that is in relation to the drivetrain or suspension.

vague315

hmmm, ok. after hearing all that from him it kinda put a scare in me, though i hadnt heard anything like this from most the guys around here. 3000 rpm just sounded a bit outrageous to me. most rpm calculators i found put me around 2300-2400rpm at 70 in 5th gear with the 4.10s

defconfire

He is wrong, max at 70 youll be just over 2500 rpm. Get the 4.10 if you still can. Machs are rev happy cars and they love those gears.

If you are really concerned with the engine over headting you can put a cooler thermostat in.

vague315

yeah, im goin with the 4.10s. theres just gonna be a delay gettin them put in since he already shipped em back...but i suppose no harm done.

03FstStngKB

You will be fine with 4.10s One thing for sure is more people than not are too conserative when it comes to getting a gear. They tend to think more about how much their cruise rpm is going to be...which is fine. I would not want to run my car muchmore than it already is...everyone has a tipping point some people think 3000 is outrageous and others think 2500 is getting steep...Why do we get the gears then? Answer=To accelerate faster. When you go with a 3.73...4.10...4.30 etc you are going to trade some gas mileage and maybe a tinsy bit of engine wear for the quicker acceleration. It is all relative though because even though you may have stock gears if you run the **** out of it your gas mileage is going to suffer anyway...why?laws of physics.

Engine wear is also not only related to high engine crusing speed but on how hard you beat on the motor. Take care of yourengine and change the oil regularly and you should not have many problems. Someengines are factory freaks and some are short fused but if you start neglecting basic maintenance then you are just asking for trouble. Just my 2 cents on this subject matter

sdtraviesosd

yea. definitely get the 4.10's. I have a GT with 4.10s and my RPMS are right around the 2500rpm park at 70mph.
Sounds to me like the guy is juts trying to "up sale." Unless you have drag radials and are launching at like 5K rpm's every weekend you will be absolutely fine with stock suspension stuff. just fyi, i was doing 25mpg with 4.10s on the highway.

vague315

yea i figured he was just tryin to "up sale" as well, though i don't necessarily doubt his reasoning. the man knows his stuff, installation wise anyways. btw...will taking the ratio, 4.10/3.55 (new gear ratio/current) multiplied time my rpms at 70 with the 3.55s, give me a rough estimate of the rpms at 70 with 4.10s? just curious

ChucksBullitt

the RPM would be around 2400 rpm i do believe. in 5th gear ofcourse

Modular_Madness96

for what youre wanting 3.73's are more than enough.I have 3.73s in my 96 GT and its not real noticeable while cruising but from a dig its nice.