should i up my gear ratio

[lovemy68coupe]

Hey guys, im new to the forum and have already learned a lot. i was wondering if you guys would know if i should up my gears in the back and if so, what to. i have a 68 mustang with a 351 windsor, edelbrock performer intake, edelbrock e-street heads, lunati voodoo 61001 cam, 1.7 rockers, holley 600, fmx transmission all going back to 2.79 gears in an 8 inch rear. the car is a weekend cruiser with a few trips to the strip a year. thanks for any help.

 

[blown65]

What kind of highway speeds do you do. I would say even going to 3.50's would be a big help.

 

[Realmongo]

What kind of highway speeds do you do. I would say even going to 3.50's would be a big help.

3.50 would not be bad. For the 8" you can get 3.45 or 3.55 ratios these days from Motive gear. I just swapped my 8" 2.80 set out for a 3.25 since I am retaining the stock 14" tires. Without OD you can only go so far without being obnoxious, as I once had a street car with 4.88 gears and you get sick of 3500 RPM at 55 MPH pretty quick!

 

[JonK]

You could plug in your tire height and see what your RPM would be at different speeds with a calc.:
Randy's Ring & Pinion. RPM Calculator.

the FMX should be 1:1 in 3rd. If you go too tall (>3.73:1??) you'll end up with a super short and fairly useless 1st gear though so keep 1st in mind when you're seeing what a taller gear will do to hwy. speeds also.
I agree with the others that going beyond 3.50:1 will probably make it not fun on a hwy. >55mph. I'd say 3.25:1<>3.50:1 will be your sweet spot depending if it's used more for cruise or race.
Jon

 

[blown65]

Exactly, for me I never drive on the Freeway, so its typically 50mph around here.

3.50's might be too much if your driving lots at 70+mph, but they would be perfect for the person that drives around 60mph or less.

 

[lovemy68coupe]

thanks a lot guys, i think i'll look around for a 9 inch posi but until then ill go with the 3.25's.

 

[PoppyMod]

thanks a lot guys, i think i'll look around for a 9 inch posi but until then ill go with the 3.25's.

Unless your going to add a lot more "grunt" (torque) don't waste the money on a 9". A 3:55 will work very nice with your combo. With 225 X 60 X 15 (25.6") wheel/tire combo, you will be turning around 3K at 70 MPH. Not too bad.

 

[Vinyl66]

3.55's would be a huge difference in acceleration over the 2.79's. I'd put 3.55 gears in the 8" rear if I were you. I have run that same combo for years with numerous runs in the 11's. No need for a 9" at your power level.

 

[tx65coupe]

I agree about the 3.50 gears being a huge improvement, but with out an OD transmission, I would want more like 3.00 or maybe 3.20 gears. The highway cruising RPM will be too high for my liking with the 3.50 gears.

 

[blown65]

3.0 gears over 2.79 gears is a total waste of time. What is it about 3000 rpm at 70mph that is bad? Ya, it might eat more fuel but I did that for 10 years with my old motor and its a non issue.

 

[blown65]

I should add, I ran 3.89's all those years.

 

[jspagna1]

Go with the 3.55's. It's a good gear for your combination and will still make it drivable on the highway. I've had mine for 10 years now and am still happy with the performance.

 

[ultrastang]

3.0 gears over 2.79 gears is a total waste of time. What is it about 3000 rpm at 70mph that is bad? Ya, it might eat more fuel but I did that for 10 years with my old motor and its a non issue.

3,000 rpm's on the highway for more than 20 or 30 miles gets to be annoying, due to the decible level inside the car.

Overdrive will make all the difference in the world and you can also run a low rear gear set without the high RPM penalty on the highway, so you get the best of both worlds.

A 2.79 rear gear and no overdrive with a 25" tall tire at 60 mph would put the rpm's at 2,250. The same senario above but with an overdrive would put the rpm's at 1,535. This make makes a huge difference in the decible level inside the car and puts a lot less stress on the engine.

With 3.55 gears and the same as above [without overdrive], the rpm's would be around 2,863. With overdrive, the rpm's would be 1,947.

RPM-charts

 

[blown65]

3,000 rpm's on the highway for more than 20 or 30 miles gets to be annoying, due to the decible level inside the car.

Overdrive will make all the difference in the world and you can also run a low rear gear set without the high RPM penalty on the highway, so you get the best of both worlds.

A 2.79 rear gear and no overdrive with a 25" tall tire at 60 mph would put the rpm's at 2,250. The same senario above but with an overdrive would put the rpm's at 1,535. This make makes a huge difference in the decible level inside the car and puts a lot less stress on the engine.

With 3.55 gears and the same as above [without overdrive], the rpm's would be around 2,863. With overdrive, the rpm's would be 1,947.

RPM-charts

Whichever, I did it for 10 years, never really bothered me that much. I'd rather have something that performed well than be a turd and be quiet.

OD would be ideal, but that wasn't his original question.

 

[PoppyMod]

Whichever, I did it for 10 years, never really bothered me that much. I'd rather have something that performed well than be a turd and be quiet.

OD would be ideal, but that wasn't his original question.

I whole heartedly agree with the last scenario. I'm running a 3:55 with 25.6" tires and a .063 5th. Super combo. Drop it to 4th and whoa baby! You got pass'in action! With the C4 and the 3:55 it was 3K at 70.