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Rear End Gear Ratio

 
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Old 01-31-2012, 08:10 PM
iMetal
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Rear End Gear Ratio

Hello all, I have a 74 F-100 302 with a C6 tranny behind it. The truck is gutless. I bought it from my cousin and he said the reason it has no power is because it originally had an FMX trans in it. So I'm looking for some advice on what gear ratio to put in the rear end of this thing. I want good take off but not to high highway rpm's (2300ish would be fine with me). Thanks in advance.
 
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Old 01-31-2012, 08:12 PM
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What size tire?
 
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Old 01-31-2012, 08:15 PM
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Originally Posted by Sleepy445FE View Post
What size tire?
LT235/75R15

Sorry, forgot to include that.
 
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Old 01-31-2012, 08:38 PM
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There has to be more to it than your axle ratio. If you want 2300 rpm on the highway with LT235/75R15 tires then you will need 2.75 or 3.00 gears. Those aren't exactly ideal for getting rid of the gutlessness. You need to figure out what ratios you have right now.

When is it gutless? 0-25? All the time? Has the engine been modified?
 
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Old 01-31-2012, 08:47 PM
iMetal
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Originally Posted by Sw1tchfoot View Post
There has to be more to it than your axle ratio. If you want 2300 rpm on the highway with LT235/75R15 tires then you will need 2.75 or 3.00 gears. Those aren't exactly ideal for getting rid of the gutlessness. You need to figure out what ratios you have right now.

When is it gutless? 0-25? All the time? Has the engine been modified?
Basically all the time. Taking about any kind of hill is just embarrassing. No mods or anything like the but the cylinders have been bored out 60 over. It also sucks gas like a tank. Again, I'm going by what my cousin has told me and he said that thing would scream when they had that FMX in it but I'm no expert on this so I'm open to suggestion.

Will I have good take off with 2.75 or 3.00?
 
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Old 01-31-2012, 09:46 PM
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Originally Posted by iMetal View Post
Basically all the time. Taking about any kind of hill is just embarrassing. No mods or anything like the but the cylinders have been bored out 60 over. It also sucks gas like a tank. Again, I'm going by what my cousin has told me and he said that thing would scream when they had that FMX in it but I'm no expert on this so I'm open to suggestion.

Will I have good take off with 2.75 or 3.00?
Do you know what RPM it turns now at say 65 MPH? That would help tell which gear you have.

With a non-OD C6 2400 RPM cruise RPM is low. To get any kind of performance you will need to set it up for around 2700 cruise RPM. I'm guessing 3.5x gears would be in the ballpark to balance performance, economy and cruise RPM but it would help to know the current cruise RPM and/or gear.
 
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Old 02-01-2012, 08:52 AM
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the auto trans if c6/c4/fmx are all 1.1 final ratio did a gear change in my 79 f150 460/c6/from 3.50[[email protected]] to a 2.75 now at 2200 [email protected] mph with 235/15 tires i would think there are other issues with this engine as they are a good power plant for their size i have had a few over the years that easily got 18 to 20mpg with them
 
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Old 02-01-2012, 11:23 AM
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I think you have a motor problem. My truck has a 2.75 ratio with a 302 C6 trans. The tire Im running is a 275/60/15. At 60-65mph Im right at 2000rpm. As far as driving around town its fine, but the highway it shifts down alot more than it should when you hit a grade. The truck has good take off on a hill or flat ground buts its nothing like what a 3.50-3.73 gears will do. As far as gears Im looking for a 3.50 gears just to give it alittle more low end because thats where I do most of my driving at. The 2.75 gear just makes my trans work to much. I would look in to the motor more because it should have more power than what you are say.
 
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Old 02-01-2012, 12:25 PM
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i wouldnt go lower than 3.50's, its a truck not a speed bike
 
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Old 02-01-2012, 02:05 PM
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I wouldnt go lower than a 3.50 gear either but as for a 2.75 in a truck I really dont see the point. I know its a good highway gear but Im just not a fan. It really works my C6 more than it should. I was just trying to explain that he needs to look at his motor more. Even with a 2.75 gear with the 235 tire he is running the truck should have more power than it does.
 
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Old 02-01-2012, 02:27 PM
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Do you know what gear you have now?

When the trans was changed, were there any other changes made?

Have you checked timing, jetting, compression, the choke, etc on the engine?

Your tires are nominally 29" in diameter.

With the 1:1 trans, you will have the following MPH per 1000 rpm for each of the following ratios, with 65 mph at the rpms shown in parentheses:

2.75 = 31 (2071)
3.00 = 29 (2260)
3.25 = 27 (2448)
3.50 = 25 (2636)

Note there will be rounding error, and possibly 200 rpm TC slip.

The change from an FMX to a C6 should have made no difference in performance. While it is true the C6 is a bit more power hungry than the FMX, the difference is mostly at peak rpms in first gear iirc from the long-lost article on the subject.

Your truck will be more responsive with 3.50 or even 4.10 gears. Those gears will give you snappy performance between fill ups, which will be more frequent.

Your truck will be more gutless with a 2.75 or 3.00. It will get better mpgs at cruise, but there will be more downshifts and it will run longer in each gear under load.

--find out what it has
--check that engine--I think that's your problem
--check the C6--oil look good, temperature ok, shifting ok????

Then decide if a gear change is what you need..
 
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Old 02-01-2012, 02:32 PM
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While you're at it, make sure the brakes aren't dragging. This can cost performance and MPG......
 
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Old 02-01-2012, 11:59 PM
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Thanks for the figures, that really helps me out alot.
 
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Old 02-02-2012, 12:05 AM
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X2... you need to check your ignition timing.

Josh
 
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Old 02-03-2012, 02:28 PM
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I run 235/75r15 on my 73 F100 stock 302 engine and 3.25 gears....it gets along great.

My 79 F150 302 has 3.00 gears which is great for highway driving. But it has engine mods and aftermarket cam....its out of it power range until 55mph. So at low speed its kinda a dog. I'm eventually going to go to 3.25 to help it out some.
 

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