Gear ratio help!!!!

Hello all. I have a 1980 F100 with the 300 that is in the process of upgrading the intake, adding a 4 barrel, and headers. The owner before me switched the 3 on the column to 4 on the floor. The rearend currently has 3.50 ratio and you can't even take off in 1st (way too granny) and by the time I hit 35 mph. I'm already in 4th so on the highway it just screams. All I'm looking for is a good running daily driver with a good mixture of fuel economy and cruising speed. What would be the best ratio for this situation? Thanks for letting me be a part of the forum and for any help that is given!
 

bubba22349

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Both the 3 speed and your 4 Speed should work the exact same way with a 3.50 rear gear since the Hi Gear is a 1 to 1 Ratio. Otherwise you could go with a 2.47 to 2.80 rear if you wanted to actually take off in that Granny Gear, this gear is usually only used to get a heavy load or trailer moving. How tall of a rear tire is on it? Best of luck
 

Tbone3366

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My roommates 91 efi motor with the 5 speed mazda, 2.73 and 28.9 inch tal tires does great gets about 22mpg and takes off with ease, getting started with a load is but possible, he had to tow my 87 f350 when her ignition module cut out on her and it held 65 just fine towing her through the flats
 
Both the 3 speed and your 4 Speed should work the exact same way with a 3.50 rear gear since the Hi Gear is a 1 to 1 Ratio. Otherwise you could go with a 2.47 to 2.80 rear if you wanted to actually take off in that Granny Gear, this gear is usually only used to get a heavy load or trailer moving. How tall of a rear tire is on it? Best of luck
I appreciate the help as I am not that knowledgeable with all this. The tires are 215/75 R15. I really don't plan on pulling or hauling anything of significance with the truck, I just don't want to max out my last gear at 35 mph.
 

Frank

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I appreciate the help as I am not that knowledgeable with all this. The tires are 215/75 R15. I really don't plan on pulling or hauling anything of significance with the truck, I just don't want to max out my last gear at 35 mph.
Hey Little Mac, welcome. Are you sure you have a 3.50 ratio? What you're describing sounds more like a 4:1+ . It's important to verify this before you make a decision. Here's an easy way to do it: Jack up one rear tire, just off the ground. Mark the tire with a sharpie or paint pen and a mark on a board or something on the ground next to the tire. Align the marks. Go under the vehicle and mark the driveshaft. You're going to count rotations, so make a mark you know you can rotate around to precisely. Rotate the tire slowly (in either direction) twice while counting the turns on the driveshaft. Two rotations of the tire will give you the ratio from the shaft count. If you have a 3.50 gear, 2 tire rotations the shaft will pass the mark 3 times and be 180* from it when you stop.
 
Hey Little Mac, welcome. Are you sure you have a 3.50 ratio? What you're describing sounds more like a 4:1+ . It's important to verify this before you make a decision. Here's an easy way to do it: Jack up one rear tire, just off the ground. Mark the tire with a sharpie or paint pen and a mark on a board or something on the ground next to the tire. Align the marks. Go under the vehicle and mark the driveshaft. You're going to count rotations, so make a mark you know you can rotate around to precisely. Rotate the tire slowly (in either direction) twice while counting the turns on the driveshaft. Two rotations of the tire will give you the ratio from the shaft count. If you have a 3.50 gear, 2 tire rotations the shaft will pass the mark 3 times and be 180* from it when you stop.
Well I thought the same thing as far as the 4:1+ but I do have it out and opened up and the pinion is marked 3.5 and when counting the teeth on each and dividing shure enough... 3.5. There is a local place with a complete rearend with 2.75, which I originally was thinking would too low, but I don't know now?? Either that or 3.0 in my current rearend??
 

bubba22349

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The 2.75 or 3.00 would likely work good as the 300 six's have plenty of bottom end Torque to turn a higher gear if your not in any hurry getting started. I drove a 1965 F350 for many years with the similar Tans to your T18 it had the NP435 with a really low Granny gear 4 Speed. I didn't use 1st at all starting in 2nd when I was empty or not towing it worked great it had a very low rear axle gear and was maxed out at 62 MPH. Later I got a 1994 F150 with the 4.9 / 300 and 5 speed it had a 3.08 rear axle gear I rarely ever carried any weight of more than a 400 Lb.'s or pulled more than a small Cermet mixer it worked very well for daily transportation and it gave great MPG. Good luck
 

Frank

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Well I thought the same thing as far as the 4:1+ but I do have it out and opened up and the pinion is marked 3.5 and when counting the teeth on each and dividing shure enough... 3.5. There is a local place with a complete rearend with 2.75, which I originally was thinking would too low, but I don't know now?? Either that or 3.0 in my current rearend??
Ok, good to know that you are sure! I have a '79 F100, stock 300 with 2.73 (or thereabouts) 3 speed on the tree. It's a good gear for the 300 since, as bubba said, they pull from idle. I have the same sized tires you posted. In 3rd it turns 1150 rpm @ 35, 1475 rpm@ 45, 1800 rpm@55. This will be the same for you in 4th if you go with 2.73. With that granny gear, this is a good combo. Mine has to lug the clutch when pulling a trailer to take off or back up, it would be ideal with the 4 speed.
 
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