Gear Ratio
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Freshman User
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Gear Ratio
Hey guys
I was wondering if one of you could tell me what would be the best gear ratio in the rear end. what i'm looking for is lower rpm's at higher speeds. Its a 94 f250 5 speed with a 460. I'm not quite sure what the gear ratio is at the moment. Right now at 65 mph I'm turning about 3500 rpm's. I think thats kind of high and makes the truck burn more gas. At the moment i'm getting about 16-19 miles per gallon and would like to better that if I can by changing the gears and lowering the rps on the highway
Would someone be kind enough to help me out?
Thanks
I was wondering if one of you could tell me what would be the best gear ratio in the rear end. what i'm looking for is lower rpm's at higher speeds. Its a 94 f250 5 speed with a 460. I'm not quite sure what the gear ratio is at the moment. Right now at 65 mph I'm turning about 3500 rpm's. I think thats kind of high and makes the truck burn more gas. At the moment i'm getting about 16-19 miles per gallon and would like to better that if I can by changing the gears and lowering the rps on the highway
Would someone be kind enough to help me out?
Thanks
FTE Legend
Joined: Jun 2006
Posts: 31,930
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From: Ottawa, Ontario
OK.. there's something seriously wrong with your numbers here, there's no way the motor would be reving that much at 65mph in overdrive, and if it was I guarantee you it would not get 16-19mpg.. more like 5mpg. So...
How are you calculating fuel milage?
Has the speedo been calibrated against a GPS?
Is the tachometer accurate?
What size tires are on the truck?
What is the axle code on the drivers door pillar?
How are you calculating fuel milage?
Has the speedo been calibrated against a GPS?
Is the tachometer accurate?
What size tires are on the truck?
What is the axle code on the drivers door pillar?
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your right the numbers are probably off. I'm just rendering a guess as to how many miles per gallon i'm getting. the speedo is off for sure cause of my tires. I'm running 265/16's. not exactly sure by how many miles per hour tho. from dead empty I can put 15 gallons in one tank and I get about 150-175 miles on that tank. i'm hoping that changing the gear ratio will give me around 200 miles on a tank.
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
OK... so get me the axle code off your door pillar, the original tires size on the same sticker, and the full tire size for what's installed now.. should be 265/75-16 or something like that. I'm guessing the truck doesn't have a tachometer and you were just guessing at the rpms too? If so no big deal.
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The original tire size should be 235/85/16 and axel 35 . Yes your right they are 265/75 16 sorry I should have made that clear. There is not much of a size difference between the two tires but enough to throw it off a little.
The truck does have a tack. I'm always watching it.
The truck does have a tack. I'm always watching it.
Senior User
Joined: Nov 2008
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From: Western NC
the tire sizes you have listed are very close in diameter, they won't make much difference at all. The axle code you listed is for a 4.10 ratio, but something isn't right here. Your revs are way too high for a 4.10 axle with an overdrive transmission. I'd jack up the back end and spin the wheel 1 rev and see how many revs the driveshaft does. Is it possible someone swapped gears in the past?
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the tire sizes you have listed are very close in diameter, they won't make much difference at all. The axle code you listed is for a 4.10 ratio, but something isn't right here. Your revs are way too high for a 4.10 axle with an overdrive transmission. I'd jack up the back end and spin the wheel 1 rev and see how many revs the driveshaft does. Is it possible someone swapped gears in the past?
I'm just wondering if there is anything I can do to lower the rpm's cause the motor has 251k on it and I want it to last a little while longer. I baby my truck and maybe over do it but its my bread and butter.
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FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
Axle code 35 is 4.10 gears. Ford Axle Code Chart .: Articles
According to this calculator Tire Size Calculator - tire & wheel plus sizing there is almost no difference in diameter between those tires at 31.6-31-7", so the speedo should be accurate. It wouldn't be a bad idea to clock it against a GPS just to see if it is though.
And according to this calculator Potential Engine Speed (RPM) Calculator your engine speed should be roughly 2150rpm with the ZF's 0.76 overdrive ratio.
If your tach really is indicating 3500rpm then it's faulty and needs to be replaced. Is the gauge cluster original to the truck? The tach in my truck was indicating high when I first got it so I swapped in another tach section from a junkyard cluster and that fixed the problem. The tach section will unplug from the cluster body after the cover is removed so it's pretty easy to change.
And to get back to your original question, a gear change to 3.55 would drop engine speed down to 1858rpm which will make a considerable difference in fuel milage.. probably bring it up into the 13-14mpg range. You could go even higher(numerically lower) with the gearing but it will begin to impact how well the truck hauls, so keep this in consideraton.
According to this calculator Tire Size Calculator - tire & wheel plus sizing there is almost no difference in diameter between those tires at 31.6-31-7", so the speedo should be accurate. It wouldn't be a bad idea to clock it against a GPS just to see if it is though.
And according to this calculator Potential Engine Speed (RPM) Calculator your engine speed should be roughly 2150rpm with the ZF's 0.76 overdrive ratio.
If your tach really is indicating 3500rpm then it's faulty and needs to be replaced. Is the gauge cluster original to the truck? The tach in my truck was indicating high when I first got it so I swapped in another tach section from a junkyard cluster and that fixed the problem. The tach section will unplug from the cluster body after the cover is removed so it's pretty easy to change.
And to get back to your original question, a gear change to 3.55 would drop engine speed down to 1858rpm which will make a considerable difference in fuel milage.. probably bring it up into the 13-14mpg range. You could go even higher(numerically lower) with the gearing but it will begin to impact how well the truck hauls, so keep this in consideraton.
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Axle code 35 is 4.10 gears. Ford Axle Code Chart .: Articles
According to this calculatorTire Size Calculator - tire & wheel plus sizing there is almost no difference in diameter between those tires at 31.6-31-7".
And according to this calculatorPotential Engine Speed (RPM) Calculator your engine speed should be roughly 2150rpm with the ZF's 0.76 overdrive ratio. If your tach really is indicating 3500rpm then it's faulty and needs to be replaced. Is the gauge cluster original to the truck? The tach in my truck was indicating high when I first got it so I swapped in another tach section from a junkyard cluster and that fixed the problem. The tach section will unplug from the cluster body after the cover is removed so it's pretty easy to change.
According to this calculatorTire Size Calculator - tire & wheel plus sizing there is almost no difference in diameter between those tires at 31.6-31-7".
And according to this calculatorPotential Engine Speed (RPM) Calculator your engine speed should be roughly 2150rpm with the ZF's 0.76 overdrive ratio. If your tach really is indicating 3500rpm then it's faulty and needs to be replaced. Is the gauge cluster original to the truck? The tach in my truck was indicating high when I first got it so I swapped in another tach section from a junkyard cluster and that fixed the problem. The tach section will unplug from the cluster body after the cover is removed so it's pretty easy to change.
Thanks guys for your help and i'll post the results as soon as i'm done. Lucky for us the scrap yard is open on sunday here so one way or the other i'll figure something out...
