- Thread starter Leroy
- Start date
My Ford F150 and my son's Bronco both have 3.55 read ends - the F150 gets about 16 MPG and the 26 year old Bronco gets about 14. IF I was doing a lot of towing the 3.55 would seem a bit high geared but that's not the case. The highest geared vehicle I ever had was a 1964 F250 with a 4.88 rear end, that little 223 was SCREAMING as 65 MPH.
RandyB(MI)
Well-known Member
Mike is correct but another thing to consider is the towing capacity if that is what you are doing. Towing capacity and possibly even GVWR will be higher with the 3.92. This is just from a "legal" standpoint as in the real world, the difference between the two (all else being equal) you will likely never notice and neither will the truck. Get the 3.55 if you have a choice.
old
Well-known Member
- Location
- Camdenton Missouri
The larger the number the power your top end will be and it will use more fuel in most cases. My 1980 Chev Heavy half pickup has 3:07 gears in it. 4th gear more or less is over drive but that is just fine I get up words or 17MPG.
Fred Werring
Well-known Member
3.92 for towing, 3.55 for mileage, although they're close enough I doubt you'd see much difference.
As an aside
Had a 66 Fairlane, was looking for a little more gear than the 3.27 that was in it.
Swapped a guy my pumpkin for one with a set of 4.68's.
It was quick up to about 65, at 75mph it was done.
Oh yeah he also neglected to tell me he'd welded the spider gears...found out first time trying to back into a parking spot.
Fred
As an aside
Had a 66 Fairlane, was looking for a little more gear than the 3.27 that was in it.
Swapped a guy my pumpkin for one with a set of 4.68's.
It was quick up to about 65, at 75mph it was done.
Oh yeah he also neglected to tell me he'd welded the spider gears...found out first time trying to back into a parking spot.
Fred
Steve@Advance
Well-known Member
Looking at the numbers, what you see are "3.55 to 1" ratio. What that is saying is for every time the drive shaft turns 3.55 turns, the axle turns 1 turn. So the lower the number, the slower the engine turns.
That, however, does not automatically mean better mileage with a slower turning engine. There are many factors to consider, mainly engine size verses rolling resistance. An overloaded vehicle with a too high gear ratio will get just as bad mileage as an empty vehicle with a too low ratio!
When the factory assigns a particular gear ratio, they go for a happy medium between empty weight and what the vehicle will probably be used for. They also consider engine characteristics, such as power band, torque ratings, emissions, overdrive, locking converter, etc. Throw in years of experience, unlimited funds for research, and a bunch of trial and error. But it's still really a guessing game, especially for trucks, not knowing how it will be loaded or driven.
As others have said, as close as the numbers are, it will be of minimal difference.
That, however, does not automatically mean better mileage with a slower turning engine. There are many factors to consider, mainly engine size verses rolling resistance. An overloaded vehicle with a too high gear ratio will get just as bad mileage as an empty vehicle with a too low ratio!
When the factory assigns a particular gear ratio, they go for a happy medium between empty weight and what the vehicle will probably be used for. They also consider engine characteristics, such as power band, torque ratings, emissions, overdrive, locking converter, etc. Throw in years of experience, unlimited funds for research, and a bunch of trial and error. But it's still really a guessing game, especially for trucks, not knowing how it will be loaded or driven.
As others have said, as close as the numbers are, it will be of minimal difference.
Chuck (CA)
Well-known Member
I put some 4.56 gears in my '57 BelAir when I was a kid. Ran like a bat out of h*ll but the engine was screaming at 70 MPH. My mom and I drove the same freeway home and sometimes she would go sailing past me in her '64 Lemans with a six cylinder. She never could understand why I would put all the money into a car and couldn't go fast.
Delt for truck, to get tomorrow. 2003 Dakota quad cab with 4.7 auto 4 wheel drive. I don't know yet what it has. It should have as much towing power as the 2000 club cab 4.7 auto 2 wheel drive I had with the 3.55. I thought the 3.92 should be more pulling power than the 3.55 but was not sure. I now have a 2001 regular cab 3.9 auto 2 wheel drive with I think the 3.55 and it just does not have what I need for trailer towing that the one that got wrecked did easy so this should pull the loads as easy as the one that got wrecked. Never had 4 wheel drive and the regular cab just does not have enough room in, at grocery today had to put some things in bed. Did not have that problem with the club cab. And yesterday could have used the room for 3 passengers.
K Effective
Well-known Member
My 1986 Chevy K30 came with 4.10 gears, 350 with four barrel carb. Sitting in the driveway overnight, engine off, it burned 5 gallons of gas. Coasting down hill in neutral with a tail wind I could eke 10 mpg out of it. But, it would push snow into a two-story high pile and climb a tree if you got a tire up...
All other things being equal the 3.55 will get a little better gas mileage on the interstate. The 3.92 will pull a little better and accelerate a tad better, but will use a bit more fuel. If you don't plan on doing much towing and it's just a driver the 3.55 should treat you fine.
Greg
Greg
I know how you feel. I had an 82 1 ton with a 454 and 4.56 gears. Thing was a freight train, but was wound out at 65 and got horrible gas mileage. Wouldn't get over 10 mpg going down hill with the key turned off. Good thing it had two gas tanks. It needed them both just to make it to then next gas station.
Greg
Greg
Goose
Well-known Member
Yeah. In the day, I had a 1980 C30 crew cab dually with 4:10's. Empty, it got about 11 mpg. Hook a trailer behind it, loaded or empty, and you were down around 8.
With it and my trailer, I once hauled a '67 GTO from Lincoln, NE to Minneapolis. Hauling that Goat to Minneapolis and pulling the empty trailer back, I got a grand total of 6.8 mpg. But, it was in August and I had the hammer down with the AC on. I was trying to make time, not gas mileage. I swung through Sioux Falls on the way back to see if I could find a rebuilder worth backhauling, but I struck out.
With it and my trailer, I once hauled a '67 GTO from Lincoln, NE to Minneapolis. Hauling that Goat to Minneapolis and pulling the empty trailer back, I got a grand total of 6.8 mpg. But, it was in August and I had the hammer down with the AC on. I was trying to make time, not gas mileage. I swung through Sioux Falls on the way back to see if I could find a rebuilder worth backhauling, but I struck out.
Tire size has a lot to do with the overall gear ratio. To find out the engine rpm at a given speed here is how you figure it. You need the overall gear ratio, maybe 3.55 for axle, times .8 for overdrive in high gear equals 2.85. Multiply by constant 168 or 477.12. multiply by speed, say 60 mph 28627.2. Divide by loaded wheel radius, maybe 16" equals 1789.2 rpm. You need to plug in the numbers for your drive train, but this will tell you the rpm in a few seconds with a calculator. I have had this formular around for years, from my hot rod period back in the 60s. Comes in handy from time to time. It can be changed around to find out what gear ratio you want for a certain rpm, and what tire radius you want too.
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