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with your current setup absolutely. the lower gears will help that problem
You have that mixed up, it's the other way around.
Originally Posted by Thor_1
So I have the 275/55R20 tires with the 3.55 rear axle. According to the above chart I have best gas mileage, poor performance. Why would Ford set it up this way? Wouldn't a good gas/good performer be a better setup?
Mine is even more extreme at 3.15 with the same tires. I'm assuming it's for better MPG especially at highway speeds, as that's what most people are concerned about. Yet my truck acceleration is very nice for my needs. I think saving fuel this way adds up to more stress on the drivetrain, but the trucks seem to handle it fine.
Last edited by Pullsmyboat; 01-27-2019 at 01:20 PM.
The motor will rev higher than before at highway speed. I believe this uses more fuel.
That depends on how much manifold pressure (throttle depth) is required to maintain cruise speed. What you say is generally correct, but if the gears are too high (numerically low) you can find youself on the backside of the curve. Also there is a considerable loss of torque to the ground when tall gears are coupled with tall tires.
From a practical/seat-of-the-pants standpoint, you will love the driveability with the lower gears. Even if it costs you a little mpg. Also when compensating for larger tires, you need to do more than just get back to "even" due to the increased weight and rolling resistance, especially if the truck was under-geared to begin with. If the gear swap is in you comfort range ($$) do it, assuming you're getting the tires of course. Actually do this; Put the tires on and drive it for a week. Now do the 4.10 swap. You'll grin ear to ear.
-- If you're really doing a LOT of high speed freeway driving you might consider 3.73's, but I'd be going 4.10 without hesitation. Of the 3 re-gears I've done, one had me on the fence between 4.10 and 4.56. I somehow talked myself into the 4.10's and almost immediately wished I'd chosen the deeper gears. There's an accurate old saying, "Don't fear the gear".
This is far from scientific but I can give my anecdotal experience.
My stock 6.2 Raptor on factory 34.5 BFGs got 17.4 highway - just driving not being mpg cautious. IIRC, it hovered around 14 combined but I'm in DC traffic and I drive it like a go-cart.
I'm now on heavy 37" MT ATZs. I've added at least a thousand pounds in aftermarket gear. I'm running 4.88s because I like crisp throttle response. I can absolutely ROAST the 37s on dry pavement and I'm getting 11.5 mpg combined - still driving like an idiot. I drove to Florida on winter gas and was getting ~15 hwy. I'm about due for tires and I plan on getting something less knobby. I bet just switching to a less aggressive tread pattern will regain at least 1mpg.
My point is, tread pattern will have more of an mpg impact than an extra 200rpm at hwy speed.
Makes sense. But then the question remains; if fuel economy is not a significant factor as it relates to a gear ratio change resulting in 800 higher rpm at hwy speeds, why does Ford use such a high gear (3.15) in the first place?
Last edited by Pullsmyboat; 01-27-2019 at 03:53 PM.
Makes sense. But then the question remains; if fuel economy is not a significant factor as it relates to a gear ratio change resulting in 800 higher rpm at hwy speeds, why does Ford use such a high gear (3.15) in the first place?
here is my answer to that. not sure if it's right or not. it is cheaper to get the higher gear ratio. and ford knows that people that know nothing about it will get the lower gear ratio because it is cheaper, and they will sell more trucks. that is my opinion on it
Why would it be cheaper? An aftermarket ring and pinion set is around $200 retail. I imagine the cost to Ford is a fraction of that, and no different between a 3.15 and 4.11. If anything 3.15 theoretically puts more stress on the drivetrain which could result in parts failure. This would be more expensive for Ford if it happened during the warranty period.
Last edited by Pullsmyboat; 01-27-2019 at 04:21 PM.
Makes sense. But then the question remains; if fuel economy is not a significant factor as it relates to a gear ratio change resulting in 800 higher rpm at hwy speeds, why does Ford use such a high gear (3.15) in the first place?
CAFE. Corporate Average Fuel Economy. They have a fleet average number that has to be met (federal mandate) and it gets tighter every year. Parts cost is not a factor. With the small/light OEM tires & wheels, high gears = better MPG. With the big tires, high gears will still probably = somewhat better MPG, but the performance is going to be poor at best. If fuel economy is an ultimate goal, DON'T put the big tires on it. They are going to hurt you far worse than gearing down.
I have stock 275/55/20 (32") tires on a 2015 3.5 EB 2WD Lariat. Considering putting on 275/65/20 (34") or 285/65/20 (34.5") tires. Been reading here and found out gear ratio is a factor in this decision. My door sticker says L5 which is "3.15 ratio, electronic locking differential". This chart puts my stock 32" tire and 3.15 ratio nowhere near anything "good", it's simply off the chart. Never mind if I go to a bigger wheel.. My truck performs impressively in both gas mileage and power compared to other vehicles I've driven. Am I missing something?
With some background info, that chart is completely useless...what transmission ratio is it using? what engine? If it's not specific to your setup, it's useless.
Originally Posted by Pullsmyboat
Thanks for the suggestions. All this is new to me. I've been reading and now have a better understanding. This video was helpful to put things in perspective.
Considering that now I have 3.15 running 32" tires, would 3.73 or 3.91 be a good choice for 34" tires?
The labor from a gear swap is a considerable part of the expense. I wouldn't cheap out on the gear set with an ebay special. Do some research, but Ford or Revolution gears would be a good place to start. Find a good, reputable gear shop in your area, tell them what you want to do and ask what gears/parts they recommend. Using cheap, no name gears, or gears that the shop has had issues with in the past can often be an issue when it comes to warranting their work. (i.e. they might say it isn't their fault, the work was done correctly, but the parts you provide are poor quality and that's the issue).
As for as the lower gears reducing the fuel economy...the tire size is a big part of that equation. If the tires are too big for the gear ratio, then you'll either be running a lower transmission gear or running more throttle, both of with will increase fuel consumption. So lets assume with your 6R80 that you have to run 5th gear with the 3.15s to comfortably maintain highway speed. This is the same effective drive ratio as 4.10s in 6th gear. From there, if you need to downshift you will actually be at a lower drive ratio (i.e. higher rpm) than the 4.10s with the same number of downshifts. You'll also be using more throttle to get the truck moving (less torque in 1st and 2nd gear than the 4.10s). I have seen in multiple cases where re-gearing with larger tires yields higher mpg than being geared too high with larger tires. But of course it will never be as good as small tires with high gearing.
I would personally go with 4.10s for 34s, but honestly I think you will be satisfied with 3.73s if you go that route. I'm not sure if there is anything available for the 9.75 between those (like a 3.92).
01-27-2019, 01:15 PM
