Taller tires / gearing question

I am wanting to get taller tires for my truck. What I'm running now are Cooper Discoverer A/T3's 235 70R17 . My rear axel ratio is 3.55. So, what would be the tallest I could go to without buying new gears for the axel? The truck is a half ton 2012 f150 XL V8 5.0L. I'm doing this because if I'm correct; taller tires equal better mileage and clearance?

I'm going to be purchasing E rated tires and airbags for a new FWC Hawk to be installed. Thanks
 

v_man

Explorer
I would agree that taller tires equals better clearance , but not sure where you get the idea that they translate into better gas mileage . It's the opposite . Pushing your vehicle higher into sky does not increase your mileage . Putting larger and heavier rotating masses on the four corners of your vehicle without the proper gearing is not ideal for the motor, transmission, fuel economy , or braking ...

Going up a few sizes is not that big of a deal though and is pretty common without regearing , what size tires are you considering?
 

v_man

Explorer
Also a quick rule of thumb is this:

Old Gear Ratio × New Tire Size ÷ Old Tire Size = New Gear Ratio

So you have 3.55 gears, you have 30 inch tires right now , and let's say you want to move up to 35 inch tires ...

3.55 x 35 / 30 = 4.14 , so 4.10 gears would be a good starting point when considering a re gear for the 35" tire size...
 

1stDeuce

Explorer
Even if you stay with a more narrow tire, like a 235/80R17 or 255/80R17, your mileage is not going to improve, and likely it's going to get a little worse. The only way taller tires can improve mileage is if you have something like an M1008 GM military truck, where the factory axle gearing is 4.56 and there isn't an overdrive, so at 55mph the engine is running 3000rpm's. Your truck isn't like that at all, and so taller tires just make it work a little harder to go the same speed. They will slow the engine down for a given speed, but on a modern vehicle, that's not a good thing, since your engine is probably already spinning well below torque peak when you're at cruising speed.

On another note, I'm running (~33") 255/85R16 Maxx's on my TJ and I LOVE them. (255/80R17 is the same width and OD) Maxx's are pretty spendy though... If you can live with a 32" tire, you can get really good pricing on AT3's in 235/80R17, and they seem to be excellent tires too, and would likely give better mileage than the Maxx's.
 
Even if you stay with a more narrow tire, like a 235/80R17 or 255/80R17, your mileage is not going to improve, and likely it's going to get a little worse. The only way taller tires can improve mileage is if you have something like an M1008 GM military truck, where the factory axle gearing is 4.56 and there isn't an overdrive, so at 55mph the engine is running 3000rpm's. Your truck isn't like that at all, and so taller tires just make it work a little harder to go the same speed. They will slow the engine down for a given speed, but on a modern vehicle, that's not a good thing, since your engine is probably already spinning well below torque peak when you're at cruising speed.

On another note, I'm running (~33") 255/85R16 Maxx's on my TJ and I LOVE them. (255/80R17 is the same width and OD) Maxx's are pretty spendy though... If you can live with a 32" tire, you can get really good pricing on AT3's in 235/80R17, and they seem to be excellent tires too, and would likely give better mileage than the Maxx's.[/

Thanks Deuce, will the 255/80R17 32" work with my 3.55 gearing? My current tires are 235/75R17. Will a 31" Or 32" affect mileage that much? I'm not looking for additional width, just added height.
I used to own a 1986 M1008. I ran Cooper 10 plys on it too. It hated going faster than 60. Sad to see her go.
I've looked at and attempted to understand how the numbers on the sidewall relate to inches, height and width. Mathematics is by far my worst subject.........
 
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Buckshot62

Observer
Taller tires

Online you can find metric to inch tire conversion charts. (I'm not computer smart enough to link one on here for you or I would.)Would that help you ?
 

SageSam

New member
I have a 2014 F-150, 5.0 with the 3.55 and I recently jumped from 235/80-17 to 285/70-17 (roughly 31" to 33"). There is a definite drop in power and mpg. The power drop isn't that big of a deal, as the 302 is more than sufficient. I honestly don't know how bad the mpg loss is due to the speedo being off, but it's at least 2mpg. I haven't gotten the CPU flashed or tuned, so it may be an issue that can be addressed with re calibration of the shift points.

I went with the St Maxx's, and while I love the tire itself and the increased size/clearance, I'd probably go with a smaller tire if I had to do it over again.
 

tommudd

Explorer
With the load you will be hauling plus larger tires regearing would benefit you greatly
Unloaded driving around say Florida on flat roads then no you could get by.
 
With the load you will be hauling plus larger tires regearing would benefit you greatly
Unloaded driving around say Florida on flat roads then no you could get by.

Would you go higher or lower in your gearing if you were going up in tire size and if so, how much, is there a rule of thumb or a gearing/tire ratio calculator?
I plan on driving on National Forest roads and BLM land and as little pavement as possible. Thanks
 
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v_man

Explorer
is there a rule of thumb or a gearing/tire ratio calculator?

100 acre , with all due respect , have you read any of the posts on this thread ? There is a gear calculator , a rule of thumb formula, and a metric to standard tire size calculator shared with you in previous posts ...

Also , not sure if you're aware of this , but you will need to regear you're front and rear diffs since you have the 4x4 . Just throwing it out there, but I wouldn't do any tire purchasing or re-gearing until you do your homework on the subject . It's all out there on the net, countless people in the past have had the same questions as you ....
 
100 acre , with all due respect , have you read any of the posts on this thread ? There is a gear calculator , a rule of thumb formula, and a metric to standard tire size calculator shared with you in previous posts ...

Also , not sure if you're aware of this , but you will need to regear you're front and rear diffs since you have the 4x4 . Just throwing it out there, but I wouldn't do any tire purchasing or re-gearing until you do your homework on the subject . It's all out there on the net, countless people in the past have had the same questions as you ....

Thank you, I will look more. Some of the charts are a bit confusing and I am not feeling all that well, but I will study it more. I do appreciate your candor and help.
 

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