Tire Size and Gear Ratio
 

OK, I need the help of some of you genius' out there. My 07 F250 came with 265X70RX17 tires. I recently bought a set of take offs from an 07King Ranch F350, the size is 275X70RX18. These tires are taller of course but I don't remember how much. What I'd like to know is with these tires what is my effective gear ratio change? I have 3.73 gears now, should I change the rear end? Would it be better for towing? I pull an 11,500# 5er and have noticed a definite change in pulling hills, the truck seems to be working a bit harder and my fuel mileage seems to have gone down when towing.

Whatcha think?

Steve

I find it weird you are noticing that big of a difference... not a huge tire size change, especially for a 6.0. I went from 31.5s to 35s with my little 5.4 with 3.73s and i felt the difference, but its not much worse driving (just mpg). If you want to regear you could... but its about a grand an axle, so double it if you have 4x4.

I went from stock tires to 35x12.5 R20's and I also have 373 gearing and have not had any issues at all when towing etc... I have an 8,000lb bumper tow and I also put 2 quads on my deck (about 1200+ pounds) and I have no troubles at all with power or temps with engine or tranny.

Its not a big difference but I can tell, the spedo changed by about 4MPH at 70mph. I can tell that it is working a bit harder pulling hills with the trailer hooked up. Dont get me wrong, it pulls like a mule but the mule is working harder. I was just wondering what my effective gear ratio is now that I have gone to the bigger tires and wheels. I know there are members here who can do the math and figure that kind of thing out, I certainly am not one of them.
Steve

Ah, yea my speedo is off by 6-7mph too. If you Google it there are a bunch of effective gear ratio calculators

Assuming the tires are truly the size they're said to be (and they won't be, but it makes the math prettier), you had a 4.91% increase in tire size, changing from 638 revs a mile to 608 revs a mile; at actual 65mph you speedo will now read 61.8mph (3.2mph slow); and you'd need 3.91:1 gears to convert back to your original stock ratio. 34.5" tires is where you need 4.10s to get back to the stock ratio for your truck, and that's the next option in gears. IMO not worth it just because of the tires, the only reason to regear would be if you're not satisfied with how it was pulling before. Considering how many trucks came with both your new sized tires and 34" tires on stock 20s, you're still within the range of what Ford produced from the factory.

That being said, I noticed a difference between my stock 31.5" tires like you had and the 34s I have now. Not huge, but the biggest difference was in trying to get rolling.

Bryan, Thanks and thanks to everyone for the replies. So long as I'm not lugging the motor I'm fine with the way things are.

Steve

my truck still has stock 3.73 gears in it. i have 37x13.5x18 on my truck. i pull a 8000lb bumper pull camper an my truck works it butt off pulling up hills. that bein said , it has never once complained about it at all. the trans gears down an on up the hill i go.

after i get finished with my DIY external oil cooler install im goin to change my gears an put 4.56 gears in it. so say thats to much gear, but i want it to pull like a freight train an i plan on goin to bigger tires.

Steve, you can always go into your nearest Ford dealer to have the speedometer (and the whole PCM strategy for that matter) reprogrammed to reflect the change in your tire size. Since this tire size/axle ratio is an existing option for these trucks, it's as simple as selecting programmable parameters with IDS, and then selecting the tire size/axle ratio combo in the list of options and downloading it.

^ do you know how much they charge for that? should i flash the stock programming on before i go?,

Likely whatever the flat diagnostic fee is in your geographical location.

Thank you for posting! This was exactly what I was going ask about, as I'm getting ready to take the plunge and move from 35" tires with 3.73 gears up to 37" Humvee Freedom Fighter tires. ;) I'm completely satisfied with the performance of my EX at the moment and I'm hoping moving up to 37's won't force me to do a gear change. "With the cost savings of using Humvee tires I could do a gear change for free" (This is me talking to my wife). Sorry for the hijack. :-drink

I had the dealer reprogram the computer last time I had the truck serviced and it didn't change the speedo at all, it still reads about 4 mph slow at 70 mph verified by my GPS. I guess its something I'll have to live with?

Steve

And this is AFTER the tire swap?

Yes, there was no change at all, perhaps the reprogramming didn't take? I'm almost due for service again and I'm going to ask them about it at that time.

Steve