As someone stated earlier, tire size has a definite impact on gearing choices. I made a table that shows RPM at 70 MPH, with various tire sizes and gear ratios in overdrive. Ford gear choices for stock tires put the RPM in the 1,900 - 2,000 RPM range. Bear in mind, tire size is the actual size, not manufacturer's claims. If you want to know your actual tire size, take a measurement from the center of the hub to the pavement and double it. For performance gearing, I think you can go up to about 2,300 without much of a penalty in mileage. The problem with going up to larger tires without re-gearing is easily shown in the table. If you're going to re-gear for larger rubber, I think you need to stay in the 2,000 - 2,300 RPM range, 2,000 as a minimum.
The chart was posted because gearing and tire diameter are relative. If you're running 30" tires with 3.55 gears and change to 4.10s, then yes you'll have some loss of mileage. If you're running 34" tires with 3.55 gears, then you'll probably pick up some mileage. Perhaps if you gave us your current gear ratio and tire size, we could better answer your original question. Also what would be your main objective in lower gearing? Towing, acceleration, just because, etc.?
also i probably should have mentioned this sooner but this stock wheels and suspension, so this is for more of a street/strip application. and i have also been thinking maybe, 373s but 410s are $20 cheaper.
I see four different OEM tire sizes for a 2003 F-150 2WD. Can you narrow that down for me. Also what engine are we talking about? 5.4, 4.6 or 5.4 Supercharged? For 2003, neither of these engines in stock form were high revving, so that should be considered too. Has the engine been modified and capable of more revs.
ok i have 3.55s running 275-30-24s which are 30.5" tall so thats a 269 difference in rpms thats not going to be that much in mpg loss is it? plus ill gain better acceleration with the 24's and will i have better pick up say doing 65 mph and i get on it it will pick up speed quicker right?
Is that the manufacturer's claimed diameter? If so, you'd probably be closer to actual at 30". You'd probably notice some loss of mileage on the highway, but you should have better acceleration. I don't know that it will be as noticeable at speed as on take off.
Years ago, I put 4.10s in my 92 bronco with the stock 235/75-15s and 3.55s which were 29" tall. It accelerated more aggressively, and mileage didn't suffer that much. It was a speed density unit (old EFI) and didn't get good mileage to start with, so that might not be saying much. I installed them in anticipation of putting 32" and a lift after that. It was still geared a bit lower than stock, but not quite as extreme. After I converted it to mass air and put built 351 in it, it probably could have used 4.56s.
With a stock engine, I think you'd be just as happy with 3.73s and probably not notice any mileage difference. If there were a gear set between those two, if would probably be ideal for your tire size.
With a cam, you'd probably need the 4.10s. I don't know the specs, but I'm guessing it expanded the rpm range. With a more aggressive profile from stock, a higher stall torque converter might help more than gears.
Changing gears is precise work. Ford uses cast iron spacers to position the carrier. You'd need a couple of narrower spacers to accommodate shims to properly set the backlash. You need access to a press, dial indicator and an inch pound torque wrench.
Find an article on the process and make a realistic assessment as to your capabilities. Getting it wrong could damage the gears. Bearings can be pressed off and reused, but new would be a better choice if you truck has over 50,000 miles or you already hear noise from the rear end. Personally, I'd put new bearings in with my install.
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