I have an 89 F250 with 4:10 gearing and I need to try and gear it up a little. One place I read said to go to taller tires, so my question is what size would be needed? Is this a great jump or what? For a dually I was told to go to 19.5 wheels but that doesn't help me here.

Thanks Hank

 

4.10 gearing the max I would go with would be a 33" tire.

 

That is true about changing to a taller tire to raise (numerically lower) the effective gear ratio, which will decrease engine rpms at any given vehicle speed. I'm guessing you probably don't have an overdrive trans are looking to be able to cruise on the freeway at a higher speed without wrapping out the engine so far, which I can understand because I had the same issue with my 4.10 dually with no overdrive.

I do agree with 95F3504x4 about keeping the tire size to 33" or under so as not to lose too much gearing.

Going to 19.5" wheels doesn't necessarily solve your tire height issue, since tires for 19.5" wheels can be had in the same tire height as tires for 16" wheels. It's also an expensive fix and not necessary on a dually, since you have 4 tires sharing the load on the rear end. Most people convert a single rear wheel truck to 19.5" wheels for the greater load carrying capacity of the commercial tires available in that wheel size, since it's the rear 2 tires' load capacity that limits the weight carrying capacity of a single rear wheel truck. Basically, they want higher rated tires so they can overload their truck more safely...

At any rate, your stock tires are probably 235/85-16's, or maybe 215's or 225's. My dually had 235's stock. The problem with the dually trucks is the tire width on the rears. You can only go so wide before they get too close to eachother and rub at the bottom when loaded.

I say that about the width because with metric tires, the width and the height are related, as one increases, so does the other (with the same profile percent). '85' is the profile percent in the tires mentioned above. If you have 215/85-16's now and that is the stock size, you may be able to go to 235's and be ok. If you have 235's now and that is the stock size, you may be able to go to 245's or 255's, but probably not much more than that without spacers between the wheels.

Wheel width for the tire is also something to consider. Dually wheels are usually kinda skinny. On my old dually, I ran the stock 235/85-16's on the rear and 265/75-16's on the front (same diameter, just wider) and they didn't fit that great on the wheel, they were a little too wide for that wheel. They did work, but I didn't like how the tires were bulging at the sidewalls and the wheel was set back into the tire. They seemed not to have good sidewall stiffness on curvy roads. I later switched back to 235's on the front and was happy with it. I don't remember exactly how wide those wheels were, I'm guessing 7" or 7.5".

Here's a formula for determining the height of a metric tire in inches:

((2 x SectionWidth x AspectRatio) / 2540) + WheelDiameter.

For a 235/85-16 tire, the equation looks like this:

((2 x 235 x 85) / 2540) + 16 = ~31.5" (rounded to the nearest half inch).

To determine width, divide the SectionWidth by 25.4, which is how many millimeters there are in an inch. A 235mm tires is ~9.25" wide.

 

So Cal its an 89 SC 2wd 5sp man with the 351 engine. I have made aan FIPK for it, added the MSD ignition box, wires and coil and Gibson catback. It runs well just no milage with or without towing. can get about 10-11 empty and 6-7 towing a 4000lb TT. its got the granny low 1st so everything starts out in 2d. Just trying to figure out how to maybe improve it without the big $$ to regear.

Hank

 

Hank, since your 5speed ZF has an overdrive 5th, I would think your rpms should be ok with the 4.10's if your stock tire size is 235/85-16. A 31.5" tire with 4.10 axle and ~.75:1 OD ratio in a manual trans gives an engine speed of ~2100rpm at 65mph. That's not too high to be concerned about . Actually, with a 351, 2100rpms is below that engine's powerband, which is fine in an OD gear, as long as you drop it out of OD to pull hills when loaded.

The 351 in my current F350 single rear wheel truck reaches peak torque at 2800 rpm and with 4.56 gears & 33" tires with a C6 that adds some rpm's via torque converter slip and no OD gear, I'm spinning ~3200rpms at 65mph. I try to keep speeds down to around 55mph (~2750rpms) to save gas, which also happens to put me right around my engine's max torque output .

 

i have a 86 f250 with a 300/6 with a t19 4 speed the truck the door tag said 215/85/16 i dont uderstand why ford would put such little tires on a f250 /i put on 245/75/16s /the truck has a semifloating 3.55 rear end /it gets between 12 and 13 empty is that reasonabel mileage
thank you
lrd56