I recently got a set of 235/75-15s for the rear and 205/75-15 fronts for my 55 F100 (stock suspension). The 235s are inflated to max psi of 45 lbs and I'm finding the truck to riding a bit rough since I not carrying any load. For those running with 235/75-15s, what pressure do you have them at? My fronts are running at 33 lbs. I think I will lower the fronts to 28 and the rears to 35 and see if the ride smooths out.

 

My 2c, fwiw, is 205-75's are a bit small for the front, so you'll want a little more air to make up for it. With the majority of the weight over the front end, more air is good. It'll probably handle better with at least 30 pounds in front. For the rears, if you're not hauling anything, lower is fine. I'd probably try 30-30 and see how that feels. 45 is definitely too much. Again, that's just my 2c.

 

I have 225:75/R15s all around with a Volare suspension front and PS. I run 28 in front and 24 in the rear. Without PS I think 30 will work for you in front but if you aren't carrying loads try about 24-26 out back. Makes for a much smoother ride!

 

I have 235/75-15's and run them all at 32. I tried 28 and 30, it was a little softer but a lot more scrubbing on "high speed cornering", LOL. Wear pattern at 30-32 is very uniform. I agree, 205's on the front will likely wear out quickly. No problem steering with the 235's on mine.

An easy way to figure out the best pressure is to get a part of your driveway wet, not standing water, just enough to transfer to the tires. Drive slowly thru the wet area onto a dry area. If you see the whole width of the tread, your pressure is OK. If the middle of the tread isn't showing, you need more pressure. If the edges don't show, you have too much. With stiff truck tires it doesn't always work, tho.

 

Thanks for the feedback. The front tires (205s) are 5 yr old Diamondbacks that I reversed the whitewalls inward (to match the brand new 235s). Although the tread looks great, maybe age could be a factor. Would 215s better suit the fronts. The wheel is 5 in wide so I can't go too large. Thanks again. Steve

 

The Max inflation pressure stamped on the sidewall is just that the MAXIMUM pressure that the tire should ever be inflated to. The hot pressure after driving 10 miles in the summer or 30 miles in the winter will be as much as 10# above the cold standing pressure. I'd inflate tires that are within the section range for the wheel width to about 27-28# cold, and recheck them after driving for a while. bleed them back down to 30# hot. Be sure you are using an accurate tire gauge and treat it like a fine watch to keep it accurate. A digital readout gauge (10 - 15.00) or a brand name, like Longacre, large dial racer's gauge (approx 30.00) will be the most accurate. The traditional inexpensive sliding stem pocket gauges are usually so inaccurate you might as well be kicking the sidewall to determine the pressure.

To determine the correct size tire to use, go to the manufacturer or a large tire distributor web site like Tire Rack.com and look up the specs on the brand and model tire you are wanting to buy. In the spec table should be information on wheel width range and ideal width (sometimes labeled "measured width") for each size. It's an old wives tale that you should use a wider tire for more weight carrying, Tires have a weight (per tire) load rating and as long as you don't exceed that capacity it is more important to match tire size to wheel width. If you don't know know your truck's per wheel weight, go to a place that has a certified truck scale. Be sure you are carrying a typical load for your truck, people and cargo. Tell them you want 2 weights. First drive you truck onto the platform so it is approximately centered and get the total weight. Now pull forwards or back until either only your front or rear wheels are on the platform and the other end is on the ground. Get the weight for that end of the truck, doesn't matter if it is front or back as we'll see in a moment. Now make a chart of wheel carrying weights:
At the top put the total weight for your truck: Total weight = xxxx#
Next line put either the front or rear axle weight, (which ever was on the platform, lets assume it was the rear wheels here): Rear axle weight = yyyy#
Next line subtract the rear axle weight from the total weight xxxx - yyyy = zzzz# and label it: Front axle weight= zzzz#
Next divide Rear axle weight yyyy by 2 and label the result on the 4th line: Rear wheel load (per tire)= (yyyy/2)
Finallydivide the Front axle weight zzzz by 2 and label the result on the 5th line: Front wheel load (per tire) = (zzzz/2)

I highly recommend making up a permanent label or laminated card of this info and mounting or carrying it in your truck or wallet, it's useful info in a lot of situations.

 

^what he said

 

If you want to be **** about getting the exact tire pressure, do like we do with our racing tires: get a probe style tire temperature meter. Drive the truck non stop for about 20 minutes >40 MPH on a moderate to warm day with little to no breeze. Slowing down as little as possible, immediately stop and record 3 temperature readings across the face of the tire tread: inside, center and outside. Do this for all 4 tires as quickly as possible to minimize atmospheric cooling affect. Now compare the 3 recorded temps for each tire. We are not concerned with the exact temps, as long as they are well above ambient air temp, just the difference across the tread. If the two outer temps are more than a few degrees above the center temp, you need to raise the hot pressure you are running about 4#. If the center is hotter than the edges, reduce the hot pressure by 4#. Now repeat the test to verify the new settings are correct. If the temperature step across the tread in order, say inside (hotest) center, outside (coolest) you have an alignment issue, either caster or are toed out. If in order the other way you likely have too little camber, are overdriving your turns or have too much toe in.
We use a very expensive recording tire temperature meter to check our race tires. It automatically records the 3 temps for all 4 tires as you probe them, and saves for recall analysis up to 50 sets of readings.

 

Sorry Dood, didn't mean to steal your thunder! (glad you agree tho)

 

Here is what works for me. I read somewhere that at one time one of the tire manufacturers suggested this procedure. It works for me. A properly inflated tire will gain four pounds of air after a few miles at highway speed. If a tire gains less than four pounds it is overinflated, if it gains more it is under inflated. I tried it on my 40 coupe and found that my seat of the pants told me the same, I had them right on. I then adjusted the inflation amounts on each of my 53 F100 tires to be different. Based on how much they gained or lost I found the proper psi, at least it feels correct to me. I am sure that the experts will dismiss this, but it does work for me, providing a good ride and handling on my rides.

 

Ax, I really appreciate the information. Ever since I had the 235/75-15s installed on the rear, I'm now getting more severe vibration on the front when I reach 55 mph and up. I'm beginning to wonder if the raked stance with 235s and 205/75-15s in the front are causing this? Probably need to recheck toe-in and caster.

 

By putting the much larger tire on the rear you have effectively decreased the caster angle. Camber is built in to a straight axle and rarely needs adjustment. However, to increase caster you may now need axle shims...

 

Mid Fifty sells tapered alignment shims. Even a stock front suspension could use a shim on each side (install so the axle tilts back) to increase the caster for today's roads and driving speeds. If you have removed leafs, used lowering springs, or increased the rake you may need to add 2 sets of shims. If using two sets you need to replace the spring centerbolts with ones with longer heads (MF sells these also). Be sure to search on here for info on how to disassemble the springs or change the centerbolts safely. To replace the centerbolts, clamp the spring stack together with 2 LARGE C-clamps, one on each side of the centerbolt, before loosening centerbolt nut. Do not remove clamps until new centerbolt is installed and nut tightened!

 

Many thanks. Everything is stock so I probably need to look at installing a shim on each side. How is that done? Although I study my maint manual, I'm not very knowledgeable in this area.

 

32 psi is a good starting point, adjust from there.........it's not rocket science. FWIW, I run 205/15's on the front of my F-1, seems to work fine.