Tire Inflation Question

[MTTwister]

Finally got the new tires, now for proper inflation pressures.

Using Ball park figures:
Truck TARE weight @ 7000# ( 6940 +/-).
Assuming the front is a little heavier than the rear, let's say front is 3600# and rear is 3400#'s.

Based on reading other threads, I'm supposed to inflate on a per tire load basis.

So in the Rear - that would make a load per tire of 1700, right?
If the tires are rated 3400# at 80 PSI, then I would theoritically run them at 50%, or 40PSI. That seems really low to me.

50PSI is 62.5% and 56PSI would be 70% ( about close to what I could "live with" - sidewall distortion getting close to the pavement, etc) .

There was mentioned a site that had a Load / PSI Curve for tires. Can anyone point me to one of those?

And, any other advice welcomed.
Thanks all.

[DmaxMaverick]

Your front/rear axles aren't equally loaded. You need to know what weight is actually on each axle (each tire). 40 PSI is not too low, if the weight you have on that tire is no more than 50% of it's rating. While unloaded, the weight differential can be as much as 70/30% (F/R), so the pressure has to be adjusted accordingly. Loaded, well, that would depend on how it's loaded. You have to know how each tire is loaded. Otherwise, be sure to adjust your tire pressures on the safe side. Also, choose a pressure according to fuel mileage, not tire mileage. I used to adjust tire pressure for longest treadlife, but, (especially lately), it will usually cost more in fuel than you save on tire cost. The formula I've come to for best fuel mileage and ride comfort is the actual loading, plus 50% of the remaining capacity. This also creates a substantial safety margin.

[NH2112]

I'd draw a chalk line across the tread and adjust pressure so it wears off evenly while driving, and use that as your starting point. Then adjust pressure (usually upward) to achieve the highest mileage. As DMaxMaverick said, nowadays tires are cheaper than fuel so sacrifice tire life to save money at the pump.

[JohnC]

My '93 came with a nifty chart for tire pressures. I've uploaded a scan of it to the gallery here (http://www.thedieselpageforums.com/photopost/data/2/medium/tp.jpg). Interesting, the gallery created 2 files, one smaller than the original and one larger. The smaller one is illegible and the bigger one is, well, bigger. Don't see how the larger one could be any better than the original... Click on the small one to see the readable one. (Can't figure out how to link to the bigger one).

[MTTwister]

For Radial - used as Singles

PSI -> 36 44 51 58 65 73 80
Vs Load \/
LT245/75/16 E 1742 1987 2207 2428 2623 2847 3042

(The 265/75 was a Range C tire in the chart. )
Don't think I c[FONT=Times New Roman][SIZE=3]ould bring myself to run a tire ( E ) at 36 or 44#

[afgunn]

MTTwister,
I agree with you on the E load tires. I have Michelin E load rating tires on the front of the vehicle in the sig and tried running 40 and 45 psi but, the tires were just too soft and I have had to go to 50 psi as a minimum - I may have to go even higher. I could feel the front end "washing" over bumps/curbs/dips when making a turn. And I could see too much of the tread on the edges showing contact with the pavement - scuffing. And I do not do any hauling or towing on my K1500 and very little highway driving. I certainly would not run less than 45 psi in these tires.

The chaulk mark is an excellent idea! You will be able to see quickly how the tire is making contact with the pavement and adjust TP accordingly. Usually, you need less air in the rear tires (unless hauling or towing) or the tires will wear out in the middle and the edges will still have good tread. Too little TP in the front will cause the edges of the tires to wear out and still have good tread in the middle. Regular rotation is a big help too unless you have a dually then, no rotation.

I base my tire pressure on how the tires wear or better how they are making contact with the pavement. If you have driven on the highway for 30 min or more and stop and see the outside edges of your rear tires have not been making contact with the pavement then, lower the TP in the rear tires. Usually on the front this will be the rated pressure on the sticker and/or up to 3 psi higher esp. on the front tires as they will wear on the edges quicker unless you put some extra air in them. I talked to a taxi company once that put the max pressure stamped on the tire plus 4 to 5 psi to keep the tires from wearing on the edges so bad. But, remember, they do almost all city driving and little highway driving.