Quote:
Originally Posted by mikeyjd
P195/75R14...814 rev's per mile
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Now you're venturing into my area of expertise.
First, revs per mile is correctly based on measured tire circumference, not the measured radius when loaded. So if a 195/70R14 measures 80.12" new circ, then the actual r/mile is around 790.9.
Don't use the advertised r/mile. Distance travelled doesn't change based on load. It does change based on wear. But to avoid speeding tickets, use new radius, which is easier to measure unloaded, using a seamstresses tape measure made of cloth, not metal.
Next, calculating RPM: The calculations are for manuals, or automatics if the torque converter has a lockup feature and is locked.
A slightly taller tire with slightly more circumference is like a slightly taller (smaller number ) gear ratio, it gives less engine RPM per MPH.
If you start at 195/75R14, then try 205/75R14, you won't see any change on your gauges, unless you recalibrate the speedometer. Factory tachometers aren't very accurate, but they seem consistent.
But going from 195/75R14 to 205/75R14 will have you risking a speeding ticket, will increase your rolling drag, and your aero drag unless you lower your suspension to compensate, may help you get away with carrying slightly more weight, and will be less stable at the limit unless you get wider wheels.
In this case, for 195/75R14s, there are load range D options available, if that's of interest. The extra plies means a harsher ride, but they really do offer a little more stability and a little more resistance to punctures.
I see this whole tire experimentation business as jumping over dollars to pick up pennies. I'm not willing to give up any of the vehicles' other abilities, from load carrying to cornering grip to braking power, just in the feeble hope of maybe 1 MPG at best.
Oh, and to the flirting with max load rating, I know a guy with a '94 Dodge Ram 2500 pickup truck. It has single rear wheels, not a dually. It is a Cummins, so it has the Dana 80 rear axle, which is rated to carry a vertical dead weight load of 11,000 pounds. But his tires are only rated to 3,042# each.
Even so, every month he loads his gooseneck trailer so much that the rear of the pickup squats to the bumpstops, some 10" down from no trailer. I kid you not, I exaggerate not. Then he loads it some more.
The truck is 4WD, and that means even more front end weight. Even so, I've seen the front tires come off the ground when he tries to get moving from a dead stop.
And so loaded, he's well over 6100# on the rear axle of this pickup. He hauls this way, trips to the next valley over, to buy feed for his cows, tractors, et cetera. He's been getting away with this for years, with never an accident, never a blowout even on cheap Chinese tires. He runs them at 80 psi, because that's what the tire sidewalls call for.
This amazed me, and I've decided tires are under-rated the same as most other truck parts are. I've seen 85-MPH-rated tures not fail at a sustained 120 MPH. I've seen Drifters get away with 90 psi, even shredding the tires, but no blowouts, on tires rated to half that much air pressure.
Exceeding ratings opens you up to liability issues, but if you're careful, you can get away with an awful lot.
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