my tyre pressure versus fuel economy test.

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i have a rental car, an 05 sentra "s" edition with an automatic trans and bridgestone potenza tyres. the car has about 37,000 on the clock. this car features a digital miles per gallon gauge that calculates my average miles per gallon untill i reset the gauge. for instance, if i reset the gauge and drive for 30 miles, it calculates the average mileage for the 30 miles i drove. i have been doing some testing on tire pressure versus fuel economy tonight on an empty highway road where there is no traffic and i can set the cruise control and watch the mpg gauge. the praticular road i travel on has a 10 mile strip of nothing but straight road. i kept my speed at exactly 40mph with overdrive on and the engine rpm's are just about exactly 1500 rpm's. i had no stops and no traffic to draft behind during my tests. no interference or obstructions at all. i used known land marks to have a rolling start at 40mph and noted the average mpg at another known landmark at the end of the stretch of road. first test was with the tyres at 30psi and a relatively cold car after the 20 mile round trip i averaged 38.6mpg. next i immidiately the same testover again with the exact same conditions, and averaged 40.4mpg. i attribute the increase in mileage due to the overall warming up of the car components like the verious oils greases and tires. finally i did the same test again but this time i inflated the tyres to 44psi, which is the max recomended pressure on the sidewall. other than that 1 change, everything else was identical. i averaged 46.1 mpg. i would like youre opinions, if you will.
 

oli

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What were the hot tire pressures after the first run? Did you measure after the run with 44psi to see if they stayed there? Ideally you should have taken one final run with 30 psi again to help rule out any other variables. What do you normally see for mpgs with this car? 44psi is not the maximum "recomended" pressure. It is the maximum pressure.
 

Master ACiD

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sence they dont allow exceding 44psi, i figure that it is the max recomended pressure. i think we are getting into semantics here, which isnt the point of this thread. i neglected to measure the tyre pressures after the 30psi runs to see how much the pressure rose. foolish of me i know but the thought excaped me at the time. i also neglected to measure the pressure after the 44psi run. again, i apologise for this. normally if i leave the mpg gauge alone and do not reset it for a good while i see around 35mpg with mixed city and highway driving of roughly 50-50. keep in mind i drive like an old man as this miles per gallon gauge facinates me and i get quite en enjoyment out of attaining the highest mpg possible. i really wish i had a perminant car with one of these gauges. ill miss this rental car when time comes to give it back. i think the car is rated for 24/28 by the epa. thats pretty easy to get if i drive like a typical gas-brake-gas-brake south florida driver.
 
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My previous set of tires had a max psi of 35, I kept it right at that and got good MPG, it would drop noticeably around 32psi. My current set has a max psi of 44, and I do not find any differnce bewteen 35 and 40 psi, MPG wise. I haven't tried stepping up to 44, but I may just to test it and see if there's a difference. I think the ride would be too harsh by then, though. Not only does it increase your MPG, but I like the better response and handling that high psi gives.
 
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When the tire is 'over inflated' the center of the tread will buldge out, thus resulting in a smaller contact patch with the road. I inflate my tires (placard 35) to 38 psi, but will not do too much over. That can lead to uneven tire wear, and loss of traction too!
 
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My Tracker is body on frame with somewhat stiff suspension. I have noticed a little more "stiffness" on big bumps. Most would probably think it rides rough, but compared to my '92 Wrangler trail Jeep, it rides like a Caddy.
 
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I increased the mileage in my 1998 Chevy Tracker 4door 3-4MPG by going from the placards 26psi up to the tire's maximum of 35psi. It's definitely an easy way to save $$$. [Happy]
 
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When you add a lot of pressure to the tire, it's diameter is increased, but the speedometer/odometer doesn't know this. You are travelling faster at the same engine RPMs. You are covering more miles than the odometer would indicate. This throws the readings off, but it would seem that the MPG increase is even greater than you listed.
 
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quote:
I increased the mileage in my 1998 Chevy Tracker 4door 3-4MPG by going from the placards 26psi up to the tire's maximum of 35psi.
Will your tires wear evenly with max tire pressure ?? [I dont know] Is it safe to drive 80 MPH down the highway for many hours with Max PSI tire pressure ?? [I dont know]
 
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quote:
Originally posted by mechtech: When you add a lot of pressure to the tire, it's diameter is increased, but the speedometer/odometer doesn't know this. You are travelling faster at the same engine RPMs. You are covering more miles than the odometer would indicate. This throws the readings off, but it would seem that the MPG increase is even greater than you listed.
Um, I have a problem with this statement. If you can measure this them you probably have some pretty serious equipment and a huge budget. Diameter really only increase or decrease significantly if you increase or decrease tire or rim size (0+ sizing). 3% difference is the standard before things really start affecting speedometer readings significantly.
 
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quote:
Originally posted by LT4 Vette:
quote:
I increased the mileage in my 1998 Chevy Tracker 4door 3-4MPG by going from the placards 26psi up to the tire's maximum of 35psi.
Will your tires wear evenly with max tire pressure ?? [I dont know] Is it safe to drive 80 MPH down the highway for many hours with Max PSI tire pressure ?? [I dont know]

Actually, it is recommended that you increase pressure (+3 PSI or so) if you drive at highway speeds for extended periods of time. Increased air pressure allows for better heat dissipation. The max pressure stamped on the sidewall deals with the tires ability to handle load, whether that load be cargo or driving (cornering, braking, acceleration)forces. All tires will gain pressure at speed but only up to a certain point before they equalize, usually up to 6 PSI or so. If you think about things, lower pressures allow the tire (and vehicle) to slip a little during cornering (slip angle). If you increase pressure in a vehicle with a higher center of gravity, you will be more likely to induce roll over as the tire will grip a little more before slipping. The pressures are also set to help offset over and under steer in vehicles. Higher rear pressures in a RWD vehicle will help reduce oversteer. Higher pressures in a FWD vehicle will help offset understeer. Higher pressures, combined with high speed for extended periods will results in the centers of the tires wearing out faster than the rest as the contact patch is greatly reduced at speed. There is no real simple answer as there are many variables involved.
 
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quote:
Originally posted by Hajoca: When the tire is 'over inflated' the center of the tread will buldge out, thus resulting in a smaller contact patch with the road. I inflate my tires (placard 35) to 38 psi, but will not do too much over. That can lead to uneven tire wear, and loss of traction too!
I was sorta worried about that with my Goodyear Integrity tires on my Corolla, but after 20,000 miles of 36PSI all around (vs 30PSI placard), the shoulders are still wearing faster than the center by a good amount. If I didn't know better, it would at first glance appear that they were chronically underinflated...
 
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quote:
Originally posted by GarrettSocling: I was sorta worried about that with my Goodyear Integrity tires on my Corolla, but after 20,000 miles of 36PSI all around (vs 30PSI placard), the shoulders are still wearing faster than the center by a good amount. If I didn't know better, it would at first glance appear that they were chronically underinflated...
Just for reference, this tires have been regulalrly rotated, right?...and all 4 look the same? If that's not true, you should be aware that front tires tend to wear the shoulders and rear tires tend to wear the centers, so not rotating could result in front tires that are worn on the shoulder even though they are inflated more than the placard. But assuming the tires have been regularly rotated and the wear is not on one shoulder (which would be an alignment problem), there are a couple of possibilities: 1) The rims could be wide and that would cause the tire to "dearch" and the footprint would be "flatter" than normal. 2) The vehicle has a lot of caster and that induces a lot of camber on turning. 3) There is a lot of "city" driving (lots of turns per mile), as opposed "country" driving (many miles between corners) 4) "Spirited" driving as opposed to "gentle" driving. As was said before: "There is no real simple answer as there are many variables involved." Just make sure you have enough inflation pressure. Load carrying is a primary consideration and overloaded tire failures can sometimes have tragic results.
 
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I've only rotated them twice in the past 20,000 miles, and now that you mention it one of the shoulders on each of the front is worn slightly more than the backs (slight alignment problem on each front recently?), didn't notice much difference regarding wear on the backs. Probably a combination of things, as you posted...the upside being I can run higher pressures in the front without worrying about 'adding' uneven wear?
 
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quote:
Originally posted by GarrettSocling: I've only rotated them twice in the past 20,000 miles.....
So if you rotated them now, that would be once every 7,000 miles, which is according to schedule.
quote:
..... and now that you mention it one of the shoulders on each of the front is worn slightly more than the backs (slight alignment problem on each front recently?).....
If you can "see" it, it's not a recent problem!
quote:
....didn't notice much difference regarding wear on the backs. Probably a combination of things, as you posted...the upside being I can run higher pressures in the front without worrying about 'adding' uneven wear?
IMHO, going higher in inflation pressure causes more improvement in overall wear rate than might be lost due to uneven wear!
 
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I would be concerned filling the tire to the max on the sidewall. Once under way the pressure will exceed the max by a few psi. Hypermilers run their tires up to 50 without a second thought and don't seem to have blowout problems so who knows? After a bit of experimentation I settled on 3 psi above manufacturer's recommendation. I was looking for a compromise between better handling and maintaining a decent ride.
 
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