We all know that more air pressure in the tires will improve fuel economy. Obviously for the hypermilers this is an easy thing to do that doesn't require benchmarking - you simply set the tire pressure as high as you feel safe doing so. I've seen wildly different claims however on how much tire pressure influences fuel economy, so this past weekend I used my ScanGauge and 97 Civic HX on 195/55-15 Yokohama ES100's to see what kind of results I would get.

The setup: The location is a 2-mile long road that bisects a farm. It's a public road, but very lightly used since all it does it connect two little-used north-south roads at a rather inconvenient place to make the crossover. There's no stop signs for the entire 2 miles, and according to my barometric sensor-equipped GPS, it's nearly 100% flat from the eastmost point to about the halfway point right at 1400' ASL. It then extremely gradually slopes down over the next mile to 1362' ASL according to my GPS. By my math, that works out to a 0.7% grade for that second half, so the west-bound fuel economy would be a little higher than the eastbound.

Since I had to leave room both for acceleration and deceleration on both ends, the measured section was about 1.3 miles long within about the middle of the 2 mile total stretch. There were no cars within 500’ ahead or behind me, so drafting currents were not an issue. Other than the small slope at one end of the road, it was about as close to a perfect test grounds as it gets, and the 48mph test speed should be a good compromise between aero drag, mechanical drag, and demonstrating effects of A/C at around-town speeds.

I actually did three tests. Starting hot tire pressure was 46psi. I made 3 runs in each direction with the air conditioning off, then 3 runs in each direction with it set to full. I then lowered the air pressures to 35psi, figuring that’s as low as you’d likely find anyone’s tire pressures that cares about fuel economy. I was getting very hot and tired of driving back and forth at this point so it was just 2 runs in each direction. A screencap of the complete results can be seen below:



Here’s what the full run averages were:

46psi tire pressure, A/C off: 59.7mpg
46psi tire pressure, A/C on: 48.7mpg
35psi tire pressure, A/C off: 57.6mpg

The results definitely surprised me. For one thing, the A/C brought down my fuel economy by far more than I had expected. Driving with the A/C off represents a 22% improvement in my car! Tire pressure on the other hand was much less of an improvement than I had hoped. 46psi was only a 3.6% improvement over 35psi, but on the upside, I honestly can’t tell the difference between those tire pressures on smooth Arizona roads anyway, so I guess that’s a free boost without sacrificing driver comfort to me. So the question of the day is:

Has anyone else attempted controlled tests like this either for A/C use or tire pressures? I’m mainly interested in tire pressure results since I’m thinking that I’d see a greater benefit if I actually had a lower grip tire. I’m using Yokohama ES100’s in 195/55-15, which is a larger width than stock for my car, plus it’s classified as a “summer” high performance tire, rather than an all-season.

 

Hi AZCivic:

___Krousdb posted something similar a few weeks ago in one of the hybrid forums?

2004 Prius II - (Not sure if these were his old or new GY Integrity's?)

62/62 PSI. 98.8 mpg at 77F.

44/42 PSI. 97.4 mpg at 82F.

35/33 PSI. 96.0 mpg at 86F.

___Good Luck

___Wayne R. Gerdes
___Waynegerdes@earthlink.net

 

I hadn't done any AC or pressure tests but back in early '04 there was alot of confusion and contradictions of which was better in an HCH between full throttle, moderate and slow accellerations.
You know, regarding the full throttle accellerations to avoid "pumping losses" promoted at Insight Central.
I didn't know either way as a new hybrid owner so I did a series of similar tests as yours.
Back & forth over the same road.

The results might be still here in the GH archive but it determined that slow, gradual accelleration netted twice the FE as full throttle, and seemed pretty linear inbetween.

AC- I'm still holding out even though its hotter than have been in years.
I bring my 6-pack cooler with ice in water, and a dabbing cloth.
May be crazy but I'm bound and determined to break my 900mile tank barrier this summer. So far I've been able to keep my wife out of it and at 1/2 tank it looks promising.

 

Well it's good to know my testing seems to be consistent with the testing of others. Hot_Georgia, I did accelleration testing as well, although mine was mainly designed around the idea of shifting at a fixed RPM and trying different throttle positions. You can't really "force" the CVT to let you give full throttle but limit engine speeds to 2500rpm, so it makes sense that you saw dramatically different results. Mine were pretty much inconclusive. I couldn't get enough repeatability to call it a valid test, unfortunately, hence only publishing the steady speed results above. Maybe the next-gen HX will come with an FCD? One can wish.

 

These results were with new OEM Goodyear Integretys. Results with the worn out tires was 4 - 5 MPG better. But I cant tell for sure due to the display limitation.

 

Were the worn tires the same Goodyear Intergritys as the new tires?
--Walter

 

Well in the world of amateur road racing, a lot of guys will buy the cheap race tires and have them shaved before even taking them to the race track. The reason being if the tire only has 4/32" of tread instead of 8/32" of tread, the treadblocks don't "squirm" as much during cornering and they're generally a lot more predictable. I can only guess that brand new tires with full treadblocks move around a lot more and thus generate more friction than very worn tires with extremely short treadblocks. It would make sense to me that a very worn tire could have lower rolling resistance than a brand new one.

Uh oh... could this be the start of a new trend? Hypermilers having their tires shaved down to half-depth before even having them mounted up?