Tire Inflation Field Report

Ok Folks;

I do a 160 mile round-trip daily commute, on mostly flat terrain, on a wide-open 4-lane highway with very little traffic (relatively speaking). I bought my used 2000-model insight with high hopes for high mpg. My daily commute was perfect for coaxing high MPG out of an Insight.

However, I was having a great deal of trouble getting the car to achieve a reasonable mpg number. I couldn't get anywhere near 65mph and 70mpg. In fact, I usually had to slow down to 55-59mph to get above 62mpg. I found insightcentral and only got depressed at seeing the mpg numbers others were achieving. My manual transmission car's lifetime mpg rating is only 55.7mpg: I worried that somehow I had bought a lemon. This went on for a few weeks and I was very disappointed.

I had some things done to the car that helped quite a bit. About a month ago, I had a brake problem fixed and that bought me about 8 mpg. But that's when I was getting way down into the 50mpg range, and that boosted me to about 56-58mpg at 60-65mph. Better, but still not what I had hoped for. "65 at 65" was my goal.

When I saw the discussion about tire pressures here on insightcentral, I thought you guys were loony tunes. :wink: 50psi in a 42-psi rated tire? I expected to hear tales of tire explosion, but after watching and reading for a few weeks, all I heard was good stuff back from the "50psi crowd".

What the discussion about tire pressure DID do was make me focus on the tires, and what I found was sobering: every one of my tires had no more than 32psi in it: they had been set at the last service to that by someone who didn't know these were higher pressure tires. I grabbed one of those little "stick pressure meters" and decided to inflate the tires on up to nameplate rating, 42psi or so.

The difference in the mpg and speed I could achieve was clear: I picked up another 5-6mpg, but more importantly, I picked that improvement up UNIFORMLY: I could ease on up to 60mph, and still get 60mpg: My goal of "65 at 65" was within shooting distance!

This was a revelation: the overinflation of the tire made a tremendous difference in mpg, and allowed me to keep that mpg up at a higher speed. I enjoyed the increase for a tankful (about a week) but by the end of the second tankful, (into the second week) my mpg achievements were gone. In fact, I had another problem: I couldn't seem to keep my battery charged up: I was now depleting it during small amounts of stop-and-go city driving, and it was NOT getting charged up on the long highway drive home. More importantly, my mpg was dropping way BELOW 50mpg at 60-65mph. It's a scary thing to see 48mpg, a depleted battery AND bad performance... :shock:

My first thought was that my battery pack was dying or something. But then it occurred to me that it had been two weeks since I had checked the tires: my experience from cycling tought me that high pressure tires do lose air faster, and I hadn't checked. Worse yet, when I did check the tires last, I used one of the cheap, inaccurate "stick" gauges. I knew better than this and got my nice dial pressure gauge that would hold the pressure reading after you took it so you could bring the gauge face up into the light and see it clearer.

So: I had to get gas yesterday, and I'd just found a gas station with reasonable prices ($1.53/gal here in central NC) and free air (an extreme rarity these days). I filled up and moved over to the air hose, and this time I had my accurate dial pressure gauge with me. What I found stunned me: three of the tires had only dropped down to about 38psi (warm) from the 44 (cold) I had set them to. A fourth tire had dropped down to 23psi warm! What worried me most is that it was possible the tire had never reached the 44psi I tried for, because of the inaccuracy of the "stick" gauge.

So, anyway, I topped off all the tires, and this time I "went for broke": put them all at a solid 50psi, as read by the accurate gauge.

I expected the ride to get very noisy and uncomfortable: it was no worse than 40-44psi. I did find myself having to be more careful going over railroad tracks or with unavoidable bumps and potholes. A tad bit of an increase in road noise, but nothing significant. I was cautiously pleased.

Then I got out on the highway for the drive home, and the ride of the car was completely different! At 50psi (as measured by the accurate gauge) the car would kind-of "glide" along on the highway. Effects from drafting or "surfing" were much more noticeable on the instantaneous mpg bargraph. I had set one of my tripmeters to record only the highway mileage: this was to be the litmus test: what would happen on the 73miles of highway between me and home?

Arriving home that night, I saw what I didn't think possible: I averaged 65mph and managed 70.1mpg. I did a "little" bit of drafting and surfing, but not a huge amount. I had a tailwind part of the way, but not very much. So the results seemed good and not a fluke of wind or traffic. More importantly, my battery charged up nicely, even though I required some assists on some of the hills going home. I was amazed at the difference.
So, here are my preliminary conclusions:

1. 44psi is a good place to put your tires, but 50psi is even better. I've had no trouble with that in the tires, save for a harder ride over railroad tracks and potholes. I just slow down for those objects and it helps.
2. Check your air pressure frequently, at least once a week.
3. You can have only one tire lose it's pressure and it'll make all the difference. I had only 1 tire go down and I thought my battery pack had failed!
4. An important trick to getting high mileage in the Insight is "proper tire air pressure management".
5. Finding a gas station with free air is worth paying a few extra cents per gallon because it costs 50cents most places and the air compressor there won't run long enough to get to all four tires.
6. Buy a nice dial reading air pressure gauge, one with a pressure release button that lets the meter "hold" the pressure reading after you take it off the tire.
7. Check that the tire valves are tightly screwed down in the valve stems. There's a tool you can get at bike shops to do this with, just ask for something you can use to tighten "schrader-styler tube valves".
8. I wasn't actually sure I believed in this talk about "lean burn mode", but getting the tires up to 44psi, and then on to 50psi, helped me see this on the mpg bargraph. I don't know if the Honda engineers are laughing at us or not, ;-) but I can definitely tell when the car goes into NOX purge mode, and I can see a difference in how the car behaves when I let off on the gas and "feather" the pedal into the higher mpg range.
9. I'll wrap up my accolades with a WARNING: at the much higher tire pressures, the car does not handle well on road seams. And I mean ANY road seam or imperfection. In fact, it wants to "jump off" the road if you hit a seam at the outside white line. So my only warning from "50psi land" is to always "drive the car": don't tool along using only one wheel, don't talk on the cellphone, and pay attention to the road. But then, all Insight drivers know they have to drive this way anyway, right?


Well, I apologize for my verbose message, but I'm just putting out information with more detail, something I hope is of help to those of you out there who, like me, agonize about pushing the tires beyond "nameplate pressure" and who also know that when the tires heat up, you get another 2-4psi, added reason for concern. I went on up to 50psi and it really helped on the highway to reach the mileage numbers I needed. More importantly, I found that when one or more tires drops a lot of pressure, that can look like a malfunction elsewhere in the car. So get a good tire gauge and watch that pressure, at least once a week.

JKB