Slippery Slope

[Don Jorgensen]

So just before we took the Tercel on the 2,500 mile trek, I changed the tranaxel and rear differential over from 89-90 gear lube to Mobile 1 synthetic. First drained the units, cleaned the magnetic plug ends and flushed/cleaned with cheap ATF then drained and filled with the synthetic.
Now I had no way of comparing the improvements to mileage or much of anything for that matter until I did my coasting route down Academy Boulevard. Normally the stop light catches me a Wyoming so when I get green I accelerate to 35 mph and put him in neutral. I have a mile of downhill that only has 1 stop light that I usually cruise through. Now with the 80-90 gear lube, the Tercel would slowly gain speed and top out about 42-45 mph, just the speed limit. This would be the case in summer and in winter with the gear lube viscosity restriction, he would get to 38-39 on this stretch coasting.
Lo and behold, Now he zips right past 45 mph and I actually have to feather the brakes to stay under the speed limit. I know this is not very scientific. I don't care.

[Petros]

That is a good simple test. If you know the slope angle and the test weight of the car, you can calculate total drag of the car, and how many pounds reduction in drag (and with speed, you can than calculator hp required).

I used to do cost-down tests. you find a level stretch of hwy, and measure time it takes to cost from say 70 to 60, or 60 to 50, etc. from there you get the average drag at each speed. And reduce it to hp or lbs drag. To be real accurate you need to average many tests from both directions, it takes a lot of time. but on long cross country trips I would use the stop watch function in a phone or watch, and do the calculations in my head to break the boredom on long flat crossings.

the lower the drag, from all sources, the better the fuel economy at any give speed. And the higher the top speed too, but that would be illegal for most of us.

[takza]

Here's a coast down test on tire pressures:

http://www.metrompg.com/posts/tire-pres ... stance.htm


Moral of the story...no point in going over the sidewall max.

[Ron B.]

...no point in going over the sidewall max.

I wonder what the sidewall max was on the tires of this car. The last line of the advertising copy reads, "Just run the tires up to 70 pounds and hang on"! http://tinyurl.com/lcfgjx

[4x4wagon]

According to my father (66) you're suppose to go by the sidewall pressure because "the tires have a different thickness than the ones that the car came with, so you follow the sidewall and not car for the correct pressure" when asked what to do if you have one tire that says 44psi, and another on the same axle with 35psi on the sidewall, he just walks away and ask why I "have to argue with everything he says"


I believe the recommended vehicle pressure is there because the maufacturer has calculated tire size vs vehicle weight to determan what the tire pressure should be for even wear of the tire tread.


kindof off topic I know, just throwing it out there.

[Ron B.]

According to my father (66) you're suppose to go by the sidewall pressure because "the tires have a different thickness than the ones that the car came with, so you follow the sidewall and not car for the correct pressure" when asked what to do if you have one tire that says 44psi, and another on the same axle with 35psi on the sidewall, he just walks away and ask why I "have to argue with everything he says"


I believe the recommended vehicle pressure is there because the maufacturer has calculated tire size vs vehicle weight to determan what the tire pressure should be for even wear of the tire tread.


kindof off topic I know, just throwing it out there.

Well, I'm only 64 years old but I know that you shouldn't inflate your tires to the maximum PSI numbers listed on the sidewalls. They are the maximum pressure at which a tire can safely be inflated before it is possibly damaged or worse....you may be damaged when it explodes in your face! The following is a quote from the Tire Tech section of the Tire Rack website:

"The pounds per square inch (psi) pressure number branded on the tire's sidewall identifies the maximum cold inflation pressure that specific tire is rated to hold. However, the tire's maximum pressure is not necessarily the correct pressure for every vehicle upon which the tire can be used (almost all vehicle manufacturers' recommended tire inflation pressures are less than the tires' maximum pressure).

Therefore when checking and adjusting tire inflation pressures, the "right" inflation pressures are those provided by the vehicle manufacturer, not the "maximum" inflation pressure branded on the tire's sidewall. "

There is an enormous amount of useful information just waiting for you at: http://www.tirerack.com/about/techcenter.jsp

[4x4wagon]

Oh I personally know all this. I was just stating what my father says when inflating a tire. Also, I had to teach my friend how to put air in a tire. The case with his '02 Taurus, one front tire had 64psi, another had 60. "but it still looks low" Luckily I was able to get to the car before he caused too bad of damage.

[Ron B.]

Oh I personally know all this.....

You know all of the information tucked away in all of those Tech Tips? I'm impressed.

[4x4wagon]

You know all of the information tucked away in all of those Tech Tips? I'm impressed.

Ok so maybe I don't know everything in all those tips. I was reffering to what you had posted.

[hberdan]

OK, these old wagons have a recommended tire pressure of what? 26 psi, iirc, as noted on the sticker inside the glove box. But I'm not running my modern tires on those numbers, that's plain silly.
When I did buy 4 new Bridgestones 175/70R13s at Costco a few years back they absolutely refused to inflate the tires higher than that, and they said they would not inflate higher than vehicle recommendations! I drove the car with underinflated 26 psi steel belted radials as it swayed and swung and wallowed thru intersections to the nearest gas station and ran em up to 32 psi; normal as I understand it, and after a while I settled on 35 psi as a good number overall. Good mileage and handling, no feeling of under or over inflation. In fact, I run all my summer tires in that range, and always have.
So, on these old Tercels I think you need to disregard the manufacturer's recommendations...and use common sense.
I run my snow tires at 32 psi, or even a little more, which once again is above the t.p. recommendations on the glove box.

[takza]

I generally run all my tires near to or at the sidewall max....for mpg reasons and for good handling. I back off some if the ride gets too stiff.

Truth about radial tires is that different tires and different brands are made differently...and it's more of an safety issue if you run them low than if you have them near the sidewall max. My guess is that most radial tires are actually somewhere between radial and bias ply....and there are variations...plus variations in tread compound.

The higher the tire's series...as in 80 or 70...the more effective a higher tire pressure is.

What's in the manual is for the OEM tires and for comfort...not for another brand and best handling.


Chalk test (someone's post):

"I put a chalk mark on each tire, from the edge of the tread/sidewall transition to just past the edge of the "sidewall tread" as I call it, and slam the thing through a few hard corners. If any of the chalk marks get scrubbed off more than halfway down the "sidewall tread", then I add 5psi to both tires on that end (front or rear). The added pressure seems to keep the tire from "rolling over" in turns and improves handling. Yeah, I know. It's a pain. Sure feels good when I'm done though."

[4x4wagon]

maybe we should all just get these, and cal it a day.

[takza]

So just before we took the Tercel on the 2,500 mile trek, I changed the tranaxel and rear differential over from 89-90 gear lube to Mobile 1 synthetic. First drained the units, cleaned the magnetic plug ends and flushed/cleaned with cheap ATF then drained and filled with the synthetic.
Now I had no way of comparing the improvements to mileage or much of anything for that matter until I did my coasting route down Academy Boulevard. Normally the stop light catches me a Wyoming so when I get green I accelerate to 35 mph and put him in neutral. I have a mile of downhill that only has 1 stop light that I usually cruise through. Now with the 80-90 gear lube, the Tercel would slowly gain speed and top out about 42-45 mph, just the speed limit. This would be the case in summer and in winter with the gear lube viscosity restriction, he would get to 38-39 on this stretch coasting.
Lo and behold, Now he zips right past 45 mph and I actually have to feather the brakes to stay under the speed limit. I know this is not very scientific. I don't care.

Interesting that you documented a difference...I might just spring for synth gear oil next time. I'd also add this stuff:

http://www.lubegard.com/automotive/gear_fluid_supp.html

Sold at NAPA.

[Don Jorgensen]

Okay so this thread changed from gear lube to tire pressure.
Before leaving on the trip, I aired up the tires to 34#s all around. The tirs actually have 36#s maximum printed on the sidewall and 1,235 lbs. carrying capacitiy. I tried to keep our luggage and stuff under a ton.
Going from about 6,000 feet altitude down to 600 put the tires to about 26 or 27 when I checked them in Eminence, MO. I also noted that the Pringles cans of chips had deflated so the plastic reseal lids would stay on.
Also aired the tires when they were cold, now that is relative because I suspect the drive way and the tires themselves were way above 90 degrees at that time here in ABQ.

[takza]

That also works the other way...going from lower elevations into the mountains...you need to drop the pressure some...espec if driving thru cuts on rock shards...as I found out.