2 questions.

[tyb525]

That doesn't make any sense and I didn't say anything close to that. That last part of what you said is just what I said. The middle number is a percent of the width which determines the height, but it doesn't actually tell us the height of the tire.

Yes the rim size does affect the height of the tire, but not the width of the tread. A 155/80/13 would be just as wide as a 155/80/15, but obviously the second one would be taller because the rim is bigger.

Check if the sizes compare for yourself: http://www.miata.net/garage/tirecalc.html

Again, if you only know the width of the tire, and the rim size, you can't determine the diameter of the tire without knowing what the aspect ratio is.

From UNECE: "Tyre size designation
－Section width、Aspect ratio、（Const. code）、Nominal rim dia."
http://www.unece.org/fileadmin/DAM/trans/doc/2007/wp29grrf/TYREgtr-02-08e.pdf

The 2nd paragraph in this article explains it all, again: http://www.gglotus.org/ggtech/tire-DOTnotes/tirerat.htm


http://en.wikipedia.org/wiki/Tire_code

[air-cooled or diesel]

srry 4 the misunderstanding, wasnt posted toward you, hopefully that will clear up that

[theman53]

155's are profile of 80%, (take a tape measure)

even on a 15" wheel?

never heard of a 155/40/15

isnt that nice of you
math shouldnt lie, in fact a 155/40/15 would seem to be the same total measurement as a 155/80/13, may be so. you can see if profile is an 80 though. a 40 will be distinctly shorter.

Please use a better description. This hurts trying to read it. I cannot tell what you are saying and if you were taking another shot or not after I told you not to. Please re read it and make it start making sense.

[ORCoaster]

I used the Miata calculator to try and get to the RPM's noted in the post that I was replying to.  That is how I came up with the 155/40/15 numbers.  But like I said I wasn't sure that was the correct size in the first place.  I was trying to figure it out with the wheel calculator as one method. 

As for 40 profile tires in a 15 inch wheel?  I think they are out there.  For sure the 16's.  But who runs these things.  Or maybe the better question is where?  Show cars on dollies and trailers that never touch the real streets I have to dodge potholes on daily?  The road maintenance in some towns is so poor these days I shake in the Rabbit with either my 155/80/13's or the 165/70/14's on. 

And we still don't know the middle profile number yet do we? 

[tyb525]

Well I googled 155/40/15 and it doesn't appear that anyone makes them, I think those are too narrow. Now they probably make them that low-profile in wider 15" tires.

I worked at a tire shop for 2 years (best-one tire) and saw lots of low-profile tires mostly 16's and bigger, but never anything like 155/40/15's.

Anyways if we could get the guy to take a look at his sidewall then we would know.

[rabbit_tdi]

A long time ago it was common for metric tires not to list the aspect ratio on the tire, such as : 155R15, 155R13, &c.  As far as I can tell, they were/are an 80 series tire, just were not marked (I have some in 155R12, very old, on a '77 Honda Civic).

If you would like to by some _new_ 155R15 tires, Coker Tire Company in Texas, USA has them for sale...

[R.O.R-2.0]

155's are profile of 80%, (take a tape measure)

even on a 15" wheel?

never heard of a 155/40/15

isnt that nice of you
math shouldnt lie, in fact a 155/40/15 would seem to be the same total measurement as a 155/80/13, may be so. you can see if profile is an 80 though. a 40 will be distinctly shorter.

Please use a better description. This hurts trying to read it. I cannot tell what you are saying and if you were taking another shot or not after I told you not to. Please re read it and make it start making sense.

yea, that was another shot, no question..

he would never say that i was being nice, and mean it..

they DO NOT make 155/40/15 tires..

[ORCoaster]

I have to sit here laughing as the 144/40/15 is totally fictional.  It was the only way I could get to the OP RPMs given the limitedinformation at the time.  My question later was do they even make them?  Apparently not, I even went looking.  So give it up. the guy probably just didn't read the 13 correctly and called it a 15.  Or like it may be that the tires are really old and he is driving on dangerous rubber.  Perhaps that is why we are not hearing from him as to the magic middle number.

ROFL

[jaed_43725]

I will look into the ACN trans, and the 3.67. Might be able to get a free 3.67 swap, as I am sure some gas guy would love my trans.

Also I want my alternator to start charging right off the bat because I hate having to wait 30 seconds for the oil to circulate and then blip the throttle to 2K. I would prefer charge at idle so when I turn on my HID's and such I am not draining the battery like crazy.

So if someone can help me with that, that would be awesome.

[TylerDurden]

You might put a smaller pulley on the alt.

[ORCoaster]

Jaed,  I have an amp gauge on my 81 and I have that sissy alternator that cranks out 55 amps it think.  When I hit the glow plugs and start it up my gauge pops up to 30 amps charging when the light goes off.  I don't wait 30 seconds either as giving the engine a bit of throttle to start seems to get it going better in the first place.  I also have an oil gauge off the top of the head and I can watch it climb all the while the engine warms up as I sit there for like you say.  30 seconds. 

If I pull the lights on at idle the amp gauge jumps a bit to say 45 but comes back to near zero after say a minute.  A well charged battery will allow that.  A battery that is going out or charges some and goes dead quick will always seem to pull on the alternator. 

So i wouldn't worry about the charge at idle if your battery is good and I wouldn't hesitate to give a bit of fuel at start up to make the alternator turn on ASAP.  Not from what I see going on with my monitoring equipment.  And if you have ever been inside the engine after it has set a while there is still oil all over the place in there.  The crank lobes meet the shims and there is generally oil waiting there before the pump starts delivery.  If you are worried about dry metal parts I would look into some Lucas oil treatment.      That seems to let it hang better than oil alone.

[8v-of-fury]

Yes, the oil clings to everything.. So unless it has sat 3 years there is not really any harm in giving it a blip immediately. Some diesels (big 8 wheel John Deer tractors) start and immediately rev to full throttle for a second to prime everything good and ready. There is no harm revving up the engine for a quick second.

[jaed_43725]

OK. So my alternator is dead. I am putting a new one on and hopefully all the issues I had with it not charging until 2K will go away. If not I will ask a vehicle electric pro to check it out.

[tyb525]

My alt is fine and the batt light doesn't go off till I blip it to around 2k. In fact it's a GM alt.

[R.O.R-2.0]

OK. So my alternator is dead. I am putting a new one on and hopefully all the issues I had with it not charging until 2K will go away. If not I will ask a vehicle electric pro to check it out.

having to blip the throttle on a VW to get it charging is perfectly normal..

if you dont want to blip the throttle every time, then swap your charging system from a V-belt alternator to a 90a or 120a serpentine alternator, and appropriate brackets and pulleys to make it work..

my 86 GTI is the only VW i own that does not take the initial blip of throttle to excite the alt, but thats because it has a 120a TDI alt, and serpentine belt drive setup..

some take less throttle to get the alt working, but ALL v-belt alternators take a throttle blip.. its a way of life, much like death and paying taxes..