how much does weight affect times?

Well yes....and no.

First off, taken to the extreme you are looking at a 6 foot 58lber taking the world record in the mile down to 2:34.57. You need to have a certain amount of muscle, body fat, and of course blood volume (that and vital body organs and such) to run period, let alone run fast.

I think overall weight is often confused with race weight. Everyone has an ideal race weight and an ideal training weight which would be a few pounds more. I have had the opportunity to pose this question to some current greats such as Matt Downin and Meb Keflezighi and the answers were rather scarey. Downin felt as though a few pounds difference could cause a noticible affect in his performance. Wetmore is quoted in Running With the Buffaloes as saying that 'leanness is underrated'...and that 'distance runners should look like skeletons with condoms pulled over their heads.' Again, pretty scarey.

When your body fat goes down, your blood volume muscle ratio goes up, allowing your muscles to have easier access to fuel and oxygen. The danger is in getting too thin for too long and starving your body and breaking down, and for women especially, eating disorders.

The leaner the meaner.

I don't have any formulas to offer. Only some observations.

From a physics standpoint one has to consider the power/weight ratio. More power pulling less weight equals a faster runner. Look at Geb and KK, those guys are not emaciated at all, but they look very strong for their low weights. Joe Falcon was very strong for his weight, I've heard that at 115 lbs., he could bench press over 200 (not that bench pressing is directly correlated to running, but it's just illustrative of his strength.

So, if you're a tall runner (like me) you have to find the level where you are muscularly as strong as you can be at the lightest possible weight. I think Paul McMullen had to lose muscle tissue before he got down to his optimal racing weight because he's a mesomorph (mostly muscle). The Kenyans look skinny and weak, but they have really strong, long legs, pulling minimal weight (I'm not sure how stride length affects the equation). The North Africans and Ethiopians seem to be more like the Joe Falcon model, a short, light runner who's rather strong. Pre seemed to fit this model (he was always a little overweight later in his career).

So how were Ryun, Ovett, and the tall Anglo guys so fast? Kardong and Bachelor???? Shorter was REALLY skinny when he was in top form (he had to be b/c he was tall).

Just observations and speculation.

If a runner is at ideal weight and puts on a couple of extra pounds, it won't slow him down at all. But if a runner is ten pounds over ideal weight, then that will be a factor. It's not going to slow his or her mile down by 10 seconds, though. It might slow it down by three to four seconds over a mile - and about 25 - 30 seconds over 10k.

Females are in danger, however, if they take the low weight business to the extreme - it will mess them up. Most don't, thank goodness, but those who do usually "get away" with it for a few years. Their times improve dramtically, which tends to reinforce the behavior. Eventually the bill comes due and there's a lot of physical and psychological problems left to contend with.

Moral of the story: get light, but don't go to extremes. Protect your health.

Liquori said that he usually raced at 144, but his best times (13:15 5000) came when he dropped to 139-140. I believe he was 5'11". I read this in his book, "Real Running" - not a bad read. You can buy it used at Amazon.