Tyre sizes question

[chillyuk]

My Ridgeback hybrid has 700x38c fairly chunky treaded tyres fitted. I have purchased a pair of 700x35c less chunky treaded tyres to fit when I get round to it. Is the difference in size likely to be so small I wont feel a difference. I only really do road riding. To be honest I have never really understood the second number anyway. It can't be the width of the tyre because pumped up the wheel rims determines the width, so is it the height of the tyre from the rim to the top of the tread. As I am fairly chunky, some might even say a fat old git, would I actually be better with the larger tyres?

Years ago I went to the bike shop, bought a pair of 27x1 1/4, fitted and rode them. It's all so bloomin' complicated nowadays, with one wheel that seems to accept any tyre size from skinny to balloons

 

[youngoldbloke]

3 mm Difference - be amazed if you felt any difference - more likely feel difference due to tread. Second number is width - rim does not determine width. See: http://www.sheldonbrown.com/tire-sizing.html
Lots of elderley, rotund, persons ride on tyres much thinner than yours, so the choice is yours ......

 

[jimheaney57]

That Sheldon Brown link has me confused I thought I had tyre/rim sizes worked out until I read that