One of the biggest problems I found when I was diagnosed was that it was hard to find out what the "normal" blood glucose (BG) levels are. Actually, at first I didn't even measure my BG as I received the standard advice for a type 2 diabetic who has not yet been prescribed any drugs: eat lots of complex carbohydrates, little fat and don't get a BG meter. For a couple of weeks I followed that advice but I also did my own investigation, starting with Gretchen Becker's excellent book, "Type 2 Diabetes: The First Year". This changed my mind about the dietary advice I had been given and made me decide to buy a meter. I'm glad I did as my fasting blood glucose (FBG) was not improved since diagnosis, still being 8.0mmol/l (144mg/dL) or more.
So what are normal levels? To be honest, I still wonder because there are so many different answers to a seemingly simple question.
Diabetes UK say that before meals normal BG is between 3.5 and 5.5mmol/l (63 to 99mg/dL) and below 8.0mmol/l (144mg/dL) two hours after eating. On the same page they give more relaxed numbers for diabetics to aim for. I prefer to aim for the normal range at all times as I believe from my own research that this will lower the risk of diabetic complications to near normal levels. However, for diabetics that use insulin or oral BG-lowering medications there is logic in aiming higher to avoid hypoglycemia.
Diabetes.co.uk have slightly different "normal" levels. They suggest that before meals a normal person's BG is between 4.0 and 5.9mmol/l (72 to 106mg/dL), a little higher than Diabetes UK. Post-meal they are a little more stringent, suggesting 7.8mmol/l (140mg/dL). This lower two-hour post-meal number is interesting as some studies have shown that damage to the body starts, or increases greatly, when 7.8mmol/l is reached. However, reading down the linked page you see that for diabetics they still say to aim for values that are higher than these numbers.
If you take a look at the Wikipedia article, the normal blood ranges are shown as quite different from those of the above two diabetes organisations. The normal value is given as 4.0mmol/l (72mg/dL), though this fluctuates throughout the day. The article goes on to say that the normal test result before meals would be 3.9 to 7.2mmol/l (70 to 130 mg/dL). This is a substantially larger range than already mentioned. Wikipedia specifies that a post-meal reading of below 10mmol/l (180mg/dL) is normal. This is a large different indeed - I need to increase my carbohydrate intake by a substantial percentage to see the difference between 7.8 and 10mmol/l (and a 10 on my meter would make me very nervous indeed!)
Netdoctor says similar things to Wikipedia, as does the ADA (American Diabetes Association). These numbers seem to be the established answer given by medical practitioners.
Lower numbers are provided as targets for diabetics by the great web site, Blood Sugar 101. Here the suggestion is that 5.6mmol/l (100mg/dL) should be the maximum for fasting levels and the two-hour test should give a result of no more than 6.7mmol/l (120mg/dL). There's quite a bit of research to support these values and I heartily recommend that you look around the Blood Sugar 101 web site and read it.
All of this left me pretty confused about what targets I should aim for. If I follow my doctor's advice I will be happy with BG levels that would put me into the "damaging" range indicated by others. I decided to play safe. My fasting levels are what they are. Sometimes they are a little higher than 5.5mmol/l (99mg/dL), other times they are below. Post-meal is easier to control. I always aim to be below 7.0mmol/l (136mg/dL) one hour after eating and below 6.0mmol/l (108mg/dL) two hours after eating. Very occasionally I'll stray above those numbers by 0.1 or 0.2mmol/l. If I do, I find a ten-minute run or 30 minute walk usually lowers my BG considerably.