On my 2007 450r the front sprocket has 15 teeth how much top end in mph will I loose if i come down a tooth. ( A estimate will do) Also will it be good for me to do I want it for dune hill climbing to have a little more power going up.

 

You will lose top end but I can't say exactly how much.
I have a 2004 450R and it comes stock with 14/38 gearing.
For trail riding I drop the front to a 13 tooth. For dune riding I drop the front to a 13 and the rear to a 37 tooth.

 

I checked today and the front had been geared down to a 13 tooth so I'll consider loosing a tooth in the rear. Would you recommend it even if it's a little more expensive than the front?

 

Just so you know... DROPPING a tooth in the rear does the OPPOSITE as dropping a tooth in the front. To lower the gear ratio using the rear you want to ADD teeth.

Also the fact that there are a lot of teeth on the rear, one tooth isn't much of a difference in ratio as one added or removed from the front.

So...

Dropping teeth in front lowers the gear ratio.
Adding teeth in the front raises gear ratio.

Dropping teeth in the rear raises the gear ratio.
Adding teeth in the rear lowers the gear ratio.

 

Dropping one in the front and one in the rear at the same time lowers the gear ratio from the stock gearing, just not as much as dropping one from the front only.
It is a great combination for the 450R and many of us who own them run that gear ratio, especially in the sand.

 

06-07 450r's are 13/38 stock gearing. Your run a FCR carb, so just drop it down a tooth (12T) and that should take you were you wanna be.

 

In the one catalog they didn't make 12 tooth sprockets but I will look in to finding on.