Dropping ?two teeth-what difference will it make

[Guest]

How much difference will dropping two teeth off the rear sprocket make?
I will be replacing the chain and sprockets, and want to take the opportunity
to get a lower speed on the top end for long trips.

I already have an OEM 15 tooth front, and am currently running a 42 tooth rear sprocket. I don't want to change it too radically. Will I need to lengthen or shorten the chain if I go to a 40 tooth rear?

Worried more about being too short or having clearance problems than the gearing.

Bike is a 78 GS1000C. 630 x96 chain

Thank all you gurus for your help!

[JEEPRUSTY]

http://www.bikerenews.com/Stories_Archives/GearRpmSpeedCalculator.htm

looks like a good resource

[sharpy]

I have a 40 tooth on the rear and its great. Sits at just over 4000 at 60mph compare to 4500 revs. BUT my bike is modded and its ok for pulling in first gear from stop.

[koolaid_kid]

If you changed the rear, you would probably have to shorten the chain (depends on how much it already stretched). If you changed the front, it would not only be cheaper, you shouldn't have to change the length of the chain.
I went from a 15 to a 16 front, and I like it a lot better on the highway.

[MAC10]

Thanks everyone.

Next time I need a new chain I will go with changing the front sprocket over th rear, and do a 530 conversion. This time I will use up the OEM 15 tooth front I already have.

500 rpm is a significant rpm savings at speed, for only one tooth difference.That is probably what I will choose.

[sharpy]

Opps my bad, its a 40 tooth sprocket. sorry. edited my previous post aswell

[MAC10]

Thanks for counting Sharpy. Much appreciated.