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I have a 1980 Suzuki GS550L and it's geared for 55mph and supposed to get about 55mpg but the speed limit now being 70 it no longer gets 55mpg it now gets about 42mpg any suggestions on how to fix this dilemma?
 

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Could be it needs a tuneup. How's the air filter? Plugs? Tire pressure? Wheel bearings? Brake calipers? ...
 

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but the speed limit now being 70 it no longer gets 55mpg it now gets about 42mpg any suggestions on how to fix this dilemma?
You can not "gear out" the extra wind resistence. :sad:

A small shield might help; might not too.
Since that bike was built, a lot of gas has ethanol, which by itself will drop the mileage 4-6%.
A little loss of compression won't help either.

If you have a tach, what speed is the engine turning at, say 60 mph ??

Unless somebody comes along with that exact model who has already done some "dirty work", experimenting with different drive ratios will likely be a long and expensive process.

About the only thing that I would recommend is dropping one tooth on the front but that might make the engine run too fast and you might not like that.
 

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Your most cost effective change would be to swap the rear sprocket to lower engine RPM for a given speed, assuming anyone still makes rear sprockets for this bike in something other than the OEM size. But you are not going to see more than a slight increase in MPG, and you will loose acceleration around town. 42 MPG is pretty good. The 55 figure was when the bike was new 30 years ago, and the test used back then was hopelessly optimistic and performed under unrealistic conditions, so I guarantee no one was actually getting 55 MPG in the real world even when the bike was new and in perfect shape.
 

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I have a 1980 Suzuki GS550L and it's geared for 55mph and supposed to get about 55mpg but the speed limit now being 70 it no longer gets 55mpg it now gets about 42mpg any suggestions on how to fix this dilemma?
Watch the fuel you are using.
Ethanol, Oxygenated, etc reduces energy and reduces mpg's
 
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