Chain Sprocket ratio question

[Greg O.]

Would I gain any significant fuel mileage by changing sprockets to increase the drive ratio? It is now stock which I believe to be 15 tooth front sprocket and 41 tooth rear sprocket. I am looking to get a little better range on a tank of fuel. I now get about 44mpg if I keep the speed around 60. If I hit 70, it drops to about 34mpg. I think there is a 16 tooth available for the front, and a 40 tooth available for the rear. The bike is a 1979 GS750 L

Thanks

Greg O.

[Steve]

One reason the mpg drops at 70mph is increased engine rpm. However, wind resistance is an even bigger factor.

If those are, indeed, your current tooth counts, your ratio is 2.73. If you change both sprockets the way you indicate, it will be 2.50. This is about a 9% change. If you run 4000 rpm at a particular speed, you will now turn 3600 at that speed. If you ride solo most of the time, it will probably not be a problem, and might actually get you a couple mpg increase, but most likely less than 9%. If you frequently carry a passenger, the extra load might have you downshifting more often, minimizing your increases.

How old is your current chain? If it is about worn out, get the whole set. If the chain is rather recent, wait until replacement time.


.

[Greg O.]

Hi Steve,

I put a new chain and sprockets on about 3000 miles ago, and the o ring chain is still tight. I had it off last week to repair a tire going down. It may not be worth changing the setup for a couple miles per gallon. What I am really after is extending the range on a tank of fuel. The L only has a , if I remember, 3.8 gallon tank. this gives me 100 miles at highway speeds before I hit reserve.

Steve did you get the Email I sent?

Greg O.

[bwringer]

Gas mileage on motorcycles is almost totally determined by speed and wind resistance. In fact, even engine size makes much less difference than you'd expect -- going the same speed, there's little difference in wind resistance, and you'll get much the same gas mileage on a GS400 and a GS1100.

Gearing will make little to no difference, although if you go to extremely low gearing, it can raise RPM to the point where the bike gets a little worse mileage. Basically, get the bike as close to stock as possible for most economical operation -- stock airbox and jetting and stock sprockets.

As you've found, using routes that avoid extended periods of high speed is your best bet for getting good mileage. Pushing a windshield or fairing through the air is also hard work, and will eat into gas mileage. And practice moderation with your throttle hand. If you can... :twisted: You gotta live a little!

If you're also looking to reduce your cost per mile, you might consider upgrading to an extended mileage touring tire if any are available in your sizes and keep them inflated on the high side (36psi front/40 rear). The Dunlop 491 or Elite 3 are the undisputed mileage champs -- a set of Dunlop 491s will easily go 15,000 hard miles or more and they don't cost any more than other premium tires, such as the Pirelli Sport Demons, that only last about 6,000 - 7,000 miles.

I still have to laugh when people point at my bike at the gas pump and tell me I must be getting great mileage and saving a lot of money. Once you add up more frequent oil changes and maintenance, along with tires that last less than 1/10 as long as the 80,000 mile Michelins on my Corolla and cost nearly as much... um, no. Not saving a dime, really. Quite the opposite. Just having a lot more fun getting from point A to point B!

[Greg O.]

I still have to laugh when people point at my bike at the gas pump and tell me I must be getting great mileage and saving a lot of money. Once you add up more frequent oil changes and maintenance, along with tires that last less than 1/10 as long as the 80,000 mile Michelins on my Corolla and cost nearly as much... um, no. Not saving a dime, really. Quite the opposite. Just having a lot more fun getting from point A to point B!

BWringer,

You are so correct about the economics of a motorcycle. Especially when riding with several bikes, if you figure the passenger miles per gallon, it is not a pretty picture. But we ride for the fun of it, and not the economy of it. I would like to be able to go more miles between fill ups, but guess to do that I would have to install a larger tank, which I am not going there, or get a bike with a larger tank.

Thanks everyone for the imput, I kind of figured as much, but had never messed with the gearing, so I plan to keep it stock. I actually enjoy 60mph more than 70mph anyway. Guess I am getting old.:-D:-|

Greg O.

[chef1366]

I'd go with the 40 tooth rear. I feel 16 tooth front will be too much. It will make a little difference but not much. I did the opposite. I wanted better take off power so I went from a 40 rear to a 42.

[mriddle]

I'd go with the 40 tooth rear. I feel 16 tooth front will be too much. It will make a little difference but not much. I did the opposite. I wanted better take off power so I went from a 40 rear to a 42.

I just changed my sprockets ... went with 15/40 and it relaxed things just a little bit at freeway speeds. I always ride one up .. it was good change for me.

Mike