EcoModder Forum Sprocket calculations for motorcycles (to get better MPG and top speed).

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EcoModding Apprentice

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Mazda3 - '08 Mazda 3 S
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Quote:
 Originally Posted by ProDigit We now want to calculate our MPG gains. If with our stock riding, we got 70MPG, mainly a mix of 3/4th city and suburbs, and 1/4th highway; our new MPG should come close to: 70MPG * 15/14 * 33/28 = 88MPG The formula is derived from: MPGstock * New front sprocket / stock front sprocket * stock rear sprocket / new rear sprocket) We have gained an average of 18MPG compared to stock!!
Playing with gearing is always fun, and I often think many people could benefit from it but are too afraid to play with changing it.

However, I don't understand the part I quoted above. MPG isn't a linear equation with rpm change, ie, cutting rpm in half doesn't necessarily double mpg. Just like a 8.5% change in gearing on my bike (geared down for off road) only netted about a 2% reduction in fuel economy riding the same route and habits commuting to work.
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 01-23-2016, 02:10 AM #3 (permalink) EcoModding Apprentice   Join Date: Jun 2012 Location: Florida Posts: 125 Thanks: 0 Thanked 11 Times in 9 Posts You are correct, At lower speeds, MPG gains are greater than at higher speeds. The MPG gains are in line with the torque. Most of the time reducing torque by 10% results in 10% better MPG, or, accelerating with 10% more torque results in 10% lower MPG. Out of several hundred readouts, you'll find that MPG roughly is inversely proportional to torque, which torque is directly proportional to gearing. But with a 10% torque decrease (aka 10% lower RPM), you may see upto 20% MPG increase at lower RPMs (eg: 3k RPM), while see much less at top speeds. I guess it also depends from machine to machine. A bike or car that is undergeared (eg: does 4k RPM at 40MPH, with a 2 liter engine) will gain 50% MPG when lowering RPM to 3k RPM. A small 250cc engine that is very taxed, doing 6k RPM at 80MPH, will most likely gain only 1 or 2 MPG when upgearing it by 10%.
Wanting more for less

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Location: New South Wales, Australia
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Metric - '94 Honda Magna 250
Motorcycle
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Quote:
 Originally Posted by ProDigit We now want to combine high top speed, with great MPG. We can do this, by creating an extra overdrive; and by making our second to last gear, the same ratio as our last gear. In the example above, our Honda Rebel's 4th gear would need to have the same speed per RPM as our modified 5th gear.
This is pretty much what I did with my bike Metric (VT250C).

The only power/torque curves for this model on the internet are from Honda's factbook site (in Japanese). Due to it's small size and lack of labelling the curve only gave a rough idea of how far I could change the ratios.
As a result, I relied on personal observations during my commute. The bike has no tacho, so it was "seat of the pants" estimations.

The bike would take the whole climb through the mountains in top gear, except for one long steep hill (Lapstone hill) where 5th was just too tall.
I used this hill as my 4th gear target.

Using the gear ratio list in the owner's manual, I calculated that changing from 14/38 to 15/36 gave me a 4th gear that was just a bit lower than the original 5th. The sprockets had to be ordered from Japan.

Trying 15/38 first, the bike performed well and I saw an MPG improvement.
Going to 15/36 was even better on the motorway at 110kph and climbing Lapstone hill in 4th is fine, but 5th is now too tall for some of the other hills through the mountains, so I find myself using 4th more than I expected at first.

I'm happy with the new gearing, even though I'm not getting as much improvement in MPG as I'd hoped.
My next step would be aerodynamic improvements to take better advantage of the tall gearing.
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Quote:
 Originally Posted by ProDigit We now want to calculate our MPG gains. If with our stock riding, we got 70MPG, mainly a mix of 3/4th city and suburbs, and 1/4th highway; our new MPG should come close to: 70MPG * 15/14 * 33/28 = 88MPG The formula is derived from: MPGstock * New front sprocket / stock front sprocket * stock rear sprocket / new rear sprocket) We have gained an average of 18MPG compared to stock!!
Sorry. This will have no basis in reality for fuel economy gains of that magnitude just from changing the gearing.

EcoModding Apprentice

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Quote:
 Originally Posted by sendler Sorry. This will have no basis in reality for fuel economy gains of that magnitude just from changing the gearing.
Works perfect on a Motorcycle!

It's more a rule of the thumb, rather than a precise measurement, as exact fuel figures are determined by how much your ignition is advanced, or how heavy your bike is taxed while accelerating.

 01-30-2016, 11:55 AM #7 (permalink) Master EcoModder   Join Date: Feb 2010 Location: Elmira, NY Posts: 1,532 Thanks: 228 Thanked 247 Times in 209 Posts It is the combination of gearing and drag reduction that yields improved mpg. I spend a good deal of time analyzing gearing on my HPVs since my power to weight ratio is so low.
 10-02-2017, 03:46 PM #9 (permalink) EcoModding Lurker   Join Date: Oct 2017 Location: Victoria, BC Posts: 1 Thanks: 0 Thanked 0 Times in 0 Posts Where did you find the calculator chart? I used to have a bookmark for what I believe is the same place you secured the calculator chart but the URL is no longer valid. Do you have a current site for it? I ride a Kawasaki KLR 650 and greatly enjoy changing the sprockets to achieve the best torque/speed combination for my riding style. An event coming up has me wanting to compare different ratios and I would love to be able to use the calculator. Thanks in advance.
 10-02-2017, 04:04 PM #10 (permalink) Master EcoModder     Join Date: May 2011 Location: Syracuse, NY USA Posts: 2,220 Honda CBR250R FI Single - '11 Honda CBR250R 90 day: 106.36 mpg (US) Ninja 250R SE Green - '09 Kawasaki Ninja 250R SE 90 day: 65.9 mpg (US) 2001 Honda Insight stick - '01 Honda Insight manual 90 day: 60.68 mpg (US) 2009 Honda Fit auto - '09 Honda Fit Auto 90 day: 38.51 mpg (US) Kawi Ninja650 - '07 Kawasaki Ninja650 90 day: 54.85 mpg (US) PCX153 - '13 Honda PCX150 90 day: 103.38 mpg (US) 2015 Yamaha R3 - '15 Yamaha R3 90 day: 81.28 mpg (US) Thanks: 277 Thanked 657 Times in 493 Posts gearing commander? . Gearing Commander ⚙ .