Hello! I have a 1981 Yamaha SR250 Exciter and i‘m having a hard time finding this size back tire thats a knobby for this bike. the Back is tire size is 120/90-16. Thank you!
Never heard such a thing, only that low tire pressure will increase fuel consumption.
3.5 - 4.00 is the same as a 120 . 3.5 X 16 Maybe even a 4.00 X 16
Changing tire size very slightly fudges the final gearing. a larger tire gives more top end and smaller more torque. impact to fuel isn't notable.
Language is a little confusing- perhaps you just mean larger tire size on wheel as replacing a 3.50x18 with a 4.10x18 tire? - or perhaps larger wheel and tire size like using a 350x19 wheel instead of a 3.50x18 Or do you mean using a low profile tire on wheel like 70% height to width instead of 90% normal height to width? Depending on cycle - larger overall diameter of back tire will in theory increase mileage by difference in size- 10% over size 10% increase fuel economy at same engine rpm or alternately increase in speed at same engine rpm. In practice, minimal increase for next size tire. Oversize tire decrease fuel economy? Maybe on small scooter that gets a way oversize dirtbike wheel and tire without proper gearing change that doesn't allow engine to get to best power range. I used to see something similar on VW Beetles that got Baha Kits with 10x15 wheels and tires put on with stock 40/50 hp engines- top speed was reduced in 4th gear and economy suffered, best gearing for speed, cruise and economy became 3 gear on streets/highway. Engine needed 1800cc kit/two carbs of about 65/75 hp to get back into 4th gear and reach proper engine speed of 4000 rpm or higher. CJ3 Jeeps with the over size Desert Dog wheels and tires suffered same problem with stock engines- but they had lowrange 4 wheel drive gearing available for using the wheels /tires offroad effectively in sandy soil.
