Help with math?

[Okey0]

if i understand correctly, a larger diameter tire lowers your ratio. In other words, if I have 3. 73 gears, a taller tire could lower it to 3. 5. the lower ratio would mean that at the same speed, the engine is at a lower rpm, correct? If this is the case, why does everyone say they notice a drop in mpg with larger tires?

 

[SBall]

power is power

Yup, while the larger tires would have the effect of a reduced engine RPM for a given speed, to produce the horsepower required to maintain that speed you would need a corresponding increase in torque from the engine. This increase in torque would require additional fuel for each revolution of the crankshaft. Thus you have a balancing act going on. Less rotations of the crankshaft but more fuel per rotation. This leaves you with other variables that dictate fuel economy such as pumping losses, friction, etc. If the gearing ratio change puts you closer to the "sweet spot" where your particular engine is most efficient, then you'll see an improvement in fuel economy. What I've learned from other mentors here on the forum is that tender pressure with that right foot makes the most difference in fuel economy.

 

[Hohn]

if i understand correctly, a larger diameter tire lowers your ratio. In other words, if I have 3. 73 gears, a taller tire could lower it to 3. 5. the lower ratio would mean that at the same speed, the engine is at a lower rpm, correct? If this is the case, why does everyone say they notice a drop in mpg with larger tires?

Yes, taller tires will lower engine RPM, as it makes your overall gearing "taller" (lower numerically).



The drop in mpg is due to the fact that larger tires have much more rolling resistance, and take more power to turn. This usually offsets any increase in mpg you may get from taller gearing.



Besides, diesels will not get better mpg just from taller gearing, unlike most gasoline-powered vehicles. Gas engines have a very narrow range of air-fuel ratios, so when you lower RPM, the amount of fuel used goes down as well.



Diesels can very air-fuel ratios substantially. Thus, when you put taller gears in, the engine compensates (since it has to work harder) by making the fuel richer. The net result is no increase in MPG due just to gearing.



Now add back in the negative effect on mpg that larger tires have and it's pretty easy to see why larger tires almost always give you lower MPG on a CTD.



Oh-- and to calculate the effect of tire size on gearing or RPM, just do this:

(new tire revs/mile)/(old tire revs/mile) X existing gear ratio or RPM.





For example, say you switch from 265 tire that's 655 revs/mile to a 285 tire that's 622 revs/mile. If you currently have 2000rpm at 72mph, then your new rpm would be (622/655)*2000= 1899 rpm (basically 1900rpm).



Same goes for gearing. (622/655)*3. 55= 3. 37 as your new gear ratio.



A 315 tire usually has 599 revs/mile. So swapping from a 265 tire to a 315 would change gearing and rpm this way:

(599/655)*2000= 1830 rpm. You went from 2000rpm to 1830rpm cruising at 72mph.



Gearing went from 3. 55 to (599/655)*3. 55= 3. 25 effective gear ratio.





Hope this helps.



Justin

 

[Joe G.]

If you don't take the change in your odometer caused by the tire size differance then your calculation won't be for the correct number of miles. With bigger tires you go farther for each revolution.

 

[opjohnny]

In the 17" size taller is heavier and wider. The extra weight of the tire and the extra height and increase in frontal area negates any increase in mpg. The larger diameter unless accounted for will show less miles then you have traveled and you will think your getting even worse mpg.

 

[pwknapp]

Stock '96 dually with 215/85's and 4. 11's @ 70-72 mph equals 13. 2-13. 4 mpg at sea level. Bombed truck with 255/85's at same speeds equals 16. 2-16. 6 mpg at 7300 ft elevation. Go figure.