<VV> Tire diameter and mileage

[Kirby Smith]

While the larger diameter tires lead to lower engine RPM for the same 
speed, the vacuum and hence fuel flow is not likely to be the same over 
the same acceleration/decelleration trajectory.  Hence, the loss in 
milage will not be exactly proportional to tire size.  If tire rolling 
resistance were the same for both cases, then the milage loss due to 
smaller tires would be due only to added drive train friction over the 
trip.  Some of these losses are proportional to various powers of 
mechanical part surface speed, in which case the smaller tired car could 
go slower and end up with the same corrected milage.

Frankly, I think Corvair fuel economy versus tire size is a red herring 
on the path of automotive fun.  If you can routinely get away with 
operating at triple-digit speeds, use tall tires.  If you like to hold 
up the honor of the Corvair at the stoplight, use short tires.  If you 
want the best milage on the road buy another mark.  All of my Audi's get 
better milage than my 66 turbo Corsa got, and better than it will get 
when I get it back on the road.  But that doesn't stop me from spending 
endless $$ and time on my Corvair.  It is a labor of love.

kirby


walt matenkosky wrote, in part:
> Just a couple of thoughts here. Remember that for all intents and 
> purposes, you are not going to change your engine or final drive ratio. 
> Let's consider that we are going to drive at the same engine speed for 
> the sake of comparison.
> 
> If you increase the diameter of the wheel/tire combo, then you increase 
> the true distance travelled by the difference in diameter times the 
> number of wheel revolutions. Let's say you put on a tire that is 10% 
> larger than OEM, then you will travel 10% farther. Your gas mileage will 
> be better in that you will have travelled farther in the same number of 
> engine revolutions. There is a penalty, however: your acceleration will 
> suffer because it will effectively numerically decrease your final 
> drive. If you have a 3.60 axle, it will feel like a 3.10.
> 
<snip>
> 
> Walt