Quote:
Originally Posted by SeanPlunk
Not sure, but I HIGHLY doubt your Vette gets it's best mileage at 90mph. Remember wind resistance plays a hugh roll as speeds increase.
|
Wind resistance factors in to engine load/ lower vacuum. The increased wind resistance will cause your mileage to get worse not directly, but as a result of making your engine work harder. This can be seen by reading your vacuum/ boost guage. Because my car is geared so low and has such a big OD ratio, there will be more lug on the engine (at a lower rpm in 6th gear at 75mph) than at 90 mph. Once wind resistance becomes a factor again (at speeds higher than 90 mph when the vacuum guage reads less vacuum) then the engine will be working harder, and therefore be less efficient.
It's really simple, your vacuum guage takes into account EVERYTHING (excluding air to fuel ratio, but this SHOULD be constant) including wind resistance. It is simply telling you how hard your engine is working. Therefore, the more vacuum you see on the guage, the better gas mileage you will get (at the lowest rpm possible). Have you seen some new cars that display a read out of the gas mileage on the dash? Those use formulas to determine the mpg, using a reading from a vacuum guage as the variable.
With the smaller diameter tires it might be able to pull highest vacuum at 85 or so mph. I haven't been paying attention. But it's still above 80 for sure. If I were to throw Ben's tires on my car, it would go back up to 90. But, Ben's car will get the best mpg at a slightly lower speed, since he has a higher diff ratio.