Mimi:
Yes, you are asking the impossible or close to it. Your larger tires
require more power and that means more fuel.
But be aware that you are now traveling faster than your speedometer
indicates as compared to your prior condition. That means more fuel.
How much is a "huge" drop in mileage?
Your new tires have a greater frontal area and are harder to move through
the atmosphere. They are also larger and that means that your engine RPM is
less at any given speed. It may be that you have fallen out of the designed
"power band" where your engine is at it's best efficiency. Your speedometer
will register less than your actual speed all the time. There are lots of
formulas and nearly as many methods for calculating your true speed. If you
have a measured stretch of miles in your area you can pass through at a
constant indicated 60 mph with a stopwatch and will likely find that you are
doing a mile is something less than a minute (who knows if your prior
calibration was accurate?). An easy way these days is to use your GPS.
They are very accurate and will calculate your real ground speed very
closely.
For the future, you might raise your tire pressure a few pounds (less
rolling resistance) and purchase a few shares of Exxon stock.
===Carl Kunath
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-----Original Message-----
From: Mimi Jasek
Sent: Tuesday, July 31, 2012 11:12 PM
To: TexasCavers
Subject: [Texascavers] Help - speed, tire size change
Ok, I need help. I have a Ford Escape that we use as our main caving vehicle
due to great gas mileage. Due to age, miles, and need for stronger sidewalls
and better traction, we traded our old tires for new. Love them, but because
bigger, noticed huge drop in gas mileage at same speeds used to traveling,
and I seem to be staying up with or passing everyone!!
Old = P235/70R16 normal street tires
New = LT245/75R16 E Toyo OPAT OWL 120Q
Don't want a speeding ticket, and need my good mpg back, or close to it. Can
anyone tell me how much to decrease my speed, and if the reduction is the
same for all speeds? If not the same, how much decrease at what increments?
I believe my old tires were 28" diameter (spare is that) with 7.33'
circumference, and new are 30" with 7.85' circumference.
I have to believe there are more than enough math wiz/ tire savvy folks out
there to get me some answers, but if I am asking the impossible, tell me. I
do have a tachometer as well as a speedometer if that helps.
For us, mpg means a lot due to cost of gas, and determines how many trips we
make. I would really appreciate any help anyone can give me.
Thanks,
Mimi Jasek
Sent from my iPhone
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