I would guess that the increase in mass of the tires has far less impact on the increase in gas usage than the fact that the larger tire means that the vehicle is traveling faster. The amount of drag associated with even a 5 mph increase in speed at 60 mph would be plenty to cause a noticeable effect in reduced gas mileage for an SUV-type vehicle such as a Ford Escape. We're not talking about the world's most aerodynamic vehicle here, after all.
Diana ******************************************************** Diana R. Tomchick Professor Dept. of Biochemistry University of Texas Southwestern Medical Center 5323 Harry Hines Blvd. Dallas, TX 75390-8816 (214) 645-6383 (office) (214) 418-5827 (cell) diana.tomch...@utsouthwestern.edu<mailto:diana.tomch...@utsouthwestern.edu> On Jul 31, 2012, at 11:41 PM, "Nico Escamilla" <pitboun...@gmail.com<mailto:pitboun...@gmail.com>> wrote: All engines have an "RPM sweet spot" so to speak, when you get past it your fuel economy goes down.. To move the larger mass of bigger tires you need more power which requires higher rpm and hurts your pocket, like Carl said use your gps, I jumped from stock to 35" tires on my dodge and theres a difference of 7 to 10 km/h between the ground speed on the gps and the one shown on the dash of the truck. Some people resort to re-gearing the rear differential to better move larger tires but that might be expensive. Nico On Tue, Jul 31, 2012 at 11:32 PM, Carl Kunath <carl.kun...@suddenlink.net<mailto:carl.kun...@suddenlink.net>> wrote: 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 -------------------------------------------------------------------------------- -----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 --------------------------------------------------------------------- Visit our website: http://texascavers.com To unsubscribe, e-mail: texascavers-unsubscr...@texascavers.com<mailto:texascavers-unsubscr...@texascavers.com> For additional commands, e-mail: texascavers-h...@texascavers.com<mailto:texascavers-h...@texascavers.com> --------------------------------------------------------------------- Visit our website: http://texascavers.com To unsubscribe, e-mail: texascavers-unsubscr...@texascavers.com<mailto:texascavers-unsubscr...@texascavers.com> For additional commands, e-mail: texascavers-h...@texascavers.com<mailto:texascavers-h...@texascavers.com> ________________________________ UT Southwestern Medical Center The future of medicine, today.
