We have a 1948 Kaiser Special with overdrive, with a 4:10 rear end. We towed a teardrop trailer from Ann Arbor, MI thru the UP on US 2. Our tires were one size bigger than originally equipped. We ended up in the Olympic Pennisula in the state of Washington. Then a more southern route thru Colorado, back home. 7400 miles total. According to our GPS the speedometer was 5 miles an hour under the actual speed we were traveling. We ran across the scales at 5210 lbs. Our average mileage for the entire trip was 16.3 miles per gallon. We had no problems with the car. We went across three mountain ranges and our highest altitude was 10,800 feet. We climbed in 2nd gear at 23 mph. in a 7 percent grade for over 20 miles. We were climbing one mountain range and we were passed by a school bus!! The addition of overdrive on this car, made a heck of a difference. My understanding is, that 48' Specials didn't come from the factory with overdrive. When it's just the two of us and no trailer we get about 21 mpg. However our speedometer is about 8% off.