[Nissandriver]

I posted to this thread because I feel I´ve done some decent testing about the difference in mileage my car gets at 55 mph versus 80 mph.

Originally Posted by JDaly

So you are driving a taller tire. I had not realized that one could do that and get better MPG.

Yes. When I bought the car I was getting 32-35 mpg. As with all the other 4 cylinders with 5 speeds I´ve had I wished it had a 6th gear for better mileage on the highway. With the normal size tires the engine runs at 3000 rpm at 60 mph. In a less than 2500 lb. car with 140 hp and 132 lb-ft of torque I think it´s ridiculous to run the engine at such high speeds on the highway. I wish it turned 3000 rpms at 75 mph. That would rarely cause any problem. I´d probably have to use 4th gear in some places like west of Denver on I-70 but most of the time it would cruise just fine in 5th gear on four lane highways.

I´ve checked the odometer/speedometer error at least 10 times on the Interstate over 100 miles. With the taller tires for instance, I would pass mile marker 100 with my trip odometer at 100.0 miles. When I reached mile marker 200 my trip odometer would read 192.4 miles. I´ll check it occasionally over 1 mile and I see the trip odometer read a little over 0.9 miles and I do ten mile checks sometimes and see that the odometer reads less than 9.3 miles. So for speed corrections I divide by 0.93 to know my actual speed although I´m erring against what might be the true mileage and speed. 60 mph / .93 equals 64.5 mph. At 93 mph on the speedometer I should be going a little over 100 mph.

Originally Posted by JDaly

"A taller tire with a higher aspect ratio (the ratio of the tire's sidewall height to its width) can potentially lead to better gas mileage because the taller tire has a smaller contact patch with the road, which can result in less rolling resistance.

The contact patch should be the same since both tires are supposed to be 185 mm wide?, but the 75% ratio tire does appear to have a little narrower contact patch. You get slight variations in tire dimensions even though they are labeled the same.

Originally Posted by JDaly

However, it's important to note that a taller tire also has a taller gear ratio, which can result in a lower top speed and can negatively impact acceleration.

I doubt there is much difference in top speed with this car. The coefficient of drag on this car isn´t that good. Total drag probably has the most influence on that. I can tell the acceleration is negatively effected. The effect is little enough I have no concern about it. With the taller tire the car is a little higher off the ground so I´m probably increasing the drag a little bit.

Originally Posted by JDaly

Additionally, taller tires can also negatively impact the vehicle's handling and stability. So, it is important to consider all factors before making a decision to change the aspect ratio of the tire"

The taller tire does have a bad effect on the cornering. That´s not enough to bother me. It still drives good. The taller tire also had an effect on the ride quality. It rides smoother and quieter. I forgot to mention I´m using the alloy wheels off a 1999 Sentra. They are lighter than the original wheels so that should help the handling a little and have a good effect on the ride quality. I have no idea whether the taller sidewall or the lower unsprung weight improved the ride quality most. I doubt the weight difference has much effect on the mileage. I´ll measure the difference in weight some day. I also had to use spacers on the rear wheels to push them out from under to spring perch otherwise the tires would touch the underside on the perch. That´s likely to wear out my rear wheel bearings sooner. I´ve had no problem with clearance on the front wheels but I have had the rear tires rub two or three times but I was carrying several hundred pounds if not as much as 1000 lbs. in the car.

Originally Posted by JDaly

I will take the better MPG.

When I first changed the tires, I would get the same mileage calculations from how many gallons it would take to fill it compared to the reading on the trip odometer. 1 / 0.93 = 1.075. I´m now traveling 7.5% farther than the trip odometer measures and going 7.5 % faster than the speedometer measures. 35 mpg / .93 = 37.6 mpg. I´ve learned to drive the roads in my area better so I´ve gotten better mileage than when I first moved here. I´ve also replaced several engine parts trying to get it ruuning as good as possible. I do coast some times in neutral with the engine running and I keep my eye on the traffic lights trying to stay in 5th gear as much as possible. It´s kind of amazing how little brake pads I use compared to 30-35 years ago.

I´m not as exacting as many people on this forum but I think my measurements are decent. I´ve not recorded less than 41 mpg the last two years except for the two high speed trips on I-40. I´ve calculated 42 mpg two or three times, 44 mpg once, and a remarkable 47 mpg once in the past two years. I always try to use 10 gallons before I refill the tank to reduce any errors from the difference in the level I fill the tank. I´d doubt difference in the level I fill the tank is ever more than 0.2 gallons. With my best mileage, the pump indicated 10.25 gallons. I calculated 492 miles / 10.3 gallons for 47.76 mpg. If I did a poor job filling that tank and actually used 10.5 gallons that would only change the mileage to 46.86 mpg.

Originally Posted by JDaly

Just interested to share how a higher aspect ration can reduce the amount of rubber touching road.

That´s something I might argue is wrong. The first number 185 is supposed to be width of the tire in millimeters. The second number is the percentage of the width that the tire should measure from the tread to the bead? I have no idea where the width measurement is supposed to be taken, the tread or near the widest points on the sidewall?