it depends on where the sensor is as well. the gear change in my truck didn't change my mph at all because the speed sensor picks it up from the axle. since that is past the differential, it didn't care what gearing was in my rear end.

the tire size would have totally messed up my speedo. haven't done much with front wheel drive stuff so I am not sure how that works. I know some of the old cars had little plastic wheels that you could change out for gearing changes. I think that was still on a mustang though (still rear wheel drive)

you can buy programmers to fix that but they are expensive. I wouldn't have changed the gears if I would have had go that route.

my new tires are 0.4% different in circumference than the original equipment tires so i will not have my speedo recalibrated (.24mph error at 60mph). however when i report my mileage i will subtract 0.4% from the odometer reading before i divide by gallons. same as the way i have currently been adding 2.1% to my odometer for the larger than stock diameter tires i was running.

After putting on the new 13" tires, why not check how accurate the odometer reading is with a long highway drive, like 40 or 50 miles? You said you have a long commute to school.

It will help a lot
 

I have a drag race car and I can tell you we all use the smallest, lightest and narrowest tire we can get away with.
When I put the "Skinny's" up front the car was faster and even noticably easier to push around in the staging lanes. It car steered easier and did not need the power steering allowing me to save even more.
It is more than just saving the wieght it is reducing the rolling resistance by making it narrower and a taller tire helps to.
If it helps that much on a drag car I am sure a thinner, taller and lighter tire would help a lot in a car built for mileage.

first gaslog update
 

115.3 miles on 2.395 gallons. same pump at the same gas station on the same day. new personal best of 48.14mpg! more to come later this week.

I actually found the mileage loss from changing the original tires to Michelins to be dramatic. In the range of 6 MPG or close to 10%. This was on my VX when it had 37k original miles. That was the only thing I changed, and the difference was immediate and very obvious. Check my gas log and you can see where it happened.

therealtime, I have the left over original VX tires in my garage. The car is gone. It would be an interesting experiment to see how much difference they would make in your experiment.

regards
gary

Now that you bring it up i guess i should list the tires used. my 195/14s were Warrior fluent tires and the new 175/13s are general altimax rt's. I would be open to testing any other tires that someone gets to my house.

I know for a fact that oem tires that come on a car are made of special low rolling resistance compounds (only available to vehicle manufacturers) in order for the vehicle manufacturer to meet standards for fuel economy.

One of my professors used to work for michelin and he said that whenever a michelin employee would buy a new car they would immediately change to aftermarket tires (even if the same size and brand) for grip and performance boosts. This would be the opposite of what an ecomodder would try. You are suggesting reverting from higher rolling resistance tires back to oem lrr units.

I have another question
 

if changing wheels to lighter weight aluminum units is #10 on the list of gas saving tips here: https://www.gassavers.org/showthread.php?t=2584 and included on ecomodders list here: https://ecomodder.com/forum/fuel-econ...cations.php#17 how come everyone said that they expected little or no change? if this had indeed been busted wouldn't it be left off of the lists? no one has said anything about my first tank increase. if I did indeed go from 40 to 48mpg, then the wheels and tires should pay for themselves in less than 34,100 miles (probably the life of the tires). this is at $1.69/gal where I am saving around $.007/mile. If gas prices rise again then the time will be shorter (little over 14k miles if gas was $4.00/gal again).

Everyone didn't write those lists...individuals did. Every individual would write a very different list based on their experiences, knowledge, and ideas. I also disagree with much of the list that you get if you click the "201 Tips To Save Gas" tab in this site's navigation header.

Your jump in FE is definitely interesting and (assuming it stays higher in the long run) could be because of the weight of the wheels. My first guess would be that your new tires are designed/constructed with better rolling resistance than your old ones. It's tough to prove, of course, and I doubt we'll have any way to prove it; and so, this is one more data point, this one showing that wheel weight could be worth more than I think.

yea, but that "BEST TANK EVER" was on an extremely short run. I could get my best tank to about 43 (which is great for me) If I only used 2 gallons and chose the route.

before saying it made that much of a difference, run an entire tank through.

I can't say why anything is on the gas savings tip list. some of the items on there is just plain out there and crazy to me but obviously someone has seen gains from whatever is on there or it wouldn't be there.