Are these lowering my gas mileage?

Ah who cares, if you want a gas mileage queen buy a prius!

 

What I found with OEM tires was about the same, ie speedometer 2-3 mph off (read high, 62-63 showing was actually 60mph on scangauage), BUT miles traveled (odometer) was extremely accurate. After 22,000 miles on oem-size Michelins, odometer is NOT quite as accurate (tire specs are the same, but tread is .4" wider), and speedometer still reads 2-3mph higher than my calibrated scangauge.

 

when my other daily driver, 16' durango r/t with the 360 horse hemi and 8 speed automatic gets 11.5 mpg in the city, its nice to know my tacoma can get a few mpg more than my silverado did

 

You're right I meant 265/70/r17. They are cooper discoverer S/T Maxx. There isn't a single time I fill up the gas tank and don't do the math. 20-21 on the freeway if I'm at or below 2k RPMs and not too many mountain passes. Yesterday I filled up with 4.1 gallons and drove 70 miles around town. You can do the math if you want.

 

just an update as i have ran through a few tanks of gas now. I removed and sold my shell, and added 1.25" spidertrax wheel spacers. I filled up yesterday as my gas light came on. 280 miles i went on that tank. I filled up 17 gallons and the tank was full which means i still had 4 gallons to go. 280/17= 16.47 mpg. If i add in the 4 gallons i could have theoretically gone 345 miles on fumes. Not too bad. I drove it much harder on that tank also. On the highway, speeds were anywhere from 70-80. I just bought a mbrp dual exhaust, we will see how that affects it

 

I recently picked up a Taco too and have noticed the needle drops alot quicker when you're over 2,000 rpms. Anything over 65 and it goes down quick lol!

 

How do you determine the impact of tire size to speedometer/odometer readings?

 

Can start here: This helps you determine how your tire size impacts your effective gear ratio.

On the left is a speedo adjustment calculator.

The actual calculator I used was pretty complicated. I don't have the link for it but found it here on TW some place. You had to know your vehicle, gear ratios, and the model # of your transmission (because it knew how those gear ratios impacted things). It took a bit to fill in all the blanks on that calculator.

A simple way to figure out how far you'll throw off your Speedo/Odo is simply to look at the difference in rotation of the tires.

For example: A vehicle designed to run 245/70/16 sized tires (like my SR5 Taco came stock) that now runs 255/85/16 will display 65MPH when it's actually going 72.85MPH (according to the above calculator). That would put it off by about 11%. If both tires rotated the same number of times the larger tire will have traveled farther. So if both travel the same exact distance the smaller tire rotated more times to do it. This is what throws off the speedo/odometer as it simply counts rotations and uses a formula plugged in by Toyota to determine how far you went based on the 'known tire size' that they designed it for.

Anyhow, I had found exact measurements for my Cooper ST Maxx 255/85/16's and compared them to the Dunlop AT20's the truck came with. That calculation showed my Speedo/Odo would be off by 9%. So every 5,000 miles my odometer claims to have traveled I've actually gone 5,450 miles (simply added 9%). So I need to do my oil changes at 4,500 miles basically. And when calculating my MPGs (using miles driven on odometer) I need to add 9% to the miles driven.

Also, my UltraGauge allows you to calibrate it by driving known distances. I used the GPS on my phone and then drove the truck exactly "3 miles". I could then input into the UltraGauge how far I had actually driven (according to GPS) and it would compare that to what the truck claimed the distance was. That turned out to be... just about 9% off.

 

I'm rather paranoid of running out of gas, hate to walk, ya know. Anyway, I have never seen a "low on fuel" light. I guess I have one but not sure as I never get low enough for it to ever come on.

 

From what I have read, when the light comes on there is still 4 gallons of has left