MPG/ Tire size question

Okay, so I know this horse died a long time ago and has never stopped taking a beating

My question is this: Is the speedometer on our trucks calibrated to the specific tire size it's sold with?

My 2012 Base Model came from the factory with 245/ 75/ 16 tires on it. I currently run 265/ 75/ 16. If the speedo is calibrated for the original size, I'm getting close to 19 mpg with my snow tires on. If the speedo was calibrated up a size (265/ 70/ 16), then I'm getting just under 17 mpg.

Thanks

 

x2

 

It doesn't seem right, considering all the weight in my truck and that much of my driving is in the mountains running low grade ethanol blend. I'm not too concerned either way. I was just curious.

Post 25 is what I carry daily http://www.tacomaworld.com/forum/2nd-gen-tacomas/357596-what-do-you-carry-your-truck-bed-2.html

 

So 245/75/16, 265/70/16 or 265/70/17?

 

Whenever I have a GPS in mine (with 265/70/16's) my speedo is always reading 2-3 MPH faster than the GPS. So I would think it would be more accurate with 265/75/16's myself. Of course using the fill up/set tripmeter/check miles traveled vs.gallons used method on mine I was getting 21-22 mpg when I had 245/75/16's on the truck and now I rarely see 20. But with gas below $2 a gallon right now I'm not too concerned.

 

speedo reads slower with smaller tires and shows higher mpg with same. I used the tire size calculator in conjunction with figuring odometer mpg at fill ups.

 

FYI all stock 6 lug tire sizes are the same diameter.

245/75R16 265/70R16 and 265/65R17 are stock sizes and have a diameter of 30.5-30.6in

Speedo's typically read high from the factory but odometers read spot on, this is not a mistake but a way to make people drive a little slower. Do the mpg calculations properly by topping off your tank, zero'ing the ODO driving, then using miles traveled / gallons to top of.

Now if you have a different tire size than stock like the change to a 265/75R16 you need to take miles traveled x 1.034 (or add 3.4% to the miles traveled) then divide by gallons to top off.

 

When I switched from stock 265/65/17's to 275/70/17's the speedo read correctly, was always about 2 mph slow on stock tires.

 

Hmmm... If that's correct, then I'm getting 16.65ish mpg. sounds closer to what I was expecting this time of year. I bought a set of wheels from a guy and they had 265/ 70s on them and odo calculations gives me around 17.5mpg on summer gas with the same driving. I must have figured the wrong number to multiply by, assuming you are right.

Edit just to give this thread a pic. 265/ 70s shown

 

I would sleep in that truck any day.

:sexy

 

I spend a fair number of nights sleeping under the shell every year.

pre lift photo

 

Where does the odometer get the info from? I assuming it registers revolutions of something to determine the distance traveled, what's that something?

 

Speed sensor. Location may vary by engine/transmission/2wd vs 4wd. Basically it converts either driveshaft or axle revolutions to miles and feeds it to the odometer displays. If you don't have a GPS, use one of the trip odometers to compare to at least 50 miles by mile marker on an interstate to determine odometer error. Odometer (and speedometer) error will change over time with tire wear (worn tire, smaller diameter, travels slightly less per revolution). With brand new oem-size tires my speedometer is off about 3 mph at 65mph, odometer records 2% higher than actual travel regardless of mph. Most vehicles read 2-4 mph high to avoid lawsuits over accuracy (ie but my speedometer said 60, officer) plus a fudge factor for tire wear.

 

Check it with a gps. My new Pro was spot on for odo mileage reading with both the Scan Gauge and the GPS showing the same speed while the Spedo was about 2mph high at 70mph. With bigger tires you will be travelling farther and logging fewer odo miles, and your speedometer may actually be close to correct. To get true mpg's you will need to calculate out how far you actually drive when the odo reads each mile.