February 18, 2024

Bigger tires and MPG

My 4runner came with Dunlop Grantrek AT20 tires in 265/70R17. I have since changed to BFG KO2 tires in 285/70R17. I have notes a drop in fuel economy MPG, as well as poorer acceleration (and maybe a bit more braking effort is now required.

My friend Andrew has an identical 4runner with 265 KO2 tires. He says he noticed no change in fuel economy when he went from the stock tires to the KO2. He sees about 19 mpg, and that is the average over 7000 miles of driving.

First an important thing. Since my speedo is off, I also have to bump up my mpg by a correction factor. The factor looks to be 1.04. This would make 17 mpg (which is what I see) 17.7 -- so I almost get 1 mpg back by making this correction. See more on this below.

Tire weights

Dunlop Grantrek AT20 265/70r17 -- 39 pounds (my originalstock tires)
KO2 C load 265 - 46 pounds
KO2 C load 285 - 50 pounds
KO2 E load 285 - 58 pounds  -- what I now have.

Speedometer correction

The original 265 tires have 660 rev/mile. The bigger new 285 tires has 635 rev/mile. This requires a speedometer correction. I calculate this to be a multiplication of 1.04. This correction would also apply to miles per gallon since my vehicle now does not realize how fast or how far I am going. If I calculate 17 mpg (and I do), I am really getting 17.7 mpg.

When my speedometer says I am going 75, I am really going 78 mph.

Have any comments? Questions? Drop me a line!

Tom's Auto repair pages / tom@mmto.org