After two flat tires within 5000 miles on the original tires (Hankook P235/70/17) and one of those was a sidewall puncture, I put new tires (Hankook LT265/70/17) and am now getting less gas mileage, shucks. At 65mph I was easily getting 20 to 21 mpg with the old tires and now only about 18 or a few tenths over 18 with the new tires. Would not have thought there would have been that much difference especially given that the new tires are a highway tread versus the more aggressive tread on the original tires. One of the Internet calculators shows the tire diameters to be different (30.8779" for the orginal tires versus 31.6062" for the replacements) and the speedometer readings to vary from 65mph on the original tires versus 66.5331mph for the replacements (at the same engine RPM) so that is only a 2.3586% difference.

To me it doesn't add up. Has anyone had a similar experience with a tire change?

 

Not surprised at all ...

- generic web tires sizes are meaningless - the ACTUAL measured tire diameters are what you need

- rotational inertia increases with the square of the speed - and you are placing that mass farther from the center.

- Actual weights for each tire are also needed - the new one may indeed weigh more - different carcasses.

- Different carcass, rubber compounds, inflation pressures, etc can all be a factor in increasing rolling resistance

- I'm going to assume you can correct for the differences in actual speed to use in any mileage calculations ...

Small changes - but they ALL can add up.

MGD

 

^^^What he said.
Knowing exactly what tires you had/have would help in looking up the manufacturers "revs per mile".
That's a much more accurate measurement.
In addition, that extra rotating mass is killer to power and mileage.
For every pound added to your rotating mass (wheels/tires) it's like adding 10-lbs of static weight.
Just looking at the tires, if each one is 5-lbs heavier, it's like adding 200-lbs to your truck.

By the way, I'm not sure what site you're using for tire size comparison, but from what I see that should be a greater than 5% difference in diameters, not 2.4%.
235/70/17s are ~30", not 30.9".
265/70/17 should come in at 31.7".
^^^I was looking at Bridgestone Dueler AT's and BFG Rugged Trail AS's to get those numbers.

 

wider heavier tires, more rolling resistance, more weight to spin up and keep spinning

 

Do we drive trucks because of the gas mileage? I know I don't.

 

Also, did you have your speedometer/odometer recalibrated for the new tires? If you did not, your MPG calculations are way off and you are getting better MPG's than your calculations.

 

You went from narrower to wider, and from a P series to an LT tire. While an LT will give you better wear and handling, it increases rolling resistance, as does the wider tire. Of course your mileage will go down, even if your speedometer has been corrected.

One thing to make sure of is that you increase the pressure that the tires are normally kept at. You do not say what load range and max pressure the new tires are, my guess is LR C with a max of 50. If so run them at 40-45. Your OE tires were probably set at 35. Running the LT tires at 35 will further increase rolling resistance and could be dangerous. The recommended pressure for the LR C OE tires on my truck is 45, although as low as 40 would be recommended for some tires.

If your tires are a different LR and pressure, post it here and I will give you a different pressure to use.

 

Kingfish51
The Hankook Dynapro HT tires (LT265/70/17) are rated as Load Range E. As I live on a farm and drive a lot of dirt & gravel roads I needed a tire that would stand up to more than normal abuse. Also have an 18' travel trailer (3850 lbs) that we will be pulling to Alaska in June so wanted a tire that hopefully I would not have any trouble with. Have the tires inflated to 35# now as that is the recommended pressure stated on the door sticker, the tires ride a little rougher than the originals but expected that.

 

Ignore the sticker on the door. That is for P series tires, and the ones you had on your vehicle ONLY. The door sticker of an F150 that came equipped with a LR E max 80 tire would state 50. As your new ones are a LR E, if they have a max of 80. run them at 50, if 65, run them at 45. 35 on a LR E is dangerous, especially on the highway.

 

Thanks Kingfish51. Am also going to use the GPS to verify the speedometer readings. With the original tires the speedometer was spot on with the GPS. Will pump the tires up to 50psi.