Understated MPG with 33" wheels?

I have a question for those with 33" wheels on their trucks. I looked at the calculators and unless you go through regear it seems that the speed is going to be off by roughly 10%. Stated 70 MPH is actually 77 MPH, so for those that say MPG dropped dramatically which I expect with bigger wheels, isn't it a bit overstated by 10%? Like you are actually getting 10% more MPG than it says?

 

Roughly. But who puts 33” wheels on anything? I prefer 33” tires

 

You are only looking at one factor of the tire change. The effective overall gear reduction will effect fuel economy more from a stand still because the engine needs to work harder to roll the larger tires.

 

Sorry let me clarify, yes 33" tires, I was just referring to the whole thing.. And yes I expect it to go down due to the extra stress on the motor, I'm just saying the stated MPG might not be as bad as it says?

 

You are correct. The best way to get a correction factor is to check your odo against a GPS trip. I do that with Gaia.

The other thing is that it will change with wear, on any tire. For example with my 265/70R17 Wildpeaks, my odo was off by 5% when new and 3.5% now with half worn tires.

I didn't think to check this with the stock tires, but it will be a larger factor with ATs and MTs due to the greater tread depths.



Edit: But, yeah. Approximately 10% for 33s is a big difference. Plus 1mpg at 10mpg shown.

 

Mileage & engine stress would be much better with 255 Pizza Cutter tires. 255/85/16 or 255/80/17 are both a 33" tire and weigh less.

Speedo would still be off

 

Stock tire is 31", so it's more like 6%. Keep in mind size numbers like 285/75-16 or 255/80-17 are just guidelines. The actual molded tire can deviate a lot from it. Most tire mfrs publish a "revs per mile" spec for each individual tire SKU. Use that to calculate your odometer correction when checking fuel mileage.

 

@Hypetaco

The 10% you are referring to is the difference in size (and coincidently MPH), not MPG. You will not lose 10% MPG.

My Ridge Grapplers are 285/75/17 (33.8" tall) and are 10% bigger than the stock 265/65/17 (30.6" tall).

I can confirm the speedometer was off by 10% (20/22, 30/33... 70/77, etc.) I fixed that with a Hypertech calibrator, no re-gearing.

 

Interesting, the 10% was from the increased speed in the tire calculator

https://www.tacomaworld.com/tirecalc?tires=265-65r16-285-75r16

 

They are all estimates, even the quoted revs/mi from a MFG'er as they were tested at an inflation and load that is likely to not match your application. As noted above, it changes with wear.

Comparing to GPS is the real answer. Of course, estimates are okay. It is not a critical thing. We're not weighing out Plutonium here.

 

My only point is that you are roughly getting 10% more miles per gallon or hour than the system thinks you are doing because the outer diameter of the tire is different

 

No. Increasing weight,rotating mass, and increased friction will decrease fuel mileage .

 

285 70 17 KO2 C load is 51 lbs vs 58 lbs for E 255 ST Maxx, sure the Maxx is bomb proof but overkill for most people.

 

Isn't the stock OR tire 265/70-16, not 265/65-16?

Anyways I run 33's. Mpg dropped an indicated 2-3 MPG. I usually get about 1 MPG better than the truck says. None of it is enough to change anything. Bad is bad. I did find going from stock to 265/70/17 made my speedo and fuel mileage reading accurate though.

 

But they are sweet!

I got them because they were the shortest 33.
Might be the heaviest too though.

 

Yes.

 

Going from stock 265/70/16 to 285/75/16 is about 7.2% larger diameter. Your speedometer and odometer will be off by about 7% as well. The mpg numbers you see on the instrument panel will not be quite accurate since everything is calibrated for factory tires. But you can calculate by hand and account for that difference.

Lets say your odometer says you drove 310 miles and you filled up with 18 gallons of gas for 17.2 mpg. To correct for the tires 1st multiply the miles driven by 1.072. So 310 X 1.072 = 332 miles actually driven. 332 divided by 18 means you actually got 18.4 mpg.

But going to bigger, wider tires will make about 2-3 mpg difference in fuel mileage even compensating for the tires diameter. It's just not quite as bad as it appears initially.

 

So my center cluster in the dash showed 19.2 mpg . After I did the mileage check at my next fill up I was actually getting 20ish mpg. The best I've got so far is 21mpg , but my truck is tuned and a bunch other mods done to motor .My truck weighs 5,250 lbs . Thats with me and a full tank , my winch bumper and 12k winch, tool box with recovery gear . One more I'm running 285/75/16 km3's method nv 305 wheels

 

I have 285/70/17 with stock drivetrain and the best I've seen on my dash is 14.9mpg. This is both driving in city and HWY @ 70-75mph. I haven't calculated this manually yet. My speedometer is off around 3-4mph @ 70mph.