Playing With Tire Size - Fuel Economy, Hypermiling, EcoModding News and Forum

Big Dave · 11-18-2014, 02:05 PM

A few years back I experimented with LT225-75x16E tires instead of the LT235-85x16Es the truck came with. That resulted in a slight loss of MPG.

But that was before I got my GV on and sorted out. At 70 MPH I was revving 1,685 RPM with 235s and 1,825 RPM with the 225s.

Now with the GV (and its 0.8:1 drive ratio) I turn 1,325 RPM @ 70 MPH. With 225s I’d expect about 100 RPM higher. Still 35% slower engine speed at the same road speed than stock.

With the 235s I run EGT right at 600 degrees and can’t seem to coax it down much. Trials with 225s show temps 125 degrees cooler. Lower temp indicates lower fuel burn.

Lower temps are also good for extending engine service life.

If you look, all the cars that get outrageously good MPG have teeny little wheels – generally 14 inchers. That’s because they are designed to be urban cars. Lots of stop-and-go driving.

Your drive train doesn’t accelerate the vehicle away from a light. It accelerates the wheels/tires. The tires grip the road and that equal and opposite reaction thing accelerates the vehicle. You have to pour kinetic energy into the wheels before they move your vehicle.

The size of the “flywheel” is measured by the moment of inertia of that wheel. Before I stated the moment of inertia goes up with the square of the diameter.

My bad. Moment of inertia (according to my Marks handbook) goes up by the FOURTH POWER of the diameter. So 225-75s have a moment of inertia 30% less than that of 235-85s. Also the tires weigh a third less.

Thus I will expect the truck to require less fuel for stop-and-go driving (about a third of my driving) and the penalty for 100 additional RPM may be offset by cooler engine temps.

We’ll see.

BabyDiesel · 11-18-2014, 03:02 PM

Have you considered 215/85R16, Load Range E?

They are ~.9 inches taller than 225's. There would be less revs, and a decreased contact patch

Big Dave · 11-18-2014, 04:02 PM

I'm trying to get some good quality custom body work done on the truck to reduce coefficient of drag. Smaller diameter tires make that easier.

Also, the 225s lower the truck by an inch and a quarter versus the 235-85s.

aardvarcus · 11-19-2014, 07:56 AM

Big Dave,

As you compare inertias based on diameter, don’t forget that a larger diameter does not require the same angular velocity to achieve the same speed. Also the different diameters may also have different weights. I built an excel worksheet to compare wheel and tire combinations on my Tacoma. My goals were somewhat different, as I wanted the reduced gearing (jealous of your auxiliary overdrive) and wanted increased ground clearance.

What I ended up using to get accurate comparisons was the net kinetic energy stored in a wheel and tire combination at a given road speed. (I used 60 MPH) I believe this is the best and most accurate way to compare apples to apples when it comes to wheels and tires, as it accounts for the differences in weight, diameter, inertia, angular velocity, etcetera.

I went into my chart, which is based on the actual specs of Michelin LTX M/S 2 tires, and sorted it to show LT tires on 16” wheels in the sizes you were interested in, and a few other sizes for comparison that others may find interesting. I attached a screenshot of my chart, I couldn’t figure out how to paste a table without the formatting going bonkers. I would pay attention to the lines “Comparison Inertia” (which is actually Comparison Net Kinetic Energy) and Comparison Diameter which addresses gearing.

Note I have this set up to compare the other options to your current setup, 235/85R16s, which ironically is what I am running on my Tacoma. It looks like the 215/85R16s give you a 7.24% decrease in net kinetic energy while only reducing your diameter by 4.1%, so your gearing is not as affected. The 225s decrease your net kinetic energy 8.17% so slightly less, but they hit your diameter by 7.89% so your gearing is reduced more.

S Keith · 11-19-2014, 01:00 PM

Quote:

Originally Posted by Big Dave

Your drive train doesn’t accelerate the vehicle away from a light. It accelerates the wheels/tires. The tires grip the road and that equal and opposite reaction thing accelerates the vehicle. You have to pour kinetic energy into the wheels before they move your vehicle.

This is a flawed assumption. You have arbitrarily chosen a drive train component as a store of kinetic energy. Why didn't you specify the drive shaft? Why didn't you specify the 2nd gear in the transmission? Because it would be just as arbitrary.

Tire size vs. acceleration has far more to do with moment arm mechanical advantage than kinetic energy or moment of inertia of the tire/wheel. A smaller tire gives greater mechanical advantage to the engine, so for a given amount of power, you get a greater force at the contact patch and therefore greater acceleration. This is exactly the concept of drive ratios and transmissions.

Small fuel efficient cars have small tires because they are small cars. They need to be light, so they use small tires/wheels because they are lighter than big ones, and it would frankly look silly otherwise. They are also geared for optimal efficiencies for different operating speeds. My HCH2 has 195/65-15 tires on it, and it has light wheels that have minimal styling cut-outs for reduced aero drag of the wheels.

Yeah, but it's a hybrid, right? Sure, but at 60-65mph at steady state, the hybrid system is nothing more than added weight and a means of keeping the 12V battery charged. It's simply a highly efficient and optimized 1.3L 4 banger running in lean burn mode getting 50+ mpg. It was optimized for this operating condition and tire diameter. If I alter that, I'm probably going to see a decrease in efficiency either way. Of course all this assumes that I can control enough variables to make any experimental results meaningful.

I apologize if I sound dickish. I don't mean it to be that way.

NeilBlanchard · 11-19-2014, 01:48 PM

I think that the weight of tires sorta' "counts twice" since it is part of the overall vehicle weight that has to move forward and it has to be rotationally accelerated.

All weight takes a toll during acceleration. With tires, there are other factors, as well. Rolling resistance is probably the biggest one. And larger diameter tires have lower rolling resistance, all else being equal.

aardvarcus · 11-19-2014, 02:22 PM

Some relevant facts to add to the discussion. As Neil alluded to tire size is a very small part of overall tire performance, things like tread pattern play a huge role that is often overlooked.

Every single part of your vehicle is storing kinetic energy as you drive down the road. Some objects are only storing translational kinetic energy (radio, passenger, etc.), while others that are spinning are storing translational and rotational kinetic energy (drivetrain). All of this is measurable based on the items mass, the cars translational velocity, and the items angular velocity if any.

If we assume for a moment that the mass of the entire drivetrain (wheels, tires, drums/rotors, bearings, axles, gears, driveshafts, clutch, flywheel, crank, etcetera) is zero, then the rotational kinetic energy of these items is zero. Under this fictitious scenario, the only important gear ratio is from the engine to the road. The specific individual values of transmission gear, differential ratio, auxiliary gearing, and tire diameter don’t matter; it is only the net effect of all of them in series that matters.

Cars do not have infinite transmission gear ratios however, so at the end of the day the selection of your tire diameter will impose fixed limits on the lowest attainable engine to road gear ratio (in first) and the highest attainable engine to road gear ratio (in max overdrive), assuming you cannot change the differential gear, transmission, etcetera. Depending on your specific engine, car weight, drag, driving conditions, etcetera it may be beneficial to move this ratio up or down. In my scenerio, I wanted to move this ratio up (more road per rotation) where Dave wants to move the ratio down (less road per rotation). Obviously there are many other factors being affected here (weight, aerodynamics, etcetera).

Back to the zero rotational energy idealized scenario, there isn’t any difference between 30” tires and 3.73 differential gearing and 33” tires and 4.10 differential gearing, since the net gearing from engine to road stays the same (30” x 1 / 3.73=33” x 1 / 4.10).

Adding back in the pesky little problem of rotational kinetic energy, there is a big difference though. In the stated 30” & 3.73 versus 33” & 4.10 scenario, if the weight of the 30” and 33” tires was the same, the 33” tires would be spinning slower to achieve the same road speed, (re-edit fixed this part) thus the effects of the weight of the drivetrain, wheel, and part of the sidewall would have lower rotational inertia but the effects of the now larger section of the sidewall and the tread which are now farther from the center would experience an increase in rotational inertia. The net effect of this move may be positive or negative, depending on the weight of the various components. The translational inertial would be the same. In the real world, where 33” tires weigh a lot more than 30” tires, the 33” tires would have higher rotational inertia and translational inertia. This is what is illustrated in my previously posted (greatly simplified) chart. (Edit: also don't forget the significant aerodynamic effects of the 33" vs 30" tires, the vehicle body is 1.5" taller, another 1.5" of tire height is exposed, and typically taller also means wider so additional tire width is present.)

So why the focus on wheels and tires since all of the drivetrain has rotational inertia? I believe this is mainly because ties and wheels are easy to and commonly changed out, whereas changing other components are less commonly changed out. In high performance applications it is now common to see aluminum and even carbon fiber drive shafts to help reduce that rotational inertia. I have also seen aluminum brake rotors for sale.

From a less theoretical perspective, having experimented with tire diameter/size/weight on five different cars/trucks now, I can tell you that the inertia effects of heavy versus light wheels and tires can be felt. The most obvious time was when I switched from 255/85R16s to 285/70R17s on my 2001 2500HD. The 255/85R16s were actually taller at 33.4” but only weighted about 60lbs per wheel and tire. The 285/70R17s where shorter at 32.8, but were significantly heavier at about 85lbs per wheel and tire. From a purely gearing perspective, the truck should have been faster with the 285/70R17s, since they were shorter and therefore geared lower. But it was not so, the truck was significantly and noticeable slower with the 285/70R17s, due to the extra 100lbs of wheel and tire the truck had to not only spin but to spin 2% faster for the same road speed due to the reduced diameter.