Powerloss vs Tire Size

I'm in the process of leveling out my Tacoma and am trying to understand how tire weight and size affect powerloss. Thinking of getting BFG KO2s in 275/70/R17 in load class E which weighs 55.4lbs or 285/70/R17 load class C which weighs 51lbs.

My question is which of these tires would have a less dramatic effect on power. A smaller tire that weighs more or a larger tire that weighs less? Or is the difference we're talking about for these two tires so close that it's not even going to make a difference...

 

Looks like I had duplicate images post... first image should have been the only one. Apologies

 

The larger diameter tire effectively changes your axle ratio. You'll have less power period. About 5% with the 275's. About 7% with the 285's. That makes a difference in how much you can tow. The wider 285's will also increase rolling resistance, the 275's are wider than stock and will increase rolling resistance, but somewhat less.

You'll have slower acceleration, and the transmission will spend a lot more time in lower gears trying to make up the difference. This is especially true if you drive at altitude. Your truck loses about 3% power for every 1000' you drive above sea level. If you're at 10,000' that's 30% less power, in addition to the power loss of the bigger tires. You may find overdrive cannot be used at all with either tire. Lots of guys re-gear their trucks to lower axle ratios to help compensate for this.

A heavier tire that is still the same size as stock will mean a little slower acceleration, but they don't change the axle ratio, you still have the same amount of power for towing. It will take you a little longer to get up to speed, or merge into traffic from an onramp. But once up to speed there is no difference.

Compared to stock both of these tires are larger and heavier. You're going to see loss of performance and fuel mileage with both. But between the 2 I just don't see a measurable difference. You're going to have to modify the truck to make either fit and I suppose I'd go with the bigger tire.

I recommend 265/70/17 or 265/75/16 as the biggest tire for a Tacoma. They are only 1/2" shorter than 275's, 1" shorter than 285's. That is only 1/4" to 1/2" less ground clearance. But they don't require any modifications to the truck and close enough to factory size to not mess up performance or fuel milage in a significant way.

 

Personally, just having gone to E 285s, i havent noticed a thing by the butt dyno.

 

You have a few things going on here. First is weight, unsprung weight, which makes it harder for the vehicle to get going in the first place. You would probably notice a weight difference most at acceleration from a dead stop or while accelerating. Once in motion, the kinetic energy of the heavier tire would likely balance out the fact you are carrying more weight. The unsprung weight increase means your suspension has to work harder to control the cycling because when a higher amount of mass is put into the same motion as a lower amount it will exert more energy against the suspension and will require more force to counteract it.

The next consideration is gearing. Aside from the greater amount of mass the vehicle must put into motion, the added tire diameter changes the effective final drive ratio. Generally, a larger diameter tire requires a lower gear ratio to have the same final ratio as before the increase. This would play out as, lets say, at 70 MPH your vehicle currently is turning 2000 RPM in sixth. With the taller tire, it is now turning 1750. The problem is whether the engine RPM is sufficient to keep the truck moving, or is it having to down shift to make enough torque to keep the vehicle in motion. this would be true through all gears. You might find it takes more throttle to get going or the truck shifts back and forth, because once it shifts up, there is not enough torques and it shifts back.

The last item is rolling resistance. The more rolling resistance, the harder the engine will have to work to get the vehicle going and to maintain that.

A vehicle can take a tire change and still be within a reasonable parameter of performance. I think I would balance the weight possibilities and diameter against that of the factory tire. You can find gear ratio calculators on line that will tell you how much RPM change you will see from stock to each of the tire diameters you are interested in. At some point, probably at a 285, you will need to change your gear ratio because the decrease in final drive ratio has sucked the life out of your truck. Also, the more weight and drive ratio issues you introduce will increase the wear on the engine, transmission, transfer case, and axels. It will also decrease your braking via the rotating mass and the leverage a larger tire puts upon your brakes.

I'm not saying don't do it, I'm just saying be informed of the tradeoffs as you do it. I have had several trucks with big tires, have regeared, and so on. Nothing done here is done in isolation. One factor affects other systems, so you have to consider those things when making a change and decide if the outcomes, and the associated remedies are worth what you are gaining.

My truck sits on 265/70/17s Load C KOs which represent a compromise between capability and the need to regear at a minimum.

 

It's easy to calculate the moment of inertia for any tire. I'll let you look it up but the gist is that it's proportional to mass*(radius^2). So for a given weight, a larger diameter is worse. Cool, that makes sense.

Tread width and tire pressure will affect your rolling resistance, there's another couple variables. A larger diameter tire throws off your drive ratios too.

It's best to just assume that anything other than stock will affect performance, likely not for the better. Increasing tire size or weight will pretty much guarantee an increase in moment of inertia, and therefore sap more power from the engine because there is more inertia to overcome. In the real world all these variables, and likely others, come into play. How much you'll be able to tell is not well-defined either because of confirmation bias and the placebo effect etc.

All that to say: don't change tire sizes and expect more or even the same power. Do it for some other reason, like how larger tires can traverse taller obstacles, or that they look cooler haha.

 

Good post except for this! Moment of inertia never goes away, so at every speed a larger/heavier tire will be effectively heavier than stock. Kinetic energy never makes up for it. Take 1lb off your bicycle frame and ride it, then put it back and take 1lb off your bicycle wheels, and you'll really feel the difference! How much you'll notice the difference in your truck is the golden question, as we've both said!

 

I guess I wasn't think that part through
I wanted him to think it through and make a choice based on whither he can accept the all the changes he will put into motion. If more people did that, I think we would see fewer posts that start out... Man, I put Load Range E 37s and 400 lbs of armor on my 4 cyl Taco and the power and mileage just went to crap; what gives?
There is no free lunch with this stuff.

 

I went from 265/65/17 stock suspension to 275/70/17 all terrains and 1.5 inch lift in front only, and I can tell you in the street it feels fine but on the freeway the truck feels sluggish. I feel like it struggles to know what gear it should be in for the speed I’m traveling. Gas mileage went down about 1-2 mpg both city and highway. All this said, the truck looks really good haha

 

Haha I know exactly the point you were making, a little proaction and less reaction would go a long way wouldn't it?

Don't worry though, I'm "that guy" who wants everyone to literally do the math and calculate these things, because clearly society would be solved if everyone intimately understood moment of inertia

 

I have the 285 70 17 KO2 C load and still get around 17/18 mpg comb with a lift and canopy. You’ll do better than me. I think it’s a great tire, tough, good weight.
no real change mpg wise from the 265 70 17 C load I was running prior.

 

After running various tire types and sizes on Tacoma’s over the years, I notice a bigger difference with tire diameter than tire weight.

I ran 265/75/16 Cooper ST Max’s followed by 285/75/16 AT3 XLT’s. Both tires weighed the same 55lbs but the 285’s definitely hit MPG and Power more.

 

Thanks for all the thoughtful and scientific responses. Sounds like the difference between the two tires I mentioned will be negligible. This is all still really useful stuff though, because I haven't totally scrapped the idea of 265/70/17s. Goal of my post was just to try to get a better handle on how a tire with more utility off road affects day to day driveability. I don't have a real need for a larger tire, but sometimes it's hard to resist the urge of just slapping on the biggest thing that will fit once you're already messing with suspension.