Larger Tire Impact Question

For the math on lbs for rims to performance impact there have been alot of calculations/estimates done over time on various physics/car racing/etc forums. Most result in a 0-60 type acceleration equivalent weight of 1.5x on the low end to 10x depending on assumptions made. For a real world example this is a nice dataset: https://www.caranddriver.com/features/a15130598/upsized-wheels-tires/
Where a 11lb per corner (15” to 18” rims) increase results in a .3 0-60 reduction that equates to about 130-140lbs added for that Golf, or just about 3x the wheel combo weight increase theoretically added to the curb weight. Also a 1+ mpg loss for those 11lbs added per corner. Using that 3x for this threads 30lb increase per corner would be like rolling around with 360lbs (or about a 6.9 to 7.3 0-60 change, or put another way losing 21 engine hp). These changes aren’t enormous, but the change isn’t insignificant either and there are lightweight rim and tire combos that can get closer to net 0lbs overall change.

 

That is a test of different tire profiles, not weight. You are making the erroneous assumption that the weight difference is the cause of the acceleration difference. The factor for weight would be 1.5x at most, the rest being mostly rolling resistance, and some aero resistance.

This is shown by the MPG test as well. Their fuel economy test is at 75 mph constant on the freeway, so there would be no inertial factors at all. The 10% increase in fuel consumption is due to a nearly 10% increase in rolling+aero drag at that speed.

 

In my 40+ years of experience owning and operating a multitude of trucks the only positives to bigger tires I have found are the improved cosmetics and enhanced off road performance due to the added traction and increased road clearance. Primarily attributable to the higher gearing with the larger tire diameter, the negatives are lower gas mileage, incorrect speedometer and odometer readings, won't hold high gear as well on hills, slower acceleration, and decreased responsiveness or more sluggish feeling to throttle input. Additionally, there is typically more road noise, rougher ride, won't corner or handle as well, stop as quickly, etc. These negatives are all subtle but additive and obviously increase in effect the larger the tire. Unless you are doing a lot of off-roading, the negatives greatly outweigh the positives, with the exception of the looks!

 

A lot of these can be mitigated through tire selection. Dont get a 35x12.5. Do a 35x10.5 or 11.5(or a 285/75R18) and you save weight and rolling resistance. I would bet that if you can keep the 35 close to the same width as the stock tire and minimize your weight increase, the MPG change will be small.

Also, the power issue is not a huge deal with these Twin turbo V6's and 10 speed transmissions. I went from 275/55R20's to 315/70R17's on my F150 Ecoboost and it has little issue with them at all. I can tow my travel trailer at 70 mph in 6th gear turning 2000 rpm with the 315/70's on it. And that is in Utah at 6-7000' of elevation. They produce enough torque, and do it at a low enough RPM, that it really doesnt bother them too much.

The hit was FAR more noticeable going to a measily 285/70R17 on my Lexus GX that has an NA V8.

 

A common misconception, but untrue. The part about tire selection is true, but if you focus on width or weight you'll miss the most important parts.

Rolling resistance will decline with width if everything else is the same. Aero drag will go up a bit, which will impact highway mpg though. Tires with lighter load ratings will usually have more flexible (and less durable) sidewalls, which would help with mpg. Weight will make some difference but it's in the noise; maybe 2% mpg overall for a 40 lb/tire difference. That's actually how much heavier my tires are vs stock.

The biggest factor is the materials and construction and how they effect hysteresis... but no one measures this on truck tires! We just have to guess or go off a collection of anecdotes. The tires I have are excellent in this respect, but no longer made. The best proxy I know is to look at the speed rating of tires that are the same size and load rating. Higher is better... it means the tire can run faster before it overheats... and the heat being generated is the same energy that is causing rolling resistance. For E load tires an S speed rating is usually the best.