I have a 2018 f150 RCSB 2wd and I’ve been stuck a couple times here lately because I work construction and often drive to the job site. Instead of trading in for a 4wd now loosing money I was thinking about putting some maxxis buckshot 2 on the back wheels. I’ve done some research and know you need to reprogram the speedo for it to read properly and was wondering if the speedo is read of the front or back wheels

 

I believe it's measured at the transmission output shaft (rear wheels). That said, you may throw an ABS/TSM/VSS code depending on how much bigger they are.

 

I wouldn’t be going to much bigger maxxis only make a couple sizes that would fit. my stock tires are the 245/70r17 Michelin street tires and there about 30.5 ish inches and I would probably be going to a 265/70 maxxis which is about 31.5 inches or even a 285/70 which I think is about 32.7 ish tall that couldn’t be that big of a change could it? I’m not very knowledgeable about tires

 

There are a few simple ways to DIY, one some tuning units allow tire diameter correction.Two FORScan can program for tire diameter. I used FORScan, and set it according to GPS recorded speed.
Otherwise a dealer should be able to do this. KM

 

From what i understand, the dealerships lost the ability to do this on trucks from the last couple model years. FORScan will do it, there are a couple of calibrators that you can buy from rough country and hypertech that'll do it, most tuners have a way to change the tire size as well.

There's no technical problem running different tire sizes on the back of a 2wd truck, but it might look funny to have an aggressive mud tire out back, and not have it up front.

 

Long before 4x4 trucks were commonplace, we put larger mud grips on a 4x2. Trucks without mudgrips looked funny. It wasn't until I started to drive in the sand that I increased the tire size on all 4. And learned that a wider footprint was necessary.

 

Funny how times have changed.

 

I think you are right...A few years ago my local dealers (scam artists) said they could......But every time I paid them to change my speedo (11-14 trks) they came back with no change & read exactly the same as before !!
Also another concern is the shift points for the transmission??
Tire calculators suggest you stay within 3% of your original tire size or it will affect your braking?? ( I could never get my head around that?? )

 

The truck uses the tire size calculation, fundamentally, to determine the truck's effective final drive ratio. That's a combination of the rear end gearing and the tire size, and it uses that to calculate speed (for the speedo) and distance traveled (for the odometer). It also uses that calculated final drive in the shift tables for the transmission, where it determines when to shift based on things like throttle input, load, speed, etc. When you change your tire size, your effective final drive changes. So from the transmission shift point perspective - if the truck thinks your tire size is the stock 32.1" size, and it was going to make a shift at 2,000rpm, but you've upped your tire size to 34.5", the transmission is going to make its shift 140rpm off of where it should have. I know that doesn't sound like a lot, but it does affect the way the truck feels.

As for braking, when you increase the diameter of the tire, the brakes have to work harder to stop it. It's a leverage problem. Think about it this way - if you have a bicycle upside down on the ground, and you spin one of the wheels really hard, try and stop the wheel using your hands by grabbing the wheel close to the axle. It's f-ing hard and you can't do it. Now do it again but grab the wheel closer to the tire, it's much easier to stop it. Then do it with a wheel barrow with a small wheel, the tire is much easier to stop. Same thing with the brakes on your truck, a smaller diameter wheel is much easier to stop.

Also, typically when you increase tire diameter you increase tire weight (substantially, too, when going from a stock size p metric tire to much bigger LT tire), and that increase in weight is almost all at the point that's the farthest away from the brakes, and that has an impact on braking performance. Going from a stock tire, they weigh ~42lbs maybe. Put on an LT tire 35x12x5, now you're in a tire that weighs 65-75lbs. That's a big increase in rotational weight as well as an increase in tire diameter. You'll have to push a little harder on the brakes to get those tires to stop in the same distance.