I THINK I have narrowed down my tire choices to 3. BFG AT 285/65/18's, BFG AT 305/65/18's, and ProComp Xtreme AT 325/60/18's.


1.With stock tires (275/65/18 BFG's) my speedo reads about 3 mph faster than what the city speedometer reads that the police put on the sides of the road (speedo says 55- really going 52). My question is this. How will 285's affect the speedo? 305's? 325's?

2. What kind of power and mpg loss will I see from 285's? 305's? 325's?

Thanks in advance!!!

 

Anyone?

 

First of all, I doubt that the roadside speedo is all that accurate.

Even if it is, factory speedos are only accurate to about 2-3 mph anyway.

If you want to find out how far off your speedo is with new tires, divide the new tire diameter by the old tire diameter (should be a bit larger than 1.0 around 1.05 to 1.1). Multiply that ratio by what ever speed you want, and you'll get your new speed. Also, your odometer will be off as well. Take your current mileage, subtract your milage from the moment you put the new bigger tires on, multiply by the diameter ratio (new/old), and add to the mileage from when you put the new tires on. THis will give you your actual mileage.

example:
Old tire diameter: 31 in
New tire diameter: 33 in
Put tires on at 35000 miles
My odometer currently says: 50000 miles

Actual mileage on truck = 35000+(33/31)(50000-35000)

= 50967 miles

See how your indiated milage (looking at the odometer) is not correct? The larger the new tires you get, the larger the diameter ratio will be, and the larger the error in the speedo/odometer readings will be.

If you want to have a correct speedo/odometer, I'd suggest getting an aftermarket computer. But keep in mind that unless you have it the moment you put the tires on, your odometer will NEVER read correct.

As far as power losses, it may not really be noticeable. The truck will handle differently though because the new tires will be so much wider than the old one. You'd be surprised how much of a difference it makes. What gears do you have? Besides, if you have the 5.4L, I wouldn't worry about it anyway.

I just got new Maxxis Buckshot Mudders in 285/70-17 (33" by 11.5") and I noticed an immediate change in handling and a somewhat noticeable drag (feels like power loss). I've got 3.73 gears, but also have the 4.6L engine.

Hope this helps you!

 

Thanks! I have the 5.4 with 3.73LS. I've heard that a dealer can recalibrate your speedo and odometer. Is that true?

 

Yes it is, I'm actually having mine reflashed this weekend since the 35" muds are on now. Oh, and go with the 305/65-18

 

07fx4silver, do you have any problems with rubbing or the tires fitting on the stock 18" wheels, your truck looks great i want to put 35x12.5 on mine but not sure if they fit with just the 2.5" leveling kit, i may need to get some blocks or aal for the rear thanks

 

those 35x12.50 tires will fit with just a 2.5" leveling kit and look great

 

Your MPG will all depend on your driving habits. If you put bigger tires on you cover more ground in one revolution then you did before. As you are aware of because you asked this question it takes a little more power to get the bigger tires turning.

If you do a lot of city driving you will see less MPG
If you do a lot of interstate driving you might see better MPG because your engine RPM will be a little less with bigger tire at the same speed you would drive with your current tires. Now this is true only at actual speeds you will have to get the speedo fix for this statement to be ture.

Please understand that this does not mean you can put on a 40” tire and get 5 more miles per gallon. If you go too big with the tires and don’t regear your RPM will be so low that you transmission would be downshifting all the time to maintain speed or never even getting into O/D

See below for RPM for the tire sizes you are looking at

Tire Dia---InchPR---MPH---Gear Ratio---Engine RPM O/D
32.1------100.85---65------3.73--------1776
32.6------102.42---65------3.73--------1749
33.6------105.56---65------3.73--------1697
34.1------107.13---65------3.73--------1672


"I am by no means an expert i just play one on the radio" Walton and Johnson show

 

Here is an excellent tire calculator for what your trying to figure out, just input your stock tire size then the size your looking for and it will give you speedometer adjustment readings.
http://www.miata.net/garage/tirecalc.html

 

He asked this back in 2007. He probably bought the tires already.