Upgrading to 31's

[TheGringo]

I just bought my 2005 ford ranger edge 7 months ago and am looking to upgrade from stock tires to 31's. I was wondering if I needed to re-gear my truck and how much stress it would be putting on the engine if I did upgrade. As you can tell, I am new to this so I'm just asking for advice. Thanks!

 

[RonD]

You need to get diameter of stock tires to know what the ratio change will be with 31".

Read here to get stock tire size and current axle ratio from your drivers door label: http://www.therangerstation.com/tech_library/axle_codes.shtml

Then go here to get "actual ratio" after tire change: http://www.therangerstation.com/tech_library/Gear_Tire_RatioChange.shtml

Example
new tire diameter 31
Old tire diameter 28
Axle ratio 3.73 <<< 3.73 is the most common ratio on Rangers

Actual ratio after change 3.37

The lower the axle ratio number the less the torque you will have but higher the speed for RPM
So lower number means less low-end torque, pulling power, and lower RPMs at say 70MPH, so better MPG, but with the heavier wheels/tires and increased ride height you won't see an improvement, lol.

Larger tires always causes lower ratio, and speedometer will be off as well.

New ratio number 4.13
This number is the axle ratio you would change to if you wanted to get back to original 3.73 ratio with the 31" tires,
4.13 isn't an available ratio but 4.10 is common so you would change to a 4.10 ratio
And .03 difference is negligible
So a change to 4.10 would get current performance back to what it is now and speedo would be correct again

Axle ratio is the Ring and Pinion gears in the differential, on 4x4s you have to change both front and rear ring and pinion gears, their ratios must match.

 

[cjk5063]

Which direction will the speedo be off? The PO of my truck put on 31"s and I doubt the ratio was changed.

Sent from my SAMSUNG-SM-N900A using Tapatalk

 

[RonD]

Speedo would read lower than actual speed as tire diameter increases.

Look at a tire's tread on a flat line.

So for example if I roll a 28" tire one revolution it will roll same exact distance every time.
Math for that is 3.14(pi) x Diameter = distance rolled
So 3.14 x 28 = 87.92 inches<< since 28 is inches answer is inches

Speedometer is setup for axle ratio and tire diameter, so if stock tire was 28" then speedo expects vehicle to travel 88"(7.33ft) each time axle rotate 1 time.
That 720 rotations for each mile, 5,280ft(mile) / 7.33ft = 720

If I used 31" tire then
3.14 x 31 = 97.34 inches per rotation, 8ft per rotation
5,280ft / 8 = 660 rotations per mile

So 60 rotations less per mile traveled.

If speedo is showing 60mph, or 1 mile a minute, then with 28" tire setup that is 720 revolutions per minute.
31" tire is traveling over 1 mile in that 720 revolutions, 60 revolutions more, 480ft more
So actual speed or distance traveled is higher than speedo or odo will show.
People that use odometer to calculate MPG will often freak out after adding taller tires, lol.
Because odometer no longer shows true miles traveled on xx gallons of fuel, miles will be lower than actually traveled