Help with tire sizing - Diesel Forum

danuary · 10-31-2009, 12:39 PM

Hi,

I have an '08 F250 PSD w/ the OE Contis (LT275/70R18E) and 3.73. The tires are just about done for, after only 25k miles (I ended up in a ditch in the weather we had in Colorado this week!)

Looking for suggestions for replacements. I'd like to get larger tires, on the same wheels, not doing a lift. Anyone know how big is reasonable to expect? If I did go to larger tires, should I be concerned about the tranny? I'm not thinking I'd be going *that* much bigger...

Any thoughts, comments much appreciated...

**SmokeyWren** · 10-31-2009, 04:37 PM

Your wheels are 8" wide. You can probably get by with tires that require 8.5" wide wheels, but not wider.

BFGoodrich AllTerrain is a very popular replacement tire. They come in
LT265/65R18, which requires minimum 7.5" wide wheels
LT285/65R18 which requires minimum 8" wide wheels
Lt305/65R18 which requires minimum 8.5" wide wheels.

So 285/65R18 is the biggest tire that BFGoodrich says you can run. But some folks run LT305/65R18 with no problems except the hassle of some folks pointing out that your tires are a bit too big for your wheels.

Toyo has an LT285/75R18 in the Open Country A/T tread. Those puppies are 34.8" diameter, or taller than most other so-called 35s. And that's the biggest tire Toyo makes in that tread that they say will work on your 8" wide rims.

**jsmichael** · 10-31-2009, 10:19 PM

Quote:

Originally Posted by danuary

Hi,

I have an '08 F250 PSD w/ the OE Contis (LT275/70R18E) and 3.73. The tires are just about done for, after only 25k miles (I ended up in a ditch in the weather we had in Colorado this week!)

Looking for suggestions for replacements. I'd like to get larger tires, on the same wheels, not doing a lift. Anyone know how big is reasonable to expect? If I did go to larger tires, should I be concerned about the tranny? I'm not thinking I'd be going *that* much bigger...

Any thoughts, comments much appreciated...

Those Continental tires that Ford used on the '08 Super Duty are worthless tires in the northern climates where the roads are slippery. Ford ought to be ashamed of themselves for selling $40-50K trucks in the northern climates with those tires.

I purchased my truck in February 2008 and drove it for less than an hour before I realized those tires are dangerous for driving on slippery roads. At the time the only tires for the 18" rims were Michelin and Goodyear, and I chose the Goodyear because of past experience with the Wranglers on our Expedition.

Today you have a wider selection of tires to choose from. I had BFGs on my '99 Super Duty (285/75R16), and those were good tires for winter driving. I don't think you can go wrong with the BFGs for your '08 Super Duty on the 18" rim, but there are a lot of guys who swear by the Michelins also.

Bigshow · 11-01-2009, 12:32 AM

There is a new tire from Goodyear, the Duratrac, that I would consider. I would also look at the Toyo AT's. Depending on what type of roads you drive - we are finding the BFG AT's to be wearing out a bit too soon with any amount of gravel, etc. They are a very good all around tire, as their popularity attests to that!
The Toyo MT's I have are awesome in every way with the exception of the ice/snow when the temperature is close to freezing. When its colder and the snow/ice are not as wet, they are fine.

danuary · 11-01-2009, 12:42 AM

Appreciate all the responses! After a day of research, I've settled on the Goodyear Duratracs -- they're available in 295/65R18, so I'm going to give that a shot and see how it works out. Seems to be a nice way to get a little wider tire on that rim...

Thanks again!

**SmokeyWren** · 11-01-2009, 09:09 AM

Quote:

Originally Posted by danuary

...they're available in 295/65R18, ...

Sounds good. And Goodyear says those tires fit on an 8" wide rim.
Goodyear Wrangler DuraTrac
Then scroll down to the bottom of the screen and click on "View tire sizes available..."

Those are not only wider, they are also an inch more diameter than your stock tires. So your speedo, odo, and trip meter will be slower by about 5 percent unless you have the speedo calibrated for the taller tires. And your 3.73 rear axle ratio will "feel like" about a 3.55. Theoretically the longer legs will help MPG, but the taller, wider tires will have more drag, and that drag will more than offset the gain, so expect about a one MPG loss in fuel mileage.

The Goodyear Wrangler DuraTrac is set up to add ice studs, so if you wanted some very expensive winter tires, those would do that too.

danuary · 11-01-2009, 10:18 AM

Quote:

Originally Posted by SmokeyWren

Those are not only wider, they are also an inch more diameter than your stock tires. So your speedo, odo, and trip meter will be slower by about 5 percent unless you have the speedo calibrated for the taller tires. And your 3.73 rear axle ratio will "feel like" about a 3.55. Theoretically the longer legs will help MPG, but the taller, wider tires will have more drag, and that drag will more than offset the gain, so expect about a one MPG loss in fuel mileage.

I'm just curious, why do you say they're taller? Did I miss something? Not that it's going to matter from a fittment perspective; I'm just trying to make sure I understand for the next time i need to do this.

My math on this showed:
275-70/18s = 192.5mm sidewall
285-65/18s = 191.75mm sidewall

That is to say - nearly identical. Checked the websites for GY and Conti too, Conti lists outside diameter about 3/10 of an larger than the GY's.

Thanks!