Says a klein

thats right

 

Ok so heres the deal. The tires in the rear are ALL 245/75/16. 3 of the rear tires are the same tire/tread pattern, but on the ds inside rear the tires is diff and has taller lugs. They are the same tires, but the inside tire is OBVIOUSLY higher than the outside tires. The front tirese are 235/85/16, which I THOUGHT was the right size tires for this truck? What does having a shorter tire in the rear do for mileage count and economy? I'm pissed now and i'm going to go ocd over this and get new tires I think.

 

when you run mis-matched tires on either side of the axle, you spin your spider gears inside the carrier at double the rate of speed of the axle speed. Spinning the spider gears that way will SMOKE your rear axle. Normally, the spider gears hardly turn while running down the road, unless or until a tire is spinning or you are going around a corner. Running them constantly is BAD. If you have a posi (limited slip) the clutch plates are probably gone by now.

I'd be surprised if the entire carrier assembly is not already toast. Count on spending some $$$ if it is. Costs way more to set up a new rear carrier than to put the right tires on the truck. Even if you have to run used tires, get 'er done.

Same deal with 4x4 trucks, BTW, except that you smoke the transfer case instead of the carriers.

This may help illustrate the issues:

HowStuffWorks "Open Differentials"

 

^^ as always - excellent post ^^

 

What I'm saying though is the rear tires ARE all the same size, except they are 245/75/16 and the original tires for this truck are 235/85/16. One tire in the rear is mixmatched but is still the same size as all they others.

 

If all the tires are the same size - you shouldnt have any problems at all.

 

My issue is, what is running a 75 sidewall and 245 width, as opposed to the 85 sidewall and 235 width going to do? I've heard running the duals this wide without spacers is bad for them? Even though they dont touch. And what does running the shorter sidewall do for the speedo and mileage?

 

the 245/75/16 is about the same height tire as what comes factory on these rigs 215/85/16. the 235/85/16 rub in the front when turning without trimming the plastic inner fender. the 245s will rub on back if you put a good load on it but are fine just driving.

 

Oh ok so as long as there is no load I'm good? Wont the slightly smaller sidewall eat up more gas or give me incorrect speedo readings?

 

One or two tire sizes won't make that much difference in fuel economy, but can change speedo readings for sure. Borrow a Tomtom or some other GPS and take it for a drive. See if the speedo matches the GPS. If not, make a change in tire size (seems like that is best for you), change the speedo gear, or learn to live with doing calculations in your head when you drive.

General rules of thumb:

Wider tires eat more fuel.
Taller tires eat more fuel.
Heavier tires eat more fuel.
More plys eat more fuel.
Bias ply eats more fuel.
Aggressive tread eats more fuel.

So, properly sized, skinny, light, radials with street tread, will give the best economy in most cases. If fuel economy is the desired goal, purchase tires accordingly. On the other hand, if you need to haul heavy loads, drive through mud or snow, pull a trailer, need to gear lower, etc., then pick tires that will help in that regard and stop worrying about fuel mileage.