I have a 2006 GMC Sierra 1500 extended cab 4x2. I have 4 extremely worn tires. We are traveling from Texas panhandle to Carlsbad NM so I need to replace them. The problem is I can only afford used tires at the moment and only 2. I am having a hard time finding the same size. What would be the effect of putting 245/ 75/ R17 on the front and leaving the 245/ 70 / r17 on the back? Also when I eventually get all 245 75 R17 how bad would it be if my spare was still 245/70/R17 until I could find one more?
Along with what @Zaid said, which I agree with, there are two other aspects to worry about:
- Antilock Brake System (ABS)
- Traction control (I'm not sure your Sierra has this, but it bears mentioning)
The problem with the ABS is, with that much of a difference, your ABS will believe there is a fault with the system and shut it down or might believe the smaller tires are losing traction and apply brakes to help gain traction (even though you haven't lost any).
Pretty much the same idea with traction control. If the computer believes the rear tires are losing traction, it will pull power from the engine to compensate. I don't believe your Sierra has traction control, but if it does it has drive by wire (no direct cable from your gas pedal to the throttle body), which means the computer has the control. There is no amount of pressure you can apply to the pedal when it decides to cut power which is going to overcome this. (For reference: I own an '06 Silverado Z71)
The first fear is a definite. The second not so much for you. Since Zaid did the calculations, I'm pretty sure these tires should work for you. His answer deals with whether or not they will work, while mine deals with why it is important to worry about such things.
Something to watch out for
Since you intend to buy second-hand, one thing to watch out for is the age of the tire. From personal experience in desert climate, standard tires usually don't last beyond 3-4 years. A premium tire might buy you an extra year.
See this question to know how to tell the age of a tire.
Putting on a different size
A rule of thumb used by some tire retailers is to not alter the overall diameter by more than 3%.
So the diameter of your original tire is:
17 * 25.4 + 2 * ( 245 * 0.70 ) = 800 mm
If you go for a 245/75:
17 * 25.4 + 2 * ( 245 * 0.75 ) = 776 mm
This is an increase of a whisker over 3%. The rule of thumb recommends says that 245/75 in this case is on the border.
I think you should be fine with this tire size.
Since the vehicle is a rear-wheel drive, placing the larger wheels on the front would have the advantage of maintaining normal sized wheels on the driving axle. This means the drive-train is working at its intended rotation. Placing larger wheels on the driven axle would mean the engine would have to rev lower to produce the same speed. This is probably a good idea for a long trip, but depends a bit on the characteristics of the vehicle (gear ratios and how much it is loaded).
Placing the larger (and newer) wheels on the front also has the advantage of making sure you have steering even if or when you encounter adverse weather conditions or a bad surface. Needing to quickly swerve and finding out those front wheels are bent on going straight forward is not a pleasant experience.
Since weight is transferred forward when braking, this would also give you slightly better braking. However, this should be tempered by the fact that bad wheels on the rear end could very well unbalance the truck and have you broadside across the traffic lanes. Not good.
Having bad wheels on a rear-wheel drive also increases the risk of tire spin, the rear end snaking on acceleration or simply not being capable of starting it up a steep grade.
If it were my choice to make, all things being taken into perspective I would be putting the "good" wheels on the front axle, and keeping the original size "bad" wheels on the rear. And preparing to stop for a rest whenever a summer rain-shower came anywhere near me.
That being said, making a long trip with bad wheels is never a good idea. Besides the obvious physical danger that you are exposing yourself to, you are also exposing other road users to a similar danger. What happens if your heavy truck flips into a small Japanese car at speed? (Nothing against Jap cars, but they do tend to be smaller than a light truck.)
Another consideration that you may not have thought of is: what happens if you get a blow-out -or even worse, two blow-outs- on those old tires during the journey? Even if you manage to stop the vehicle with no further damage done, is your budget up to having these tires changed at a road-side garage? You will be looking to use the spare on the first, but what about the second?
So perhaps the best recommendation would be either to set off the trip if at all possible, or look for any different means of transport your budget allows you.