What Tire Size ?

nightrider2006 · December 5, 2014

Yes, the RC 2.5 kit.

Any rubbing? I like the stance. I'm getting ready to buy a 15 and I'm trying to figure out what I want to do

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junkyardz · December 6, 2014

Routine driving, nothing special. The difference is the programming that allows upshifting and V4 operation the way the truck was designed. If you run oversize tires and don't correct the ECM, shifting is delayed and V4 operation is inhibited because the truck thinks you have a big load because your indicated speed is lower than it should be for conditions.

It's still a truck, so the mileage still drops above 70-75 mph. I rarely get over 22mpg for a tank because I cruise closer to 80 than 70. My best 400 miles per the DIC is 22.9, my overall average for my first 34K miles is 20, which has been pretty consistent since day 1.

Easy cruising around town between 40-60mph always yields the best mileage of 22-24, stop and go traffic it drops to 18, sometimes as low as 16. Overall I do about 500 miles per tank with an overall average of 20.

Bottom line is if you're running oversize tires, invest in a Hypertech speedometer calibrator. My dealer's service dept and manager has blessed it, too, because I'm only correcting the speedometer as opposed to trying any aftermarket tuning mods. Fixing the speedometer is all the truck's factory program needs to correct shifting, V4, etc. to match the way it was designed.

Man, those numbers seem unreal! I run 295/55/20 toyo mt with removed valance and leveled and get nowhere near those highway or city numbers... I couldn't get those numbers bone stock off showroom.

blackout07 · December 6, 2014

Man, those numbers seem unreal! I run 295/55/20 toyo mt with removed valance and leveled and get nowhere near those highway or city numbers... I couldn't get those numbers bone stock off showroom.

I don't see numbers like that in our stock 15 Tahoe.

5RWill · December 6, 2014

I think I get 400 miles per tank about 17-20mpg highway 13-16 city stock

Spurshot · December 6, 2014

My DIC says something like 13.7 for the 10k I've driven it. Most of that is city and with 33x12.50 mud tires.

CAtaylor · December 6, 2014

Bottom line is if you're running oversize tires, invest in a Hypertech speedometer calibrator. My dealer's service dept and manager has blessed it, too, because I'm only correcting the speedometer as opposed to trying any aftermarket tuning mods. Fixing the speedometer is all the truck's factory program needs to correct shifting, V4, etc. to match the way it was designed.

This isn't exactly correct. With taller tires the engine has a lower mechanical advantage then what you had with the factory small diameter tires so no matter how you correct the spedo, unless the wrong diameter is put in the program, you will loose mileage. I had a Hypertech on mine also, did nothing to help mileage or make the truck start using V4 again as this is all load based and the shifting was crap because it would shift I to 6th at 35 mph if you were just easing around. I could get a consistant 14.5 mpg on the highway. Only thing that correct that and give the advantage back to the engine is gears. Even with gears best I could get highway at 70 was 17.5.

DanielAcosta · December 6, 2014

This isn't exactly correct. With taller tires the engine has a lower mechanical advantage then what you had with the factory small diameter tires so no matter how you correct the spedo, unless the wrong diameter is put in the program, you will loose mileage. I had a Hypertech on mine also, did nothing to help mileage or make the truck start using V4 again as this is all load based and the shifting was crap because it would shift I to 6th at 35 mph if you were just easing around. I could get a consistant 14.5 mpg on the highway. Only thing that correct that and give the advantage back to the engine is gears. Even with gears best I could get highway at 70 was 17.5.

The specs on the 5.3 you sold with 6" lift and 4.56 gears guaranteed lower mileage. In simplest terms, you were turning 33% higher rpm for any given speed (4.56/3.42=1.33), which inverted equates to a 25% reduction in mileage (1/1.33=.75). So your 17.5 at 70 would equal 23 with a 3.42, which is about what I get when I go that slow.

My 5.3 3.42 with 34's is much milder, which is why my mileage hit compared to stock is much smaller. You mentioned 6th gear at 35, mine doesn't hit 6th til 47, IIRC stock with a 3.42 hits 6th at about 44. The oversize tires effectively changes the rear end ratio, which you accurately stated. My effective rear end ratio is 31.91 (stock height) / 34.07 (my height) times 3.42 = 3.20, so well within the factory range of 3.08 - 3.42 - 3.73, so you would expect my performance to be somewhere between a stock truck with a 3.08 and a 3.42. My tires are heavier and I have a small lift, so my mileage will drop a little, but nothing compared to the huge lift and rear end you had. 31.91 (stock) / 33.94 (your height) times 4.56 = 4.29 effective rear end, well outside the factory range of 3.08 - 3.42 - 3.73. Everyone knows the 3.08 is for mileage and 3.73 is for max towing, so you were definitely set for a big load at the expense of mileage, not to mention whatever your Black Bear did. It would have been interesting to see how it would have driven without the black bear and just correcting for the tire size and rear end change, which Hypertech provides for. I did some experimenting by inputting extreme changes in tire size, and it radically changed the shift points and V4 engagement, so Hypertech does more than just a simple speedometer/odometer correction. It's probably more accurate to state that the truck's ECM takes the changes in tire size and rear end ratio and adjusts per the mapping GM provides. Since my changes fall within the range of factory specs, I still have good results. Yours was way out there, plus an aftermarket tune, so all bets are off.

The dealer is only allowed to input stock tire changes, which are all around 32 inches tall. All the Hypertech does is give the ability to input a different tire size into the ECM, plus a rear end change. I'm guessing the Black Bear does a whole lot more, but realistically it sounds like the mileage you were getting with a 4.56 rear end was right where it should be.

Bet you're having fun with the new Duramax, congrats!

CAtaylor · December 6, 2014

The specs on the 5.3 you sold with 6" lift and 4.56 gears guaranteed lower mileage. In simplest terms, you were turning 33% higher rpm for any given speed (4.56/3.42=1.33), which inverted equates to a 25% reduction in mileage (1/1.33=.75). So your 17.5 at 70 would equal 23 with a 3.42, which is about what I get when I go that slow.

My 5.3 3.42 with 34's is much milder, which is why my mileage hit compared to stock is much smaller. You mentioned 6th gear at 35, mine doesn't hit 6th til 47, IIRC stock with a 3.42 hits 6th at about 44. The oversize tires effectively changes the rear end ratio, which you accurately stated. My effective rear end ratio is 31.91 (stock height) / 34.07 (my height) times 3.42 = 3.20, so well within the factory range of 3.08 - 3.42 - 3.73, so you would expect my performance to be somewhere between a stock truck with a 3.08 and a 3.42. My tires are heavier and I have a small lift, so my mileage will drop a little, but nothing compared to the huge lift and rear end you had. 31.91 (stock) / 33.94 (your height) times 4.56 = 4.29 effective rear end, well outside the factory range of 3.08 - 3.42 - 3.73. Everyone knows the 3.08 is for mileage and 3.73 is for max towing, so you were definitely set for a big load at the expense of mileage, not to mention whatever your Black Bear did. It would have been interesting to see how it would have driven without the black bear and just correcting for the tire size and rear end change, which Hypertech provides for. I did some experimenting by inputting extreme changes in tire size, and it radically changed the shift points and V4 engagement, so Hypertech does more than just a simple speedometer/odometer correction. It's probably more accurate to state that the truck's ECM takes the changes in tire size and rear end ratio and adjusts per the mapping GM provides. Since my changes fall within the range of factory specs, I still have good results. Yours was way out there, plus an aftermarket tune, so all bets are off.

The dealer is only allowed to input stock tire changes, which are all around 32 inches tall. All the Hypertech does is give the ability to input a different tire size into the ECM, plus a rear end change. I'm guessing the Black Bear does a whole lot more, but realistically it sounds like the mileage you were getting with a 4.56 rear end was right where it should be.

Bet you're having fun with the new Duramax, congrats!

Thanks, I do enjoy the Duramax. The 14.5 mpg was just with the 3.42 factory gears and Hypertech calibrator installed. Just the 4.56 gear install netted a 3 mpg improvement. I think that is where a lot of people don't see the benefit of regearing. The Math doesn't work out, ha! You are increasing the rpm of the engine but you are lowering the amount of work that the engine has to do by increasing torque at the wheels with the gearing advantage. If it works for you that is great and it is not the norm; I just didn't want everyone thinking they can dump $250 in a calibrator and miraculously net a 7 mpg gain from where they are now. Also just curious what tire diameter you have in the Hypertech? I see you mentioned 34.07" mentioned above. That tire probably has an actual diameter of 33.25 or less. The tires I had on mine were 35.5" but had an actual 33.86" diameter. I had to use 33.75" in the Hypertech because it only adjusted in 1/4" increments.

DanielAcosta · December 7, 2014

Thanks, I do enjoy the Duramax. The 14.5 mpg was just with the 3.42 factory gears and Hypertech calibrator installed. Just the 4.56 gear install netted a 3 mpg improvement. I think that is where a lot of people don't see the benefit of regearing. The Math doesn't work out, ha! You are increasing the rpm of the engine but you are lowering the amount of work that the engine has to do by increasing torque at the wheels with the gearing advantage. If it works for you that is great and it is not the norm; I just didn't want everyone thinking they can dump $250 in a calibrator and miraculously net a 7 mpg gain from where they are now. Also just curious what tire diameter you have in the Hypertech? I see you mentioned 34.07" mentioned above. That tire probably has an actual diameter of 33.25 or less. The tires I had on mine were 35.5" but had an actual 33.86" diameter. I had to use 33.75" in the Hypertech because it only adjusted in 1/4" increments.

My tires are 34 unladen, 33 with the weight of the truck. I experimented with the 1/4 inch increments that Hypertech offers, calibrated with two GPS's, and discovered entering 33 in the Hypertech gave the most accurate speed and distance results.

CAtaylor · December 7, 2014

My tires are 34 unladen, 33 with the weight of the truck. I experimented with the 1/4 inch increments that Hypertech offers, calibrated with two GPS's, and discovered entering 33 in the Hypertech gave the most accurate speed and distance results.

Good deal. Should be very close. Enjoy the truck.