Can This Really Be Happening

[KyRob]

My 04 Sierra 4x4 Xtra cab has 373 gears and a 5.3 engine. The stock tire was a 245/75/16 and I was consistently getting 16 to 16.5 mpg in town and a little over 19 on the highway. I bought 265/75/16 tires awhile back and checked my mileage last night and had 11.9 mpg for in town driving. Could this tire change really make this big a difference? I didn't have the computer reset and my speedomoter is almost 3 mph off. They wanted 100 bucks to do it and I thought that was kinda high and besides, I can't tell any difference in the shifting. Anyway, did I figure wrong or do I need to sell these tires and go back to the 245's?

 

Thanks,

Rob

[redvett]

Did you recalibrate for the difference in tire sizes? Also if the tire is more aggressive in tread pattern you will decrease fuel economy.

[gave20]

I have a 6.0 and had the same thing happen, even after recalculating the new size. Even after my tune I'm still not up to the 245's MPG's. For me the biggest factor is PSI and rolling resistance, if I kept my tires aired up to 60-65PSI I could get 16-17MPG no problem, I'd just replace tires every 20K miles.

[jjrobo]

I have almost the exact same truck, except mine came with 265/70-17. I got about 1 mpg lower all around than you did with my stock tires. When I switched to an agressive AT, load range "C", same size tire I lost 1.5 mpg.

[Zembonez]

Not sure if this helps you but, I lost a full 2 MPG+ by going to my 20s from the stock 17s. I went from 18.6 to 16.4 average. There wasn't any more difference in overall height than your tire swap. I can't believe how sensitive these new trucks are to tire size. My previous GM trucks seemed much less affected by this... Either that or gas was just cheaper and I didn't notice!

[pm26]

That makes some sense to me. I had a '95 Nissan pickup (4x4 V6) that originally came with 31X10.5R15 tires, which are quite large for a small truck. To get more torque and better towing performance, I downsized to P235/75R15 tires, which dropped the truck about 1 inch. However, the mileage did not improve with the smaller tires because the RPMs increased by 10% in all gears.

I also effectively lowered my 4.23? rear end gears to something like 4.63 gears, which gave me 10% more torque at the rear wheels. But the mod was well worth it. Those big tires were just too much for that truck. The mileage remained around 17 MPG HWY or city . keep in mind that the difference between the circumference of the tires is a whopping 10%!

 

With GM trucks, they get decent fuel economy at cruising speeds only by dropping the RPMs to ridiculously low levels. If these trucks were geared like my Nissan was (4000 RPM at 80 MPH in 5th gear), the mileage would be in the single digits. And the mileage would not vary with slightly different tire sizes. But at very low RPMs these trucks are apparently more sensitive to any tire size/tread change.

[cc1999]

I recently ordered several sets of tires for my fleet trucks and just for grins decided to weigh the 10 ply load range Es and compare them to an identical size 265/70/17 set of P metrics that I ordered for my personal 1/2 ton truck. There was 11 pounds difference between the E's and P's . That could hurt your milage for sure. Increasing your rotating mass is going to take more power to turn them and in turn burn more fuel. So if you went up in size and also went up in load range to C or worse E's then I think you could easily drop 2-3 mpg. I think the other part of the problem might be that you are using the trip meter that is actually loging less miles than you really traveled since it would be aff just like your speedo .

[hubbycap]

I have almost the exact same truck, except mine came with 265/70-17. I got about 1 mpg lower all around than you did with my stock tires. When I switched to an agressive AT, load range "C", same size tire I lost 1.5 mpg.

 

 

Not picking on you, jjrobo, but in-as-much-as I (we all) have gripes concerning the General Amertracs, due to their difficiencies in so many areas, one thing they are not deficient in is rolling resistance and the bottom line: MPG. GM did not choose this tire strickly on the low-bidder method. Much discussion in this forum concerns the extremes that manufacturers will go to eek out a couple of 10ths of a mpg. Tire choices are not just the luck of the draw. Fleet purchases are weighted heavily on efficiency. Unlike my own, GM's budget on testing for the most efficient combinations is large because the bragging rights on the EPA figures are equally large.

 

There is no single tire that can deliver handling, style, efficiency, load range, traction and life all at once. One shouldn't complain about a replacement tire's lack of efficiency when it was chosen to satisfy other requirements. If you want a particular performance feature, choose a tire that delivers that, and be prepared to accept the other consequences of a tire that only excels in that area.

[az'03Silverado]

Same thing with mine. I lost about 2.5mpg with a larger, heavier tire. I went from 235/70/16 to 235/85/16.

[hogpowr]

 

Going to an aggressive tire, larger around, heavier, WILL hurt economy. I feel GM choose those Ameritrac's to hit those high epa numbers.(best in class) You can go to www.tirerack.com and research the weight of the tires you are considering. An LT tire will wiegh more due to more strength than a p-metric car tire. I love my new BFG All-Terrain TA ko's, but I miss the fuel economy of the Americraps.

[tigerbait]

replaced the stock Generals w/ Revos and lost about 1.5 MPG.....probably due to the aggressive tread pattern.

[keoni1978]

I'm looking to replace my 18s with 22s. Overall it'll add about 5 lbs. Hopefully that wont do too much damage on the mpg

[pm26]

I looking to replace my 18s with 22s. Overall it'll add about 5 lbs. Hopefully that wont do too much damage on the mpg

 

Why on earth would you do that? The new tires will have very skinny sidewalls and the probability of damaging the rims when you hit a pothole will be high. Same thing for curbing a wheel. If this is a 4X4 truck, i would never even consider ridiculously large rims like these. Plus, the cost and limited choice of 22 inch tires is something that is not trivial.

[hubbycap]

I looking to replace my 18s with 22s. Overall it'll add about 5 lbs. Hopefully that wont do too much damage on the mpg

 

Why on earth would you do that? The new tires will have very skinny sidewalls and the probability of damaging the rims when you hit a pothole will be high. Same thing for curbing a wheel. If this is a 4X4 truck, i would never even consider ridiculously large rims like these. Plus, the cost and limited choice of 22 inch tires is something that is not trivial.

 

 

Hey pm26, what are you doing? We're talking about my bread and butter man! I'm crazy about the 22s. I've got two girls in college and need all the lowpro pothole bend repairs I can get. Help a working man out here. LONG LIVE THE LOW PROFILE!

[cerbomark]

also if you are using the DIC to calculate MPG s it might be off because you did n t have the computer re set for the different size. That will throw of your MPH, Miles traveled and also your MPG average.I don t think you ll lose more than 1 MPG with the tire swap you did after you get it re set.