LS Swap MPGs_What're you getting?

[Sean C]

Hey guys! Just wondering what other LS swapped trucks are getting for gas mileage? My 04 5.3 with around 300 awhp gets about 11 MPGs in the city and does worse on the interstate. I'd estimate interstate MPGs are around 9 at 80 mph (right at 2000 RPM) which is awful.

My truck has a 4L65E and the stock rear end, 3.07(?). Maybe that gear is too short for the overdrive gear?

Also, the truck in general is a brick as far as aerodynamics are concerned, any ideas on streamlining it a bit? Something in the engine bay/wheel wells maybe?

Thanks guys!

[GMC-YA68]

This sounds extremely off........ I have a stock 6.0 denali AWD with heavy azz 26" wheels on it and I get 12 city, and at least 16 highway....

[Sean C]

I completely agree. The engine was rebuilt and cammed with a goal of 400 hp. When the truck was tuned, I was told the rear wheels only hit around 300hp. I haven't seen the spec sheet yet, but that information is on its way from the tuning shop. I had figured that even with 400hp I should be getting at least 13mpg. I'm not sure what the issue is.

[BR3W CITY]

~380rwhp/~450chp w/ my heavy 4l80e,12bolt, and 15x12 rears w/drag radials (yay rolling resistance lol)

In my 66 I get about 13-15city and 17-20 on the freeway. This is also with a slightly wonky convertor/od setup right now, so I'm on the lower end of that spectrum, but I have seen the 19-21 in ideal conditions with smaller rear tires.

You cant do much for aero on these trucks, aside from getting them lowered a tad and putting hard, narrow tires on (which sucks, so don't actually do that). Your best bet is to lose weight in the truck, its the next best thing for mpg and pwr/lb

[gottattooz]

68 swb c10 with 4.8l/4L85e 3.73 gear, all stock, getting 16-18 mpg. That gear is too tall for overdrive. You need a 3.42 or higher. Lugging in overdrive eats your mpg.

-Josh

[ls1nova71]

Originally Posted by Sean C

Hey guys! Just wondering what other LS swapped trucks are getting for gas mileage? My 04 5.3 with around 300 awhp gets about 11 MPGs in the city and does worse on the interstate. I'd estimate interstate MPGs are around 9 at 80 mph (right at 2000 RPM) which is awful.

My truck has a 4L65E and the stock rear end, 3.07(?). Maybe that gear is too short for the overdrive gear?

Also, the truck in general is a brick as far as aerodynamics are concerned, any ideas on streamlining it a bit? Something in the engine bay/wheel wells maybe?

Thanks guys!

You have something way wrong! Either the tune is a hot mess, or your math isn't right or something, you really shouldn't be getting worse mileage on the highway. My truck has 3;07's and I get 17 pretty consistently.

[Clyde65]

I had the 3.08 rear gear and did real well on the hwy, not so good in city, when I changed to 3.73 gears I now average around 22 hwy and 16 city. since my truck is a brick, if the wind blows very much, my MPG suffers.

[Indian113]

On Interstate runs I get 25-28 mpg's but then I've got 1/2 the truck most of you guy's drive

[Sean C]

Thanks everyone for the input! Much appreciated. I think the 3.07 might be too short for my setup.

Originally Posted by ls1nova71

You have something way wrong! Either the tune is a hot mess, or your math isn't right or something, you really shouldn't be getting worse mileage on the highway. My truck has 3;07's and I get 17 pretty consistently.

^^^ I'm going to reset the brain of my gauges and make sure everything is calibrated correctly, and rematch everything. Also the fuel sender was acting a bit funky so I'll look into that too. The idea that mpg is worse on the highway is absurd, I know! It doesn't make much sense at all, so I'll update again once I've got everything set right.

Thanks again guys!

[Sean C]

Originally Posted by Indian113

On Interstate runs I get 25-28 mpg's but then I've got 1/2 the truck most of you guy's drive

This cracked me up

[Sean C]

Originally Posted by Clyde65

I had the 3.08 rear gear and did real well on the hwy, not so good in city, when I changed to 3.73 gears I now average around 22 hwy and 16 city. since my truck is a brick, if the wind blows very much, my MPG suffers.

Is your 4.8 stock?

[656mx]

My 87 crew cab dually got 20mpg highway and 16mpg commuting to work every day. Had 5.3 4l80e 410 gear on stock tires. Sunday driving around town light to light I would drop to 14 because my light throttle shift points were to low for the heavy truck. Pretty much doubled what I got with the big block that was in it

[clinebarger]

Originally Posted by Sean C

Is your 4.8 stock?

His 4.8L is stock, Gets down the road pretty good to! (I have driven his truck)

My 3/4 ton Suburban got 17 mpg average (empty) with 30" low rolling resistance tires & 3:73's & a light foot, Beating on it could make that average drop like a rock!

[whitels1]

My 84 K10 Long bed, with a 5.3/SM465 3:73 and 35's gets right at 15 mpg.

[Clyde65]

As Clinebarger said, all stock except shortie headers, cai and a Nelson performance tune on the Pcm. I have driven the truck to Abilene and back at 70mph and never missed a lick. I averaged 23mpg and back was better at 65.

[littlevictories]

I nabbed 26 forcing open loop and leaning it out to like 16.5:1 afr on a trip to eastern oregon. Now I have it almost stock except for DFCO turned on and pretty aggressive on it's apply. gets 16 mixed and 20 ish hwy. the 60-66's are just not very aero.

[koolkat269]

You want at least 4.10 gears but depending on your tire size 4.29 or even 4.56 might be better. You don't have enough RPMs to hit your cams sweet spot.

[Sean C]

Originally Posted by koolkat269

You want at least 4.10 gears but depending on your tire size 4.29 or even 4.56 might be better. You don't have enough RPMs to hit your cams sweet spot.

There must be some equation to back this up right?

Also, UPDATE:
my highway mpg is 15.6 (did some recalibration and remeasured)

This still seems crummy for a truck that has 283awhp, unless I'm just setting the bar too high

I'd like to swap rear end gears to something more appropriate, if in fact 15.6 is still crummy mpg for my setup. Anyone know of the calculation to choose the best gear so I don't have to do this twice?

Thanks for all the input guys! It's truly impressive how reliable this forum is

[Sean C]

Originally Posted by BR3W CITY

~380rwhp/~450chp w/ my heavy 4l80e,12bolt, and 15x12 rears w/drag radials (yay rolling resistance lol)

In my 66 I get about 13-15city and 17-20 on the freeway

If BR3W CITY is putting down 100hp more than I am, and getting better mpg, something just isn't right! Now if I had 400awhp and was getting my current mpg, I'd be fine with it, but getting crummy mpg with average power gives me a headache haha