Aerodynamics/anyone recognize this as an old post?

[tRustyK5]

I'm sure it's not as cut and dry with a gas engine, but with a diesel rpm's are way more important to fuel economy than almost any other factor. The peak efficiency (best BSFC) for a 6.2 diesel is 1800 rpm. Torque peak is 2000 rpm.

At highway speed you'll lose about 1 mpg for every 100 rpm faster it's spinng over 1800 rpm. My '83 1/2 ton gets 25-26 mpg spinning at 1840 rpm at 65 mph. My K5 got 16.5 mpg spinning at 2650 rpm (also 6.2). Weight of the vehicle doesn't seem to matter much except around town. The K5 used to get 14-15 max in town, the pick-up about 18-19 in town. The pick-up wieghs about 4200 lbs, the K5 weighed 6000 lbs.

I think with a gas engine you need to aim to run 200-400 rpm under tq peak for best economy...and obviously at a sane speed for aerodynamic drag too. 2200 rpm at 63 mph was the sweet spot for my old 355 in my Jimmy. 2500 rpm at 70 mph lost me over 3 mpg.

Rene

 

[h0wl0ngcanitbe]

ok so im regearing to 4:56 and im assuming my rpms will drop from 2700 at 65, too hopefully 2300-2400. do you know what the tq peak rpm is for a standard 5.7 k5?

 

[tRustyK5]

Tire size, tranny, current gearing?

Every small block I've ever owned has done better economy wise at 2200 than 2500 or higher. With a tame enough small block I'm sure 1900 rpm would net even more gains...at the expense of performance.

Rene

 

[NorCalKid]

ok so im regearing to 4:56 and im assuming my rpms will drop from 2700 at 65, too hopefully 2300-2400. do you know what the tq peak rpm is for a standard 5.7 k5?

Going off the specs in your profile and assuming your 35s measure exactly 35" With a 700r4 in overdrive at 65 you would be spinning around 1990 if its a 350 or 400 or 465 then you would spin about 2845

 

[h0wl0ngcanitbe]

awesome those number sound great i just need this thing to behave a little more on the free way, so im definitely goin to gear up too 4:56!

 

[h0wl0ngcanitbe]

i have 35" toyo mt open countrys, a 700r4 tranny and probably 3:08 gears, my blazer in particular was pretty stock off the line so im assuming it doesn't have the 3:73?

 

[ryoken]

my 87 came 3.73's, 700r4, 350 tbi.....

4:56 will be a decent gear as long as you don't go bigger on the meats...

here's a good gear calculator to play with... http://www.grimmjeeper.com/gears.html

I run a 700r4 with 38's and I'm going 5:13.....

 

[h0wl0ngcanitbe]

thanks for the link man this kinda stuff helps me out alot!

 

[vortec]

what those numbers don't take into account is that around here, the traffic is so bad, you're usually just doing stop and go anyway, which is terrible for mpg, and once it starts moving, if you're going slow on the highway, you're butt is getting squished. there's always someone with a bigger truck.

 

[blazin_blazer]

75 blazer frame & 82 k25 axles and sheetmetal

[ QUOTE ]
I still can't figure out what they were thinking with my 3.42's though /forums/images/graemlins/grin.gif

w/sm465, 3.42's, 38x12.50x15 tsl's(measured at @ 36'') at 60ish, the computed mpg=21, in 3rd/lastgear(3speed w/granny low)or 4th whichever you choose..it was barely above idle(@ 1000 rpms...motor was happy) once you reached speed/ and it still performed well offroad!


a fuel injected 262 I6... mmmmmmmmmmmm

 

[jonrpick]

w/sm465, 3.42's, 38x12.50x15 tsl's(measured at @ 36'') at 60ish, the computed mpg=21, in 3rd/lastgear(3speed w/granny low)or 4th whichever you choose..it was barely above idle(@ 1000 rpms...motor was happy) once you reached speed/ and it still performed well offroad!


a fuel injected 262 I6... mmmmmmmmmmmm

Do you mean, 292???

The 262 was a V8 from the '70s...