# math people, Gas Mileage at 2.5k or 3k?

Discussion in 'The Garage' started by TXsizeK5, Jun 15, 2003.

1. ### TXsizeK51/2 ton status

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What kinda gas mileage would a stock chevy (new) 350 long block with headers and TBI get running at about 65-70mph
at 2,500rpm or 70-75mph at 3,000rpm?
Thanks

2. ### tRustyK5Big meanieStaff MemberSuper ModeratorGMOTM WinnerAuthor

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Too many variables to even make an educated guess. I can say running at 2500 rpm at 65-70 will get you better mileage than 3000 rpm and 70-75 mph.

These trucks have pretty poor aerodynamics...the faster you go the harder it has to work to keep it going. FWIW my Jimmy with a well tuned carbed 350 running 65 mph (2450 rpm) used to get me about 13 mpg. Much faster and it rapidly got worse...

Rene

3. ### MousePowrd1/2 ton status

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Not very good. You should probably be happy if it hits 12mpg

I get 19 with o/d and slightly smaller tires, but 3.73's vs 4.10's probably evens things out. At 70 I run about 2000 rpm

4. ### TXsizeK51/2 ton status

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Yea im saying w/out variables though. Just theoretical formula... how much a 350 consumes at a given RPM * time traveled = gas used on trip/miles traveled = mpg?
Like a 5 hr trip at 2600rpm = how much gas on avg consumed theoretically?
Or something like that....

5. ### HarryH31 ton statusAuthor

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There are a LOT more variables involved than that. /forums/images/graemlins/eek.gif Coefficient of drag, rolling resistance of the tires, etc. also have to go into the equation. Then there are things like Brake Specific Fuel Consumption for the particular engine, head, cam cobmination, drivetrain losses, etc.

6. ### dyeager5351 ton statusPremium Member

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Anyone happen to know the coefficient of drag on these things? Camaro's are down around .30 IIRC, but the frontal area is also much smaller.

Think I figured out the truck frontal area was like 40 ft. /forums/images/graemlins/shocked.gif On top of that you'd probably have to add in the ar dam and axle.

7. ### TXsizeK51/2 ton status

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[ QUOTE ]
blah blah blah

[/ QUOTE ] /forums/images/graemlins/rolleyes.gif

j/k

I just wanted to know for comparison.

8. ### tRustyK5Big meanieStaff MemberSuper ModeratorGMOTM WinnerAuthor

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I know you were just kidding...but Harry only gave you the tip of the iceberg.

At 70-75 running about 2800 I'd easily dip below 10 mpg. When I drove to Texas and back I averaged 12.5 mpg on the way down never going much over 65. On the way back towing another Blazer I was getting 11...not much extra drag being the same shape as the tow pig. As soon as I pointed the truck west on I-70 I had a 40 mph headwind and my mileage dropped to under 5 mpg./forums/images/graemlins/eek.gif/forums/images/graemlins/eek.gif

My best guess is about 12-14 at the lower rpm and 10 or under at the higher rpm.

Go fill your truck up and drive down the freeway til you're near empty...do that at 70-75 mph and 3000 rpm. Then fill it up and return at 60-65 and 2500 rpm and tell us what the difference was.

Rene

9. ### HarryH31 ton statusAuthor

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[ QUOTE ]
Anyone happen to know the coefficient of drag on these things?

[/ QUOTE ]
It's quite similar to that of a small parachute. /forums/images/graemlins/wink.gif Jack up the truck so that the axles are really hanging out in the airstream, then add in the extra drag of the frontal area of some 12-inch-plus-wide tires, and it gets even worse. /forums/images/graemlins/frown.gif

So, to answer the smarta\$\$... /forums/images/graemlins/wink.gif Put that 350 into a low, slippery Corvette and it will get WAY better mileage than if you put the exact same engine into a 6,000 pound K5 riding on a 12 inch lift and 44's. /forums/images/graemlins/eek.gif That is why there is no "math" for figuring out the fuel mileage of an engine when provided such a small set of variables. /forums/images/graemlins/thinking.gif

10. ### TXsizeK51/2 ton status

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My gas gauge doesnt work thats why I was wondering... /forums/images/graemlins/grin.gif
I recon' i ought to get that fixed

11. ### 1972Blazer1/2 ton status

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Well look at it this way, you will probably get better milage then I do. Best ever 9mpg; worst ever 5mpg; average 7-8mpg It is hard to get a huge box to do 65-70 mph, oh and my engine is a little overkill also (1970 LT1 Block, Forged 350 crank, Forged pink rods, Forged TRW high dome pistons 10.5 to 1 compression ratio with 64 cc heads @ .030 overbore, 1970 LT1 2.02/1.60 heads fully ported and polished, Mallory unilite distributor, 3310 Holley 750 cfm, Holley Dominator Intake and a Lunati Cam with specs like this: Intake lift/duration .490"/290 degrees, Exhaust lift/duration .490"/297 degrees, Intake duration @ .050"/235 degrees, Exhaust duration @.050"/240 degrees, intake lobe centerline 107 degrees, Exhaust lobe centerline 112 degrees, this is all with a rocker arm ratio of 1.5). I figure a conservative 425 HP

12. ### rebelk5frk1/2 ton status

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Since you have no gas guage then fill up every 250 miles and you should be good. I have a 31 gallon tank and I would always fill up at around 300 miles and normally had close to a quarter tank left when my guage was broken.

13. ### MousePowrd1/2 ton status

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[ QUOTE ]
My gas gauge doesnt work thats why I was wondering... /forums/images/graemlins/grin.gif
I recon' i ought to get that fixed

[/ QUOTE ]

That has nothing to do with calculating fuel mileage. Just fill it up, drive over some Hondas on the hwy, and fill it up again. Divide miles by gallons

14. ### TXsizeK51/2 ton status

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Yea its easier to figure out with a gas gauge though without have to fillup twice and do all that hard math in your head /forums/images/graemlins/1zhelp.gif

15. ### tRustyK5Big meanieStaff MemberSuper ModeratorGMOTM WinnerAuthor

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Keep a \$3 calculator in the truck...lord knows you're gonna be at the gas station often enough anyways/forums/images/graemlins/grin.gif

Rene

16. ### 90K51/2 ton status

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mine, with 350 TBI, 700R4, 4.11s and 33s gets about 14-15 at night (65-70), 12 at 75, and 10 at 80...on a 5 hr trip, that extra few mph could mean you stop for gas, not really helping your time...or in my case, you run out of gas in the middle of nowhere (Idalou, Tx) and walk to a gas station, taking about an hour longer than the trip would take going the speed limit /forums/images/graemlins/rotfl.gif

17. ### Shagnaz1/2 ton status

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I have a bone stock TBI motor with 271,000 on the od. Just did a run from Charleston, SC to Windrock, TN on one tank of gas. I have 38's with 4.56's and a 700r4 tranny. I was moving pretty good also. Averaged @ 70mph at 1900 rpm's. It was great... @16-17 mpg!

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