How fast does everyone cruise on the highway in their truck?

[1985-GMC]

I went to Austin and back last weekend and nobody drives under 75 on I35. Most of the trip I was at 85 with a few times past 100, my 454, 4 speed, 4:10 gears and 245/75-16's. Usually I get anywhere from 7-9mpg around town but on the highway I got 10mpg. If I did the same thing and stayed at 75 it probably would have gotten 12. I was not easy on it at all, and I know this is not safe driving habits in a 34yo truck. I'll get a picture next time.

[86c20]

i do well at 70 in my c20 with 3.42 and a sm 465 gets me 20-22 depending on grade of fuel. if i go to 75 its 16ish 80 14mpg 85 10mpg

[slugish]

Originally Posted by Keith Seymore

No - seriously - drag is affected by the square of the velocity (not just the velocity alone) - so the drop in fuel economy is exponential rather than linear.

That's for all vehicles, not just those in the shape of a brick.

K

Lost me on this one Keith. Any explanation for dummies

[Redcap]

Originally Posted by 1Cobra427

I usually stick to about 60-65 - 454 engine drinks a lot plus the truck is not daily driver so when I drive it is pleasure and time to relax. Mainly I drive the country roads and avoid highway. More fun that way!!!!!

Same here. 454, 4.10 gears and 35s...I take the backroads and enjoy the ride.

[clinebarger]

70 mph 5.3/4L60E 3:73 30" tire....17mpg average.

['79squared]

I drove my 79 350/350 turbo 2/2,5 hours to Olympia Washington a couple months ago at 70/75 most of the way and she ran great the whole way there and back. No idea how many miles the motor has or how many mpg I get. And frankly I dont give a dam lol. When Im in my truck its for fun and I dont care the mpg or the cost to fill it. Thats not why I have it.

[EasySteve]

Originally Posted by slugish

Lost me on this one Keith. Any explanation for dummies

He means..the faster you go, the more the rate of fuel consumption will increase. So lets say you go 80 and get 15 mpg, then 90 could be 13, but then 100 could be like 8, because the force against the vehicle is increasing very quickly. If that helps at all...

Linear would be 80mph=15mpg, 90mph=13mpg, 100mph= 11mpg, and so on.

[Keith Seymore]

Originally Posted by slugish

Lost me on this one Keith. Any explanation for dummies

Sorry bout that!

The formula for drag is:



and the problem is that "v" - which is not just a "v" but is "v squared".

Graphically it looks like this:



and you can see that instead of increasing in a linear, or straight line fashion, the drag is increasing faster than you would expect for an increase in speed.

K

[Rich84]

Is that why it feels like the front wheels are coming off the ground at about 85mph??

[motornut]

79 4X2 has 305-2B@3.4 rear ,quite a change this summer from the 78 $X$
here most speeds top out at 80-100 (50-60)on the highway
over in Quebec tho 90-120+ (55-75) ,keeping up...no prob,
gas mileage drops like a brick lol

[Firebirdjones]

Originally Posted by clinebarger

70 mph 5.3/4L60E 3:73 30" tire....17mpg average.

70 mph in my blazer with the 6.0/4L60E and 3.73's with a 33" tire gets me 21-22 mpg highway average. I get 16-17 mpg regularly just driving around town.

[Firebirdjones]

Originally Posted by 1985-GMC

I went to Austin and back last weekend and nobody drives under 75 on I35. Most of the trip I was at 85 with a few times past 100, my 454, 4 speed, 4:10 gears and 245/75-16's. Usually I get anywhere from 7-9mpg around town but on the highway I got 10mpg. If I did the same thing and stayed at 75 it probably would have gotten 12. I was not easy on it at all, and I know this is not safe driving habits in a 34yo truck. I'll get a picture next time.

That is entirely possible. I can routinely get 14 mpg highway out of my 79 1 ton with a 502/400 turbo and 3.73 gears with a 29" tire,,,,but I have to cruise at 65 mph and no more, which is right at 2700 rpms. If I bump the speed up to 70ish, I lose about 2 mpg highway.

When I tow my 28 foot trailer loaded (about 10,000 lbs.) I cruise about 60 mph and going accross country I averaged 9-10 mpg highway.

I know what you mean though, most people here drive over the 75 mph speed limit so you have to keep your eyes peeled when cruising along at 65 or get run over. Seems everyone is in a hurry these days. Most times I take back roads if at all possible because of that and prefer a nice relaxed leasurly cruise.

[oldmann1967]

My '81 scotts has a .030 over 350. Not sure of the gear. it runs 70 like a boss. I actually had the spark advanced a little too far and it liked 70-85 a lot! I need to adjust the vacuum cruise control because it does not hold above 67mph.

[Number21]

I usually stay close to 55...though I don't do much highway/freeway driving. Something starts vibrating really bad at about 60...haven't figured out what yet.

[GM-power]

55-60, 357/th350 with 3.40 gears, past 60 I can't look at the fuel gauge.

[clinebarger]

Originally Posted by Firebirdjones

70 mph in my blazer with the 6.0/4L60E and 3.73's with a 33" tire gets me 21-22 mpg highway average. I get 16-17 mpg regularly just driving around town.

17 is my overall average......For a 8600# GVW 3/4 ton Suburban I think its great.

[78longbox4speed]

60-75 w/3:73 runs great

78 3/4 ton 2wd 4 speed 350

[Evil Trailblazer SS]

I cruise at 65 all the time on the highway. With the converter locked in Overdrive, it lopes along a 1900rpm turning 35's with 4.10 gears and averages 14mpg all highway driving...I average just shy of 11mpg average overall. Not bad for a 454TBI crew cab!
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[Edahall]

With the price of fuel these days, I keep my speed at 57 mph on my Suburban and get high 20's.

[Skunksmash]

Yeah areodynamics play a HUGE roll in how much mileage a vehicle gets. Notice how nowadays, even the seams between the door panels and the fenders and the hoods are all getting tighter on the brand new cars? Smaller seam = less drag. And these trucks are like a brick sailing through the wind. They never even considered areodynamics for these things. Would have been pointless anyway. Better that way though cause they look a lot better for it. Notice all the huge headlights and and frontends on the new trucks just sort of blend together in one big blob. Better gas mileage that way.

[Firebirdjones]

Originally Posted by Edahall

With the price of fuel these days, I keep my speed at 57 mph on my Suburban and get high 20's.

I had always wondered if anyone took that into consideration and figured as prices climbed speeds would come down.

Doesn't seem to affect anyone around here though, they all still drive like their a$$ is on fire.

[Skunksmash]

Not me. I do want to do 65/70 though and get somewhere fast. Hell even 70 you get bigrigs passing you.

[Firebirdjones]

Originally Posted by Skunksmash

Not me. I do want to do 65/70 though and get somewhere fast. Hell even 70 you get bigrigs passing you.

Yeah, even 70 doesn't scratch the surface around here. Better be in the slow lane

Doesn't bother me though, I go 65ish most of the time regardless. Some of mine are geared too steep to even do that, but that's okay with me. If they want to go faster, they can find another lane and go around.

[Rubble]

I am one of the slower guys on the road,my 86 350/465/208 with 4:10 diffs on 35's not really set up for high speed cruising.It probably averages about 10-12 mpg at 100km. My 82. 6.2/A833/208 with 3.73 on 35s does pretty good around 17-19 mpg at 100km. I usually like to sit back and enjoy the ride.I used to drive semi across north america and the company I worked for had a 65mph speed limit and even 15 years later it's still built into my foot!!!! Like I told a fella on the highway once,if your in that much of a hurry,get your a$$ outta bed an hour earlier !!!!!
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[GM-power]

Originally Posted by Rubble

I am one of the slower guys on the road,my 86 350/465/208 with 4:10 diffs on 35's not really set up for high speed cruising.It probably averages about 10-12 mpg at 100km. My 82. 6.2/A833/208 with 3.73 on 35s does pretty good around 17-19 mpg at 100km. I usually like to sit back and enjoy the ride.I used to drive semi across north america and the company I worked for had a 65mph speed limit and even 15 years later it's still built into my foot!!!! Like I told a fella on the highway once,if your in that much of a hurry,get your a$$ outta bed an hour earlier !!!!!
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In my extremely scientific experiment on speed my final result was this: I never got anywhere faster by going faster than the speed limit. In fact I was late most of the time. I totally agree with you.
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