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Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by txbluethunder, Mar 22, 2002.
with all the gear ratios around what would be a good rpm to turn when cruising down the highway?
With overdrive...about 2000 is cool for cruise rpm. Without overdrive I'd say 2500-2700 is about the upper limit of what I'll tolerate.
You can find out what engine speed is most efficient by looking up your torque specs. The rpm that the engine delivers it's highest torque is it's peak efficiency. This is what I strive to cruize at when on the highway. Mine is 2800 I belive with a stock 400.
That's a nice theory...and probably great for towing. For fuel efficiency though it doesn't hold water. Check out the October 2001 issue of Four Wheeler mag. pg 126 they did a real world test of fuel mileage using a new Toyota Tundra with the 4.7 L V-8. Steady cruise using the overdrive at 65 mph (2000 rpm) netted 21.31 mpg. The same drive at a steady 65 mph without the overdrive (2850 rpm) netted only 16.17 mpg. For consistancy they used the cruise control both times...although I'm not sure how many miles the tests were.
I do know that I got better fuel mileage with my 2.73's and 33" tires than I do now with 3.73's and 36's...although performance with the 2.73's was not exactly spectacular/forums/images/icons/smile.gif
What are the torque specs on that tundra? I don't have that magazine. And of course your going to get better gas mileage with overdrive than without. Dosn't sound like a good test.
isnt the only reason you get better mileage with OD because it lowers the RPM's?
Theoretically. Other factors such as overall gearing, aerodynamics, etc., will influence MPG as well.
For Instance, K5 305/700r4/3.08 gears/31" tires netted me at best 17MPG cruise. K5 350/Sm465/3.42's/33" tires netted me 18MPG. I'm cruising 60 at right about 2200RPM with this 350, and it's just about perfect in terms of power left to climb hills, engine RPM, etc. The 305 was too weak, but also the gears and OD "bogged" the motor down at anything under 50MPH on flat ground. 3rd gear was needed for any kind of hill, and I still couldn't pull mountain passes at 70MPH like I can now.
Peak torque in the motor I'm building now is 3000RPM, but I guarantee peak MPG won't be achieved at that RPM.
If that was the case then I would have to cruise at 4,000 rpm in my firebird!! I do get better gas milage at 3,000 rpms then I do at 2,500 in it though, but I think that has to do with the stall being at 2,500. You want to be above 1,900 rpms while cruising because thats too low and it will keep downshifting and upshifting at that point. I would say no more that 2,500-2,600 rpms. With my Jimmy (350,700r4, 4.56 gears, and 35s)
I cruise at 85 in OD at 2600 rpms. I get 15-16 mpg there and it has a q-jet on it /forums/images/icons/smile.gif
i was just refering to everything being equal and haveing OD or not the OD jsut lowers RPM and that is why you get better mileage. taking into consideration not dropping to low to bog the motor . i had 3.08 gears with 33's when i bought the truck and could not even use OD at 75 on the highway, it drop out and shift constantly.
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i was just refering to everything being equal and haveing OD or not the OD jsut lowers RPM and that is why you get better mileage
More or less...as long as the motor isn't lugging in OD.
sorry about the old thread revival, but I've read through a couple search pages....
What's optimal highway cruising range for a 400sb/th350 combo? I'm guessing in the 2000-2200 range. I've never had a tach in one of these trucks and am trying to figure out what gears/tire size to run. I'm putting in 4.10's and 36's which will allow me to cruise 56mph at 2250 rpm. I think maybe I need to find some bigger tires...
I bet on flat ground (key) a 400 can move the truck easily at lower RPM's, probably in the 1700RPM range at around 60MPH. Does depend on where your particular engine makes power. Personally I just don't like listening to the drone of the motor turning in the 2000RPM range. I can pull 25MPH in 3rd gear (870RPM) but obviously you aren't fighting aerodynamics at that speed quite like you are at 60MPH. I'm very confident that I could run 1700RPM at 60MPH and not have any problems. BUT that would put me into downshift range for mountain passes (I leave it in 3rd all the time now) and it would affect crawl.
really not much in the way of big hills around here that I need to worry about. Also, my commute to outdoor activities is around 100 miles one way on average and I'd like to keep my cruise speed around 62mph.
Edit: After looking at it, looks like my best option is to find some 38's and slow down to around 55 mph.
I say try and cruise in the 2000 to 2200 range for a Small block. I think 1800-2000 is more ideal for a BB. My blazer and burb are both at 2000rpms at 70mph on the freeway. Perfect in my mind.
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