I know it cost a lot more if you go around that speed please post speed ,ratio and MPG
if you don't know those three things do not post in the thread you're going to get it shut down again
Engine 2 please
Different gear ratio MPG around 78 miles per hour
Discussion in 'Ask An Owner Operator' started by MidMay Trucker, Feb 20, 2020.
Page 1 of 3


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Tire size, transmission ?

Curious. Following.

I don't know what to tell you as far as exact miles per gallon but here is how I understand it.
Whatever engine it is that you have, you have to find out what the optimal cruising RPM is.
Then you have to know if your transmission is direct or Overdrive. I think that all modern overdrive Transmissions have a .73 Overdrive.
Then go to roadranger.com. they have a road speed calculator they will tell you for whatever speed what your RPMs will be with a particular rear end ratio and direct or overdrive transmission. It will figure it for whichever tire size you have also.
I do not think that anyone can give you an exact miles per gallon figure. A lot of it depends on if your truck is aerodynamic if you have low resistance tires if you travel always with heavy loads or if you deadhead a lot or if you are in the mountains... There are a lot of variations.
If 78 miles an hour is the speed that you want to cruise then finding the optimal RPM will give you the best mileage that you're going to get, whatever that will be.Bean Jr. Thanks this. 
I see what you're asking.
Basically you gear the truck to run 200 rpm above peak torque at the speed you wish to cruise at
Eg: my 600 hp DD16 peak torque is at 1240 rpm so I would gear it to run 1440 rpm at 78 mph.
I have 11R24.5 tires and an 18 SPD 0.73 top Gear equals ideal rear ratio of close to 3.25 
The information you are looking for is going to be hard to find. The reason is the number of variables.
Engine HP and torque settings, Internal and final output transmission ratios, differential ratios and tire size and type. These all change RPM at speed.
Throttle at speed is also a factor. If you have to be at WOT to hold 1500 RPM, your not getting good MPG. 
Figure it out for yourself......
Transmission Ratio RPM Calculator  Spicer Parts
My guess would be X15,18 speed , 3.08 rears and 11R24.5 tires.....get goinghaycarter, Midwest Trucker, Crude Truckin' and 1 other person Thank this. 


I think there was a thread already posted earlier today about this same subject. Why we did not stay in that one I don't know.
Maybe the mods can merge them.Bigwin123 Thanks this.
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