Ratio calculator question.

1-tonmudder

1-tonmudder

Doin my part to stir the pot.
Joined
Apr 17, 2005
Location
Greeneville TN
I have my grandfathers old 84 F-150.Its nothing special,at all, and if it hadn't been his Id never looked at it twice.Truck is a 300/6 with a four spd man trans that has a 0.78:1 O/D and a 8.8 w 2.73 gears.Needles to say its a D.O.G.!!!.Truck will run almost 40 mph in first and will easily run 70/75 in third but cannot maintain speed in O/D.Forget backing a trailer w it and if your hauling any weight at all you have to double/triple clutch it to pull out.(its on its 3rd clutch).Im looking to do an axle swap to make it more driveable.I have a 3.55 and 4.10 geared 8.8's to choose from and would like to know what my RPM's will be w either ratio on a 235 tires in O/D.I know they have gear ratio calculators but all I have saw only have a 1.1 final drive ratio.Dad had it before me and would never change ratios in fear of hurting the MPG's,I think a better ratio,that you can actually use O/D,with will actually help the MPG some.
 
1-tonmudder said:
I have my grandfathers old 84 F-150.Its nothing special,at all, and if it hadn't been his Id never looked at it twice.Truck is a 300/6 with a four spd man trans that has a 0.78:1 O/D and a 8.8 w 2.73 gears.Needles to say its a D.O.G.!!!.Truck will run almost 40 mph in first and will easily run 70/75 in third but cannot maintain speed in O/D.Forget backing a trailer w it and if your hauling any weight at all you have to double/triple clutch it to pull out.(its on its 3rd clutch).Im looking to do an axle swap to make it more driveable.I have a 3.55 and 4.10 geared 8.8's to choose from and would like to know what my RPM's will be w either ratio on a 235 tires in O/D.I know they have gear ratio calculators but all I have saw only have a 1.1 final drive ratio.Dad had it before me and would never change ratios in fear of hurting the MPG's,I think a better ratio,that you can actually use O/D,with will actually help the MPG some.
Click to expand...

I use the Grimmreaper one. It’s got plenty of transmissions and you can pull them up beside each other compare.


Edit:
http://www.grimmjeeper.com/gears.html
 
Just multiply whatever 1:1 comes out to be by .78 and that'll give you your RPM in OD.

It's a simple equation though:

336 x axle gear ratio x speed
Divide by tire height
Multiply by OD ratio

A 235/75-15 is 28.88 inches tall by the math, but probably 28 to 28.5 once it's loaded.

So, 336 x 3.55 x 65= 77,532
77,532/28.5= 2720.42
2720.42 x .78= 2122 rpm at 65 mph in OD with a 28.5" tall tire.

Currently you're at 1631 rpm at 65 mph with a 28.5" tire.

4.10s would put you at 2450 rpm, same speed and tire size.
 
Last edited:
4.10
 
I would go with 3.55 if the truck will see highway speeds (65-75 mph), and 4.10 if it will just see local driving (60mph and less)
 
Assuming it's got a P235 or shorter tire on it, it's going to be a gear-banger with 4.10s. Go 3.55 unless you really want to haul with it.
 
iwaxmyjimmy said:
I use the Grimmreaper one. It’s got plenty of transmissions and you can pull them up beside each other compare.


Edit:
http://www.grimmjeeper.com/gears.html
Click to expand...

@iwaxmyjimmy beat me to it, I have spent way too much time playing around on the grimmjeeper site.....I use both sides of the chart and run the speeds from 5mph to 70mph to check shift point rpm drops vs. approx. torque curve.
 
I agree with everyone who said 3.55. If it wasnt a 300 straight six, I would say the lower gears, but those straight sixes dont like the higher rpms.....
 
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