efbeason

I have an '89 Dodge Cummins 4x4 with the three speed transmission and 4:10 gears. If I were to change to 3:54 gears, would there be a significant change in fuel mileage? I get 17 mpg now. Would changing to this gear ratio put too much strain on my transmission? Thanks guys!

apwatson50

One of my trucks with an auto 4x4 has 3.08 gears and an over drive, so 3.54s would probably work perfect, better mileage on the high way for sure. How much if any towing do you do?

efbeason

I do a bit of towing but nothing more than a gooseneck with three or four horses in it or a load of similar weight. Is it expensive to make the change? I know I would have to change the front and rear ratios.

apwatson50

Fairly expensive, would you do the work yourself? Just the gear sets are around $200-$300 a piece closer to $300, and labor I wouldn't know probably ~$200 a piece.

Mcmopar

I have pulled 17,500 lbs with my 3.54:1 ratio. Worked fine but I knew it was back there. The 4.10:1's in my Dad's truck pulled it a lot easier.

bgilbert

Did you install that 4:10 geared axle? 89's with 727 3sp auto ONLY came with 3.07 axles. The Cummins and 3sp direct drive auto plus 4:10 gears would be very slow... say 55mph screaming .

efbeason

No, I did not install the 4:10's they were in it when I bought it. The truck simply will not run any faster than 76 mph. That is it!! I don't care about running any faster, but I would like to get a bit better fuel mileage and ba able to run 70 with ease on the interstate. Would the 3:54's make a big enough difference to spend the money on changing it?

apwatson50

Do you know what rpms your running at 76?
How do you know that you have 4.10's?
Have you done any mods to the injector pump?
What year truck do you have?
3.54's are a huge difference from 4.10's as for worth the money, how much highway driving do you do?

Like bgilbert said 727 no OD and 410's would be "screamin at 55mph"

bgilbert

I'm betting you got 3.07's if you're top speed is 76. What you need is an overdrive transmission.

Alec

Yeah, I just did the math, it is very doubtful that you have 4.10's. To improve things you would need an overdrive . . .

efbeason

Thanks for all the info. Do these trucks have a stamp on the rear housing that tells what gear ratio they are?

wannadiesel

There should be a sheetmetal tag held on by two of the cover bolts. Unless the axle has been swapped out, you have the 3.07's. That was the only gear ratio available with the 3 speed automatic.

efbeason

Thanks again for all the great information. Would an overdrive transmission make a noticeable difference in the rpm's I'm running at 70 mph and would it be economical as far a fuel mileage to make the swap?

wannadiesel

An overdrive would reduce RPM by around 25%. That would put you a little under 2000 RPM at 70.

Whether or not it makes economic sense depends on the amount of highway driving you'll be doing. I wouldn't expect fuel mileage to be any better than 20 mpg empty at 70 mph due to the fact that our trucks are about as aerodynamic as a brick. Assuming $2.10 for fuel, that means you would save $0.015 per mile. An add on overdrive would easily cost you $3000 installed, meaning it would take 200,000 highway miles to pay for itself.

If you just want to have more pedal left at 70, install a 3200 RPM governor spring. The part costs less than $20, and it makes the truck feel much more responsive as well as needing less pedal travel to run at a given speed.

efbeason

Where could I get a 3200 rpm governor spring and how would I install it? Thanks so much!