Socha_62, I started my own thread so I don't get in trouble for Hi Jacking again ;-)

I have a 2001 4x4, 5.9l, auto, 3.55 gears

I used to own an early Chevy 4x4, 4spd (1:1 in fourth) that came stock with 3.08 rear end gears. The PO installed 31" tires on it. The fuel mileage was about 13 mpg. I had planned to put a lift (4") on and install larger tires (33") so I installed 3.73.

When I had the 31" tires, no lift and 3.73 rear end gears the mileage increased to 15mpg, after the lift and tires the mileage again dropped to 13 mpg.

My question is has anyone replaced the rear end gears in their truck to 4.10 or 4.56s without increasing tire size? Does anyone notice a difference in mileage between the stock 3.55 gears and stock 3.92 gears? It seems to me with the HUGE overdrive that Dodge uses that the overall final drive in o/d is too tall and the engine is turning too slow (currently about 2000rpm at 120kph or 75 mph), especially if a hotter cam has been installed.

Thoughts, questions, comments, concerns?

 

the drag racers or sled pullers might go with lower gears w/o bigger tires, but i can't see why you would want that on the street. The result would just be higher engine rpm's for the same speed.

the combination of the big overdrive and 285/75 tires sucks all the torque out of my truck and makes it real sluggish at light throttle. I would benefit from a 3.90 or 4.10. 4.50 would be way too low on the street unless you put on huge tires, lift, etc, which would make it drive like crap.

as far as changing gears, with the 4wd, we would have to change both front and rear. It'd run into some money unless you could find a set in the junk yard or swap meet. Its easier and cheaper for me just to turn off OD for non-expressway driving, and let the engine rpm stay up around 2000. With OD on and light throttle, mine will shift into OD at 40-45 and drop rpm down to 1200-1300 and just bog down.

 

The reason you would want to is that these trucks are so heavy. 3:55 gears just don't give you the mechanical advantage necessary to acheive decent performance. I think a lower gear would absolutly help you with mpg, even if you stay with stock tires. I wouldn't go to low, maybe 3.92 if your staying with stock tire size. If you plan to run around a 33" tire on your Ram, then 4.10.

Would it be worth the cost of gears (both axles in a 4x4), just for a few mpg gains? Not sure about that. If you want better performance though, I think it is well worth it.

 

Silver_Dodge, are you running 4.10s with 285/70/17s, what is your RPM at highway speeds?

Actually, If lower gears do increase mileage as they did on my Chevy (from 13 to 15mpg) the payout is about 27000 miles.

When I changed my gears last time the cost of gears and installation kits were $800. I did all the labour. So, if I expect an increase in cost to $1000. In Canada, I currently get gas for about $3.60/gallon. $1000/$3.60=277 gals of fuel needs to be saved. 27000 miles/15mpg=1800gals, 27000/13mpg=2076gals. The higher the price of fuel goes the less mileage required for the savings.

And as you mentioned a benefit in terms of performance.

This all of course assumes that an increase of 2mpg is possible with lower gearing.

Eninge RPM at 120kph

3.55 gears

 

I wish I knew my RPM, I don't have a tach. I get 16mpg on the highway though, and 12-14 in town depending on how hard I drive. I have mods other then gears though, so they are a factor too.

 

Your 16mpg would be 19mpg. (Canada uses the UK gal of 4.54 litres/gal) I took a long trip the other weekend 2000km and averaged 13.25mpg or 11mpg US gal. (calculated not overhead computer).

I do need to replace my TPS as on the way back the torque converter would unlock and lock up again when a change in throttle position occurred, but no CEL came on. I took the cruise off and was able to keep the converter locked up better/long between unlocking using my foot.

As mentioned above I would like to see average increase to 15mpg or better and I'm happy spending less than $1000 to do it as in the long run it will pay off for me. I suspect that an increase in around town mileage would occur with lower gears as well as the mechanical advantage would be greater to get the heavy truck moving.

Oh, and I calculate your tire size to work out to just under 33", with sidewall flex they are probably turning about 650 times/mile. That makes your engine RPM at 75mph: 650*4.10*0.67 (overdrive gear or it is 0.69?)*75/60=2231rpm.

I was actual looking at switching to 4.10 or 4.56s. 4.10 would give me about 2300rpm at 75mph and 4.56s 2570 rpm at 75mph. I currently run 265/75/16 tires.

 

you also got to add the performance in

im putting gears in my truck and i would take performance and saving gas over over the six hundred dollors i spent at complete off road for some 4.88 yukons with the master install kits

 

???

your still saying mpG... gallon.... and im pretty sure the size of a gallon doesnt change in Canada..... besides, even if you converted it to liters or w/e, itd still be consuming the same amount of gas.......

 

Gallons in Canada are different then the "standard" gallon used in the US...Canada uses the Imperial Gallon (pretty sure thats the name) which is equal to 1.2 US gallons. Therefore, the 26 "gallon" tank that is in the US trucks would only take 21.6 gallons if you were to fill up in Canada...You are correct that you would be consuming the same amount of fuel, but the difference comes in the calculations used...If I run 200 miles and get 10 MPG (US), then I would use 20 gallons (US)...If I fill up in Canada it would only take 16.6 gallons (UK) which calculates to 12 MPG (UK)...That is what boisvenue is saying.

 

You are correct Imperial gallons are 4.54litres. The US gallon is 3.78 litres

OR

Imperial gallon is 160 oz and US gallon was 128 oz