Last month I had 4.56 yukon gears installed in the front and rear of my truck to help turn my 35" bfg A/Ts and I think it's too much gear. I got 14.5 mpg running highway speeds 70-75 (just under 2k rpm) on a trip right before I dumped my stock 3.55 gears and after my 4.56 gears were installed I dropped down to 11 mpg highway (about 2,250rpm). My tires are getting to the point they need replaced but if I give my truck more than a quarter throttle on a wet road, or half throttle on when it's dry, my back end completely kicks out. I do have a vortech supercharger and e-fans but I didn't expect my traction to be so bad.

What I was considering is going to a 37" tire to help lower highway cruising rpm's since I have more than enough low end power, hopefully getting my MPG's back a little bit. I'm just not sure what kind of setup I would do with the back to keep it level, I have bilsteins that are made for 3-4" rear lift so a bigger block might max the shocks out, and I don't like the ride of an add-a-leaf. The front could just be a leveling kit.

What do you guys who are running 4.56's and 37's think about that combo as for MPGs?

side note: Last week I blew out O/D (this was since my MPG calculations) in my truck and I'm going to be getting a built transmission soon, so I don't know if that is affecting anything. I'm getting about 10 miles per gallon running at just over 3,000rpm without overdrive highway.

 

i have 4.88 with 35's right now. going to install 37's soon. but definatly make hwy driving a pain in the butt. also my milage has dropped dramaticly. was getting about 18-20 hwy now 12-14. have lots of power though and i dont have a supercharger.. i can only imagine you are breaking loose all the time. if its wet i can get loose at about 45-50 mph. just have to remimber not to kick it down hard.

 

btw way i have a 6" procomp with 2.5 leveling kit up front and the back has 6" ouo blocks and add a leaf.

 

If you want power, you loose mpg.

 

Sounds about like what I want to do so here's a few questions:

What did the block setup run you in cost, and did you get rid of the factory and procomp blocks doing that?
Was the add a leaf to get more height or to stiffen suspension?

I'm not a big fan of stiffer ride with an add a leaf. I wasn't sure what all needs to be done to get the back end level with the front if I do a 2 or 2.5" leveling kit up front.

 

I have not changed out the gears in my truck yet so I can not help with that part. Can say that if you recently lost overdrive your transmission and you changed the gears only a month ago the transmission was slipping some then too and would impact your gas mileage. With the steeper gear overdrive probably aslo has gotten more work since the gears were changed.

If the converter or the clutches & bands for the other gears in the transmission are damaged too then the impact to MPG from your transmission problems will be greater.

I would wait to make any other changes until after you have the transmission problems sorted out. That way you have a new baseline from a healthy engine/trans combo.

 

see some people post on here that it is needed for MPG and some say it doesn't do anything - but when I see people who actually have 6" w/ 35 and end up losing MPG from 3.55 to 4.56 it makes me not want to get the regear

 

I am pretty happy with my 3.73's and I have a 6 inch lift and 35's. If I were you, I wouldn't go lower than 4.10's. That's if you plan on sticking with 35's. I'm considering a gear swap, but it will be 4.10's. I want to keep somewhat better mileage.

 

yeah i am just going to wait till I get to Alaska since I'll be doing all highway pretty much from NY - I am not to worried about gas cause uncle sam is paying for it - I guess I am just going to go to 4.10 when I have the money and time to get it done

 

I am sorry to hear that your gears are too short for your current tires. I have had the same issue myself with an old camaro. When the gear is too short, the vehicle is louder, gets poorer mileage, engine and belt-driven accessories wear quicker... It feels better when giving it some gas at highway speed, because you have more power without downshifting; it also is much nicer from 0-20 or so. Other than that, personally, I like a taller gear. Actually, I want the tallest gear I can have without jumping in and out of high gear (6th on my '09) when driving on the highway.

 

As much as you might not want to regear, I can tell you that you need to do something for your transmission or else you will have problems like mine. I guess every truck is set up different but 3.55's are definitely not good for turning 35's. In my case I have way too much low end and loss of highway mpg's but a lot of guys that put in 4.56's don't have e-fans and a supercharger either.

I'm glad to hear some of you guys see where I'm coming from. It seems like it's a set in stone rule that with 35's you get 4.56 and 37's 4.88's so I just went with it. I think once I fix my transmission it will make a difference so we'll see about that.

 

yeah its eventually going to happen just not right now - of course with everything u have I am assuming u have a custom tune done also?

 

If your transmission is messing up then that's where your mileage went...

 

Did you make corrections to the computer for the new tire size and gears. That too would skew your calculations and possible cause the transmission problems.

 

I just checked the 70 mph cruise rpm with that tire and it's 2,242 rpm. With 4.10s, you'd be at 2,016 rpm. I don't think it's the gears costing you mileage. I look into Fatherford's observation. The torque converter could also be failing. Running 37s (the 37x12.50/17 specifically) your 70 mph cruise only drops the rpm 111 rpm. The increased weight and rolling resistance will eat up that marginal decrease in rpm. I'd start experimenting with tire pressures. You want decent mileage and still maintain a good contact patch. If you're running stock17"x7.5" rims, that's part of your traction issue right there. BFG recommends at least an 8.5" rim width for that tire.