Do I Need A Regear?

[Johnson8537]

Ok Guys here is what the current set up is:

07 Silverado 6"Lift w/20"Wheels and 35" tires (Gt4 3.73 and M30 4sp)

 

I am thinking about a rear gear or going to a 33" Tire next time I need them.

 

I am getting about 11-13MPG in town and 13-16 HWY

 

I don't tow very often if i do It is a very Light PWC and trailer.

 

 

You guys know what is best so what are my options and what is my cost going to be? Also What is BEST for my truck?

[Johnson8537]

bump?

[Matt_]

even if you're going down to 33s you should regear.

 

if you're sticking with 35s id rec 4.88s

if you may still want 33s someday, go with 4.56s

 

either option will yield better gas mileage than you currently geg, BOTH highway and city. and your trans will thank you

[Johnson8537]

even if you're going down to 33s you should regear.

 

if you're sticking with 35s id rec 4.88s

if you may still want 33s someday, go with 4.56s

 

either option will yield better gas mileage than you currently geg, BOTH highway and city. and your trans will thank you

Thanks Matt...

 

Just curious but I was told locally that re gearing would give me more power but MPG will decrease like crazy!!

(i understand it would if my foot was always in the gas..)

 

Matt What do you think I would gain MPG?

[Matt_]

people always falsely assume that higher RPMs mean higher MPG. this is not the case.... it has more to do with engine efficiency.

 

my 2005 was a highway driven dd, 70+ miles per day at speeds of 70-75 mph.

 

MY highway mpg was 11-12 with 4.10s and went UP to a solid 13-14 with the 4.88s.

 

take it for what you will, but this is first hand experience

[Stevens03]

Less strain on the motor, more MPG. (within reason). If you kept your 265 tires, and re-geared then yes, you'd see a mileage drop, but getting the more correct gear set for the tire size will put you back to a "near" factory power and mileage situation.

 

Having the 4-speed, 4.56's would be ideal for your 35" tires but like Matt suggested, the 4.88's might even be better especially since you have 20" wheels.

Good luck

[Johnson8537]

people always falsely assume that higher RPMs mean higher MPG. this is not the case.... it has more to do with engine efficiency.

 

my 2005 was a highway driven dd, 70+ miles per day at speeds of 70-75 mph.

 

MY highway mpg was 11-12 with 4.10s and went UP to a solid 13-14 with the 4.88s.

 

take it for what you will, but this is first hand experience

 

 

Good to know thanks!!

 

Where is the best place to buy parts... What are the brands most guys are running! ( I don't want to put crap in my truck.)

Any idea ballpark what this will run me?? I think I was quoted just under 1800

[mrOAPx2]

Good to know thanks!!

Where is the best place to buy parts... What are the brands most guys are running! ( I don't want to put crap in my truck.)

Any idea ballpark what this will run me?? I think I was quoted just under 1800

 

ditto. I would like to know as well. I'm in the same boat, 10/11 city 15/16 highway. bleh!

[Matt_]

just under 1800 sounds about right...it varies greatly by region though. bring your truck to a quality shop and go with the parts/brands they recommed and are experienced with.

[05silvycrew]

Well realistically if you can get 14-16 MPG on the highway and around 11-13 in town in a truck with a 6" lift and 35" tires you're not doing bad at all. Regearing is an expensive option, like you've already found out, about $1500-2000. If you're doing it to gain MPGs you're better off using that money to buy gas. If you're doing it to get some pep back in the truck and get it to stop dropping out of overdrive on the highway/freeway, then it's not a bad option.

 

 

let's do some math:

 

your stock tires were about 32" tall and stock gears are 3.73. at 70 MPH you'd be turning about 1950 RPMs

 

at 70 MPH, with your new tires and with the same gears you're now turning about 1750 RPMs. You've also added a lot more rolling resistance (with your tires) and a less aerodynamic stance (due to the lift).

 

To get back to 1950 RPMs you would need to regear to 4.10's. to make up for the added rolling resistance and aerodynamics I would suggest going 4.55's. at 70 MPH and 4.55 gears you'll now be turning about 2150 RPMs. This is about the same RPMs as a stock truck with 4.10s (I know, my truck has 4.10s). I can make 16-18 MPG on the highway in my truck, so I wouldn't expect anything higher than that with your setup.

 

say you average 12 MPG and 12,000 miles a year. you buy 1000 gallons of gas in a year

now you regear and manage 14 MPG and 12,000 miles a year. you buy 857 gallons of gas in a year.

you save 143 gallons of gas at $2.50 a gallon you save $357 a year, takes you 5 years to break even.

 

just some more numbers for you to think about.

[Matt_]

Gearing will help save the trans as well. Whats the average trans rebuilt... $1500? Getting those 35s going puts a beating on the 4L60E.

 

I see 3 huge benefits to regearing...

 

From greatest to least important

 

1) Making the truck useful again (when i had 4.10s and 35s, it was sad that my truck struggled with an empty 3k lb trailer back there... thats just not right)

2) Helping the trans

3) Better efficiency

 

 

I agree, in that I would not regear a truck based solely on MPG.

 

To the OP: i also ran a deep trans pan that had a drain/fill port and allowed for extra trans fluid. I also added a trans temp gauge to ensure I never went above the recommended temps. These are some investments that you may want to consider. Every 15k miles i did a drain/fill to get fresh fluid in the trans... which was simple since I had the drain plug in the new pan.

[Johnson8537]

Thanks Guys.... Great Info... Thats what i wanted to hear!! yeah I was doing it for MPG AND POWER seems like the truck should have more power!!

 

Might be something I do in the future!!! Thanks again for all your help!!!!!!!!!

[FargoZ71]

the gearing you have and tires is hell on that trans, way to high, i'd go with 4.56's at that tire size and be happy ;)

[Spurshot]

Just my 2 cents. Taller tires (rim diameter means very little in this) take more energy to accelerate, maintain speed and stop. Idealizing the gearing for the application helps, but the taller tire will take more power (gas) even if geared to the same rpm for a given speed.

 

The best fuel economy for highway driving will come from a shorter tire geared to a low rpm. What that does is forces the engine to a higher BMEP (brake mean effective pressure) which translated means; the engine is making higher cylinder pressures because the throttle has to open more but burning the fuel more efficiently because of the higher pressures. Yeah, this burns more fuel per power stroke of each cylinder, but the lower rpm also reduces the quantity of power strokes per mile traveled. This is only applicable to constant speed driving associated with highway travel.

 

The above principles are what Mr. Charles Lindberg taught the Army Air Corps in WW2 in order for the P38 Lightnings to get enough fuel economy to stay with the bombers on missions to Germany from England. Low rpm, large throttle settings. high fuel efficiency (hp/lb fuel) is reached at high BMEP.

[Matt_]

hmmmm... my .02 on that is that although the theory is solid, the analogy of application is flawed.

 

the drivetrain needs to convert that energy into motion. the P38 does this with a near direct drive system and prop pitch. being under geared in an automotive drivetrain reduced the ability to convert the energy into a useful form (most is loss through the cooling system). also, the higher loads due to being under geared result in more torque converter slippage.

 

every system has an optimal efficiency inherent to its design. for the case at hand, lower rpm does not always mean higher mpg/efficiency (sometimes it does..... but not when comparing 3.73s to 4.56 or 4.88s with 35s on the highway. )however i could see 4.56s getting better efficiency than 4.88s for steady highway driving based on RPM because the other factors will be very close in effectiveness

 

this is theory as well... but based on my experience of doing a lot of highway driving with both 4.10s and 4.88s on the same highway in the same truck at the same speeds and with the same tires..... id say the higher rpm is actually more efficient for this range of modification in "this" system