I am new to this forumn. I am putting a 6" lift kit on a 07 silverado with 35x12.50 tires and 18x10 wheels. I am worried about the gears , do i need to put new gears in it also. I tow a 12 ft trailor just about every day for work which is loaded down with tools. iam not worried about lossing some power because i know that will happen. However i dont want to tear up the transmission on a brand new truck. So should i move to a different gear to save the transmission and if so which gear.  

35X .12= 4.20 think thats one formula to figure optimum ratio. but then  you have to figure overdrive gearing ,ex .70 overdrive ratio x final drive ratio, ex 3.73=2.61 final drive ratio.i cant find a formula for the next set to figure out drop in ratio from tire size, but someone probbably already has what you need in real time info  

You are going to have to worry about more than tranny. That lift and tires are going to be tuff on ball joints in front end. Expect maybe 25 to 35K life span if you are lucky. (it is not that strong of a a design) then there will be brake issues because of increased leverage from bigger tires and then spring wrapup and rear wheel hop if you do not add a few more leafs back there (you do not want spacer blocks) Finally on the gear ratio, figure on at least a 4.56 here with a 4.88 even better. Do not kid yourself into believing that a 3.73 or 4.10 will be fine here. Finally better stay on real good term with dealer because it is at their choice to void your power train and warranty with these mods because they do in fact if they choose to press it. Also MPG will take a dump but deeper gears will help stem the loss some. Personally I never could understand why someone would buy a new truck and then chop it up and play with warranty. If I wanted to do all of this I would buy a clean used truck with no warranty and chop it up with the money saved and still be ahead of the game. Also be advise, GM unwisely moved ABS controller under truck in the spray, mud, stones and salt so keep this in mind when you play. They used to be safely located under the hood.  

Micky thompson has a 33/15.50/18, maybee you can gain some flotation and more service life out of the wagon you got in your drive,use what you got wisely and it will last a while. had a friend with a 2003 and he ran 35 mt's just fine, he used it as a service truck for his trucking co,put a tranny temp gauge and keep below 240 deg. didna say what kind of service youll be using for, but how u use power and when is the real Q for keeping 4wd alive  

240 is way to hot a safe limit for tranny. You want 180 to 200 normally. It is not wise to run much above that for extended periods and at 220 and above it is time to throttle back or gear down if you want long tranny life.  

i use the trailor to get tools to work, i am a carpenter so once on the job i stay there most of the day.Will the lower gears keep the tran temp down to a normal level.Im not a mechanic but i didnt think new trucks had ball joints,or maybe that was cv joints, of course i dont know what im talking about but thats why im here for some pro advice. I have already talked with dealer and they said warrenty wont be voided as long it isnt a ridiculos claim{ex. putting everything on and bringing it in because it makes a funny noise or rides a little differnt}They have honored warrenties before with lifts from what i am told.1/2 people that work there have at least a 6" lift  

why dont i want spacer blocks. If i remember correctly my kit has add a leafs and blocks with it  

Make no mistake, such mods can void warranty if they choose too (read your warranty fully) as you are in a very gray area with these mods. Long ago (thru 87) 1/2 ton trucks used to basically be built on 3/4 ton chassis and even front drive axles were the same except for different lug pattern on hubs. That all changed in 88 when that when to a new chassis and IFS. The 1/2 uses a LD front drive axle that is unique to it and the heavier 3/4 and 1 ton drive axle and suspension cannot even be installed to it. When Siverados came out, the lightened the chassis again Case in point, you will find no after market SAS kits (Solid Axle Swaps) for Silverado 1/2 ton trucks. In 07 they lightened the chassis yet again and while they claim it is stronger it has less metal on cross section and unlike old C style that is forgiving when pushed to far when this one fails it will fail and weaken permanently. The front wheel pivot in ball joints and they were not designed for the added stress of big tires and changed load centers. The result will be short ball joint and bushing life because of added stress on them. On the spacer blocks. This is the poor mans cheap way to do it. When you step on the gas the axle tries to twist up in springs from counter torque to force applied on tread face. The spacers amplify this force and cause even more wrap up and through in big tires which cause even more wrap up (because more axle force it required to turn them) and it gets interesting. If you use blocks, you do not want to go much over 2 inches or so and you do want to add a leaf or two as well to help better control axle wrap up as well as stiffen it because of higher CG. The proper way to lift it is with re-arched springs and added leafs, not big spacers. When that is not enough you lower spring attachment brackets too.   Sorry if I sound like gloom and doom here but many never consider the physics involved here and today 1/2 ton truck are basically cars with truck bodies and even have softer springs than 10 years ago so they even ride more car like too. Sure some might run 35 and more but a wise man would limit it to 33's if he plans to drive it for a while and have it last longer.  

Thanks for your input snoman you seem very educated in this field.If i downsize to a 33 tire will it solve all the problems or just make it last a little longer.I am also putting 18x10 wheels. will this also create problems since the wheel will be larger.If i do upgrade the gears  to 4.88 wil that keep the tranny at a decent temp since i do alot of towing.or is 4.88 to much. I do love my truck and dont want to jeorperdize the tranny life or any thing else for that matter. Keep the knowledge coming i am learning alot. thank you  

The downsize is good but their is a problem with the 10 inch wide wheels. In order to use them they change the offset of the wheels which changes the load center relative to ball joints and wheel bearings.  This places more offset stress on them which is further aggravated by tire size. In 07, GM did place the lower ball joint closer to the wheel on 1500 models to try to extend ball joint life. (the closer it is to load center, the less stress on it) so in theory your 07 should do a bit better in this regard but the bushing for control arms are not beefed up and those 4 bushings hold wheel "on" and in alignment. As far as axle ratios, when you figure best ratio you figure i one way for a old non OD track and another way for a modern truck with OD. You want axle ratio deep enough that so even in OD the tranny does not hunt around on most hills on interstate when not towing if you want max tranny life and cool temps. Basically a vehcile that pulls itself well in OD on hills with reserve will tow really well when not using OD. It should be able to take any interstate hill without a downshift to 2nd or needing to go WOT. If it does not do this then you trailer is too heavy or your axle gears are not deep enough. I do not buy into this running second gear with engine cranking on hiway grades that many do with new trucks with tall gears as the norm. On ratio, a 4.56 would be minimum I would suggest, this would give you about 2100 RPM at 65 in OD which is about right for that engine (5.3 are good motors but they are not known for torque below 3000 RPM and HP peak does not come until past 5000 RPM and torque peaks around 4400 RPM too. At 2100 RPM you have maybe 100 HP or so available to power truck and its accessories. A 4.88 would be turning about 2250 at same speed and have maybe 110 HP or so available. In drive, you would be turning about 3000 with 4.56's and have about 170 HP on tap and with 4.88's about 3250 and have about 190 HP on tap. (with 4.88 you would be safe to tow in OD most of the time) As a comparison, if you new truck has 3.42's in it now, with stock tires it has about 130 HP in drive at 65 and about 80 HP tops in OD. If you through in 33's with stock gears you would have about 110 HP in drive  (or same as OD with 4.88's and 33's) and about 70 HP tops in OD. (starting to see the bigger picture here and why tranny can hunt a lot at times) If you truck has 3.23 gears (as a lot do) it will have even less HP at those speeds.