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what gear ratio - need advise

T

TheAustrian

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Hi there! I need your advise!

I already asked in the introduction thread, but I think it is better to ask you this here.

Please excuse for any error I make, english is not my mother language. :bow:

I have a 79 Blazer 350cui Small Block with TH350 Automatic and NP203. As far as I know the gear ratio is 3.73 and stock tyres are 31x10,50x15. I have now 35x14,50 inch tyres fitted and have two unwanted effects now.

1. speedo goes wrong (more than 16% too slow)
2. accelaration and drive are good, but I'd like to have more performance on both on and off road. Engine puts out over 200 hp now.

My plan is to change diff gear ratio to come either as close as possible to the stock ratio, or go a step shorter. The 4.11 gear comes with the 35" tyres very close to the stock 3.73 ratio with 31" tyres (only 2.46% different). The 4.56 gear is about 7% shorter ratio rather then the stock ratio.I think with a 4.56 gear the performance must be very good, of corse top speed will be lower at higher rev. But I don't need top speed at all. And I must adjust speedo gear ratio. Is there a gear ratio set to adjust the about 7% shorter ratio?

What is your oppenion? Is it better to go with the 4.11 gear (close to stock ratio) or got with the 4.56 gear which is much shorter? Maybe too short?

Thanks for all replies in advance.
Thomas
 
I'd go with the 4.56's.. Help out the bottom end, especially with the 2.52 first gear in that 350...

And if you ever go bigger on tires, you'll appreciate the gear...

And welcome to the forum!
 
I would go with the 4.56 gears. Although the 4.11 gears would mathematially be comparable to the stock gears/tires, the truck will "feel" slower than stock because the taller tires are harded to get turning because of the added weight.
 
I would go with 4.11 . With your tansmission , your cruise rpm will be real high with the 4.56 and your transmission having a 1 to 1 ratio in high gear .

I have 4.56 with 35 inch tires , and at 70 mph I turn around 2000-2200 rpm in overdrive . Shift to 3rd gear 1 to 1 and I turn about 3000 rpm .

I would hate to have my motor turn that high for extended periods .
 
I would also go with the 4.56 gears. I have 4.56 gears with 37" tires and I wish I had 4.88 in some instances. I think 4.56 and 35" tires would be a perfect combination as long as you dont drive very fast for extended periods of time.
 
If you dont to alot of high speed driving(65 and up) then 4.56. If you use it on the freeway, then 4.11. On my old sub I put 4.56s in it with 35s, and with OD it would cruise at too high of rpms for me at freeway speeds(75-80), so I ended up going to 38s.Dan
 
first off, no apologies nessesary, your english is quite good.

well most people handled the gears, my suggestion is if its a daily driver, id stick with the 3.73 gears, if its a toy that does alot of low speed stuff, the 4.56 gears.

as for the speedometer problem, go to the local GM dealership, and talk to either the transmission specialist or the parts counter. tell them the tire diameter you are running, and what gears are in your diffs, and they will give you two gears for the speedometer cable drive mechenism that will allow the speedo to read true. you can either install them yourself.

that requires draining the trans fluid. removing the driveshaft. removing the speedo drive mechanism, removing the tailshaft housing, and swapping the gears, then putting it all together.

or pay the dealership to do it. it should only be a 2 hour labor charge, around $180.00 here where i live.
 
Thanks to all of you for your pro's and con's!

I personally also tend to the 4.56 because I want performance at all situations. The Blazer shall be a toy, driving around just for fun, go for US-Car meetings, and do off road wheelin with friends. I don't need it as a daily driver, altough I may go for office more of less often and use it in maybe in Winter time just for fun and some snow wheelin. My driving speed is anyway max. 62-70 mph, so I think it won't be a problem with too high revs. ? I simply wasn't sure if the 4.56 is a bit too hard for 35" tyres. Here in Austria/Germany are not so many people having Blazers and experience with such modifications. :k5:

Great Forum this with a lot of valuable knwoledge and cool stuff :thumb:
 
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You will be close to 3,100 rpm with 35's and 4.56 gears. Here is a calculator you can use to see how different ratios will work http://www.tyres1.com/calculators.html



K5dreamer said:
first off, no apologies nessesary, your english is quite good.
Click to expand...
He actually knows English better than many people on this board from the USA.



.
 
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You will be close to 3,100 rpm with 35's and 4.56 gears. Here is a calculator you can use to see how different ratios will work http://www.tyres1.com/calculators.html

.
Click to expand...

Thanks for the link! Nice tool! So with the 4.56 gears I will be always 200 RPM ahead compared to stock (31's and 3.73 gears). Which I believe is not so big problem when high speed is not my priority, isn't it? But another thing I am concerned a little of is, my engine is not stock anymore. It is now putting out over 210 hp and about 400 nm of torque. How would the more power go with the 4.56 gears and 35's tyres? Is it still okay for the power?

.

He actually knows English better than many people on this board from the USA.

.
Click to expand...

You've gotta be kidding ;)
 
the ability of the gears to handle the increased power depends on the quality of the gears. if you get cheap sheisse (sp) brand gears, they might give up, but if you get a quality set of gears and set them up correctly, you shouldnt have any problems.

and sadley, hes not kidding about the english. its quite sad how many people in america dont seem to know the language.

ah well, haben sie ein schones tag herr austrian.
 
Okay, I don't want spend money for "Scheisse" ;) but buy good stuff instead. I have to select carefully what I will buy since the price I am paying upon arrival is at last double of what it cost you. Is there any recommendation you could give me which brand is worth spending a lot money for? I know Richmond, Motive Gear, Superior,... but I don't know which one delivers best quality for my money. I actually do not mind if its USD 30 up or down, but do not need Racing quality or high end gears. Thanks! :bow:

Have a nice day too ;)
 
For GM trucks with corperate axles (10, 12, 14 bolt), I like Precision gears. Superior sometimes re-boxes Precision gears and sells them under their own brand name. For Dana axles, I would use Spicer gears. If spicer is not avalable, then I would go with Precision. That is what I did, and I have no complaints.
 
randy88k5 said:
For GM trucks with corperate axles (10, 12, 14 bolt), I like Precision gears. Superior sometimes re-boxes Precision gears and sells them under their own brand name. For Dana axles, I would use Spicer gears. If spicer is not avalable, then I would go with Precision. That is what I did, and I have no complaints.
Click to expand...

I have GM corporate Axels, 10 Bolt at front end and 12 Bolt with genuine GM locking differential at rear end. I assume the installation kit should be same brand as the gears are - right?
 
having the same brand gears and installation kit isnt always nessesary, but it does decrease the chance that the parts will have fitting problems. If its all from one company then it will have been designed to work together. but with it all being pretty much standardized you can shop around. somtimes one brand will have cheaper gears, but a more expensive kit. it all depends. personally if i didnt mind spending a bit more money, id just get everything from one company, and actually youll probly save money on shipping when its all said and done to have it all drop shipped at once.
 
Have been checking my axels to make sure what gear to order. My front axle is a GM corporate 10-Bolt with the oil drain screw (is it called like that?) on front cover. This one has a 7 1/2 inches ring gear (according to the axle guide here). This installation kit from Superior for example is half of the price of the Kit for the rear 12-Bolt axle. Can this be correct? I checked the prices at Summit Racing.
 
Front ten bolts will be 8.5", must be a mistake if the guide shows 7.5. Could be listed for an S10? (only thing I can think of that might have used that small of a ring gear)

If you've got the locking differential (read all about the "gov-lok") then you might really want to consider another differential carrier or even axle set if you can find them. With those size tires and any intentions of seriously using the truck offroad, it's only a (short) matter of time before the gov-lok comes apart. :(
 
the 7.5in ring gear (also the 7.625in ring gear) were in car and light duty truck duty applications. the third gen camaros for example got the small 10bolt with the 7.5 or 7.625in ring gear.

the 8.5in ring gear was in mid size and up trucks as well as some S10's and high performance cars like the buick grand national.

and then the camaros got the auzzy 9 bolt rear end ;)

thats pretty much the breakdown on the GM sub 12 bolt rears to the best of my knowledge.
 
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