Gear Ratio.. help?

[SquareBodyBoy76]

My truck is a rebuilt drivetrain away from being road ready but I'm concerned with what type of gear ratio I should be running. My current daily is a little sedan and my cruising speed is about 70mph at 2000rpm. I'd like to keep that consistent with my truck. It's a 76 k10 with a 4 inch lift and 36 inch tires on it. What gear ratio should I go with to be able to maintain that same cruising speed and rpm? I've heard that between 1800 and 2200 is a sweet spot for mileage. I know I can't expect a noticeable difference but I'd like to have SOME fuel efficiency while maintaining such a high cruising speed

[rich weyand]

2000 rpm with 36" tires at 70 mph is like a 3.06:1 axle ratio.  But you are going to be lugging the heck out of that engine with 3.06:1 and 36" tires.  That little sedan probably has overdrive.

http://www.advanced-ev.com/Calculators/TireSize/

I run 31" tires on a 3.73:1 axle ratio and run 2426 rpm at 60.  I still get 12 mpg at 70.

[jg1977c20]

that calculator wont give you the actual gear ratio in the diff - it gives you the ratio after the tire dia. which will change with a change in tire diameters , so dont go by that , you need to figure out what gears you have and the only way is to take the cover off and count ring teeth and pinion teeth - what diff do you have - my 77 c20 has the code stamped on the upper portion of the tube about 6 inches right of the pumpkin looking from the rear .

heres how the calculator works since i know my ratio is 4:10 with 31 inch tires = 3100 rpm @70 mph
                                                                                        4:10 with 36 inch tires = 2680 rpm @70 mph

now lets say my rpm is 2800 speed is 70 tires 36  the calc would tell me i have a 4.28 ratio now if i drop the tire size to 33 using the same speed and rpm the calc says i have a 3.92 - the tires are changing the end result in ratio but you still dont know what gears you have to start with .

confused ? good .

so your sedan likely has really tall gears as most cars do -your truck being 4x4 i would guess will be in the 4:10 range but find out for sure

the other thing is ...you need to decide between the truck being a 4x4 or a street truck  your ultimately going to have to sacrifice top speed for torque to turn the bigger tires or risk going thru motors and trannys unless you have really deep pockets

[timthescarrd]

MY 83 K10 has a 350 v8 700r4 trans (0.70:1 in overdrive) with 31" tires and 3.08 rear end.  I can do around 2000rpm @ 70MPH, Highway MPGs seem to be 14-15, but because of driving around town and hills, i average around 8MPGs

Trying to climb a steep hill, or tow anything and the truck bogs down.  Living in the mountains, it's a real pain.  Going up most hills I will lose speed unless I'm turning 4000RPMs

[SquareBodyBoy76]

Upon some further thought, my cruising speed will likely go down to about 60-65. I have a heavy foot but with being in such a tall truck I'll need to cut back some. I'll most likely run 3.73's or maybe a 4.10. I like speed but I'll need to use it offroad and to tow on hills and such, so it won't be much of a street truck although it'll be my daily driver.

[jg1977c20]

Upon some further thought, my cruising speed will likely go down to about 60-65. I have a heavy foot but with being in such a tall truck I'll need to cut back some. I'll most likely run 3.73's or maybe a 4.10. I like speed but I'll need to use it offroad and to tow on hills and such, so it won't be much of a street truck although it'll be my daily driver.

go with 4:10s for sure with that tire size , and you wont be disappointed towing up hills or getting out of a jam and driving around the city wont be a dog .

[Captain Swampy]

When using the calculators, measure the height of your tires. My daughters 33" BFG's are actually 31 9/16ths inch tall. She has 4.56s with a 700R4. Our local speed limit is 55 and this combo is pretty good. With a 35 or 36 inch tire it would work well at 65-70 MPH. I'm guessing you have a TH350 though.

[SquareBodyBoy76]

I forgot that part, I do have a TH350

[Irish_Alley]

http://forum.73-87chevytrucks.com/smforum/index.php?topic=30546.0
now i mud and drive my lifted truck on the roads. with the sm465 i had in my 79 i had 4.10. theres nothing wrong with the calculator rich posted it may be a little hard to understand how to use. but if you enter in your rpms, tire size and speed it will tell you your ratio with a 1:1 trans. if you have overdrive it will change the final ratio and manual and autos will have different rpms due to the slippage of the auto
the formula for finding your ratio is

rpms x tire size / 336 / MPH= ratio
i came up with that formula years ago when i was wondering what my ratio was. idk if i originally came up with it but i know i did some math for a bit of time to come up with it.

this is the calculator i use
http://www.csgnetwork.com/multirpmcalc.html
it tells you most if not all info you need, if i was to redo a lifted road truck i would op for 3.73 with the sm465 or a nv4500. if i was you since you have 3.08 ratio then i would think about ¾ rears but then you need new wheels and maybe tires if you cant find 15". this way you will probably get 4.10 ratios anyway and a slightly stronger axles, then if you can find the right rims it will be cheaper in the long run

but whats your intentions on use of the truck? is it just going to be used on the road or plan on mudding or towing?

also with my lifted truck i would get 8 mpgs with 38x16x16 and 10 mpg with 37.5x12.5x16.5 and this was with 4.10 ratio.

[hatzie]

http://forum.73-87chevytrucks.com/smforum/index.php?topic=30546.0
now i mud and drive my lifted truck on the roads. with the sm465 i had in my 79 i had 4.10. theres nothing wrong with the calculator rich posted it may be a little hard to understand how to use. but if you enter in your rpms, tire size and speed it will tell you your ratio with a 1:1 trans. if you have overdrive it will change the final ratio and manual and autos will have different rpms due to the slippage of the auto
the formula for finding your ratio is

rpms x tire size / 336 / MPH= ratio
i came up with that formula years ago when i was wondering what my ratio was. idk if i originally came up with it but i know i did some math for a bit of time to come up with it.

this is the calculator i use
http://www.csgnetwork.com/multirpmcalc.html
it tells you most if not all info you need, if i was to redo a lifted road truck i would op for 3.73 with the sm465 or a nv4500. if i was you since you have 3.08 ratio then i would think about ¾ rears but then you need new wheels and maybe tires if you cant find 15". this way you will probably get 4.10 ratios anyway and a slightly stronger axles, then if you can find the right rims it will be cheaper in the long run

but whats your intentions on use of the truck? is it just going to be used on the road or plan on mudding or towing?

also with my lifted truck i would get 8 mpgs with 38x16x16 and 10 mpg with 37.5x12.5x16.5 and this was with 4.10 ratio.

I made a spreadsheet years ago that calculates rpm based on tire size or actual diameter, axle ratios, and transmission gear ratios.  It also factors in transfer case, range box, or overdrive unit ratios into the RPM tables. 
I setup a table at the bottom that calculates speedometer gears too if needed.
The archive contains two spreadsheets
a Microsoft Excel 1997/2000 .XLS
a Open Office/Libre Office/Star Office Calc .ODS file
http://www.mediafire.com/download/zk4j8ymtctekx5r/RPM_Gear_Axle_Transmission_TransferCase.zip


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[SquareBodyBoy76]

Irish, my intentions on the truck after I finish the build are to use it as my daily driver, but I still want to have fun with it when it's appropriate. I'm not talking about serious mudding, I'm talking more of driving through trails and fields and such. Maybe a mudhole here and there, but nothing ridiculously deep. As for towing, I have a stepside bed that has an expanded metal floor. I can't haul much more than a load of trash or firewood. Otherwise the truck is simply too high with too weak of a floor to haul much else so I'd have to use a trailer. With me having a handyman business during the warmer months, I haul all kinds of stuff, from lawn equipment to tractors to four wheelers and other things of that nature. I'd like to be able to go up and down hills without worry of lugging the engine down, so I think I'll opt for a 4.10 ratio. I'm not terribly worried about gas mileage, I was just curious as to how much I could save when possible.

[Irish_Alley]

what about just dropping it in 4low when you are off road?

[Greybeard]

I agree with most of what Irish says. But there is an easier way to check gear ratios in the rear end. The reason it's easier is that there are not an infinite number of ratios available for the 12 bolt so all you need to do is get close enough. with that thought jack the back of the truck up or kneel down beside it so you can see the driveshaft clearly. Place a mark with something that is highly visible (like white chalk) perpendicular with the driveshaft near the pinion maybe on the u-joint cap. Also put a mark on the very bottom of the tire at the middle of the contact patch on the sidewall. Have someone drive very slowly for one complete revolution of the tires and count the number of times the driveshaft rotates. 4 times and an about an inch is 4.10's 3 times and less than an inch is 3.08's, 4 times and about a half revolution plus a bit is 4.56, 3 and 3/4 times is 3.73 and so on. Easy.

if you have it on blocks do the same thing, but since the likelihood is that you have an open diff, it has some differences. If you have a locker (only, not posi) you will have to spin (by hand) both tires at the same time and equally. One person can spin while another counts.

This will get you close enough to deduce the rest. It might get interesting if you have a rare 3.92 gear ratio but that is highly unlikely.

You also have to be aware that if you swap rearends for a 14 bolt you need to decide first what gear ratio you will want, there is a carrier change between 4.10 and 4.56 unless you opt for a aftermarket carrier like some type of locker- self contained, mechanical, air or hydraulic actuated.

As Irish said, 4.10's and 3.73 are by far the most common ratios in old GM 4x4's. $.56 was common in the cab and chassis trucks because a lot of them became dump trucks (dually housings anyway), and they generally had six cylinders up front. What the engine lacked in grunt the gears made up for at the loss of top end.

36 inch tires and 4.10's with a stock engine will do OK. The 36's are likely closer to 35 anyway and even less when the rolling radius is measured. And it's the loaded measurement that matters to the gearing, not what's on the sidewall. Off roaders air down for two reason's, to get more flex in the tread and sidewalls and to change their gear ratios a small amount. Smaller diameters will give larger ratio numbers. For instance, take a 4.10 to maybe a 4.20 at two inches smaller rolling radius. Which, BTW, is why I absolutely HATE the new rim sizes, there is no logical reason to put larger rims on a 4x4 IF you use the truck as a real 4x4.

In addition, just sayin here, 4" of lift and 36" tires is not a tall truck. Run it like you stole it. I used to slide my 36" tired, 4" lifted truck around corners. Not saying everyone should...but it is doable. The tires were mudders.