School me on gear ratios.

[zukking01]

I have a 79 3/4ton suburban frame and running gear. 400sb with th400 and np208. The gear ratio is the stock 4.10 with a 8 lug 44 and ff14. I have 38 swampers on it and it cruises down the freeway around 2500 rpm at 65mph. I dont have lots of throttle response but can get to 65-70 with time. I would like to get there faster and lower gears look like the choice. I dont know what ratio to look at. I want it geared for power without wrapping the engine to death on the freeway.

Fuel economy is nice but the 38 might have killed that luxury.

Wondering if the 5.38 would be too deep but dont want to settle on the 4.56 or 4.88 only to want the lower gears later.

It is for a driver that will see minimal trail use and towing.

What do you think and where would be a good place to buy from?

 

[iamsparticus]

I wouldn't think that is too low if your running 38's it will make up for it, i run 4.10s with 35's and i get better gas milage than with 31's, but the real question is why is a 3/4ton with 38's your driver, that will see minmal trail and towing? Shouldn't is be the other way around, or maybe smaller tires?

 

[zukking01]

I wouldn't think that is too low if your running 38's it will make up for it, i run 4.10s with 35's and i get better gas milage than with 31's, but the real question is why is a 3/4ton with 38's your driver, that will see minmal trail and towing? Shouldn't is be the other way around, or maybe smaller tires?

Here it comes........

I have a BUMMER for a run around town vehicle. I have a work truck but this is my fun toy to drive. The bummer had 3/4 ton and bald 35" when I bought it. Found a screaming deal on 38 so here I am. Will be replacing all tie rod and ball joints on it to make it better handling. Looking for stock height springs and want to replace gears for when I drive it now and then I dont have the "crap it wont go" thought.

 

[Jinx]

Bummers are fun.

Look for an RPM Calculator on line... It should be able to calculate ratio, ROM, MPH and tire size so you can figure out where you want to be...

 

[BCGPER]

I'll assume you're NOT running an OD trans? If that's the case, 4.88's would be my choice and put you back about where stock was. If you have by chance swapped in an OD trans, take it down one more ratio.

With that said, 2500 is a nice cruising range. If your axles are otherwise sound, you might want to take that gear money, and wake up that 350 a bit with it.

 

[LT.]

This is a real common question and I will do my best to answer it as simple as I know how. Your gears are nothing more than a ratio. So let's assume that your 4.10 gears came with a 235/85/16 tire on them. With that in mind take the 235's and equate them to a standard size we all are a little more familiar with. This would make the 235's measure out to be 31.7" tall. So we will just round them off to 32's. Take 32 over 4.10 and take your 38 and make it the top number in your new ratio. It should look something like this.
32 38
-- --
4.10 X
Now, cross multiply and divide. 4.10 X 38 / 32=X. X will be your new ratio to bring everything back to near stock. Or about a 4.88 ratio. I hope this helps.

LT.

 

[zukking01]

updated first post to tell exactly what the bummer has for running gear.

 

[zukking01]

I'll assume you're NOT running an OD trans? If that's the case, 4.88's would be my choice and put you back about where stock was. If you have by chance swapped in an OD trans, take it down one more ratio.

With that said, 2500 is a nice cruising range. If your axles are otherwise sound, you might want to take that gear money, and wake up that 350 a bit with it.

Trans is a th400 and the engine is a 400. I was thinking of converting the 400 to TBI but the dual tanks are an issue because of return lines and no fuel baffles.

Wondering if I can power up my 400 with current edelbrock intake and carb. I have dual exhaust but stock hedders. Money can be spent on this but I want to do it right the first time and not go crazy on the cash. The 400 supposedly has 19K on it and it does run well for the tire and gear size but I dont know about the 400 except for reading online that they have cooling issues so I thought about getting an electric fan as insurance. What is a good fan and where would I buy it.

 

[iamsparticus]

So sounds like the ratio you have right now is correct, yes i was right feels good oh um yah your good to go, but I liked the TBI idea, just run some more return lines, it will be worth it in the end especially for a driver