What gear ratio?

[chopped&bobbed77burb]

I vote the 5:13's . I have 4:10's with a 4 speed sm 465 and with 42" tires I'm turning 1800 rpm at 55mph . I get ok gas mileage but the burb is a dog and it really need's 5:13's . If you are going to do some towing you will want the extra gearing for passing power and climbing hill's . If you had a big block you could get away with 4:56's .

 

[Leper]

I wouldn't go steeper than 4.56. I run 4.56 with 39.50 tires and still have power in OD.(700r4) You won't be towing in OD anyway. I would see how it does before I swapped gears in it.

 

[kgblazerfive]

What RPM are you running in OD

 

[Leper]

Dunno, never run a tach. I do know, that at 75 mph I start to get rear end noise.

 

[6.2Blazer]

yes this all works and makes sense, but you usually want you OD gear to be the low RPM cruisin gear to provide better mileage. Thats why its usually at 1900-2200 RPM in OD, to try n get some decent mileage while on the freeway. 2400-2800 RPM is more into the powerband but could be costing mileage. And odds are you will be downshifting at any hill anyways. So I guess it all comes down to what mileage you are looking for and engine output. If I was doing alot of freeway miles in a DD with small block I would want to stay at 2000-2200 on the freeway. If I just did short jaunts then 2500-2700 would be OK.

Lower engine rpm does not directly correlate to better fuel economy. Every engine has a certain rpm range where it works most efficiently, cruising on the lower rpm side of the range will result in better mileage but as soon as you drop out of that range then mileage will begin to suffer. The reason is the engines are now "lugging", or whatever you want to call it, and generally working harder to maintain that speed.

I will admit that 2000-2200 on the highway (let's say 65 mph) isn't too hateful for most engines, especially older ones. Though I can say that my last two DD's would have felt pretty doggy and downshifted out of OD a lot on the highway if they were only running around 1,900-2,000 at this speed.