What gear ratio do I need?

[spacetransient]

I am new to the forum but have just gotten through a state of Maryland Inspection so I am now intimately familiar with my '86 GMC 2WD heavy half. I have a 700r4 transmission that I like and a good running 6.2 military diesel. I have a 2.73 rear.
    First and second gear do fine. Third gear lugs until I pick up speed by 5mph or so. Third to overdrive wants to lug as well, again until I pick up another 5mph or so. I think it shifts into overdrive at about 60, maybe a little higher but will noticeably lug below 65 when accelerating or going up a hill. That said it runs strong over 70mph with great gas mileage. Problem is I do not need to be going over 75 for the most part. 2.73 seems too low for me. I do fill up the bed and make dump runs with up to 1200 lbs of load but that is it.
   What gear ratio do you think would be my best trade-off for performance vs gas mileage given I will probably never be pulling a trailer and maybe be doing dump duty once or twice a month?  Normally I do not carry more than 500 lbs and do about 60-65% in town and 35-40% highway. Once a year I will make a long trip.  I am not sure a 3.08 ratio is enough of a difference and eventually I may want to go to 16-17 inch tires thereby decreasing the gear ratio a little later. So I am inclined toward a 3.23 but would appreciate some feedback. Do any of you have similar experiences with satisfactory solutions? Thanks for your thoughts.

[Grim 82]

I don't think swapping to a 3.08 would be worth the effort either. If you can find a 3.42 great, if you can find a 3.73 that's even better, and would be my choice since you have the overdrive trans. What tire height are you running now?

[VileZambonie]

I may want to go to 16-17 inch tires thereby decreasing the gear ratio a little later

 

[Irish_Alley]

Go with the 3.73 as PNG as your not going with like 38s or something like that

[spacetransient]

Looks like agreement at 3.73 and thanks for the info- I would have thought that to be too low for around town.

[spacetransient]

I went about this a different way and decided to determine optimum RPM's for my diesel at 74 mph as I will typically be going 70-74 mph at a 65 mph speed limit. Most people generally agree the optimum power range for this engine is in the 1850-2000 rpm's. According to the experts at drivetrain.com, the 3.08 rear gear will give me 1948 RPM @ 74 mph with 235/75/15 (27.5" diameter) tires and the 6.2 diesel. This is about perfect. In town acceleration should increase somewhat with the 12% increase in gear ratio while still giving me a near optimum power/mpg at normal highway speeds.

[chevymanrob]

i personally dont think you'll even notice a difference going from a 2.73 to a 3.08. i swapped from a 3.55 to a 4.11 and it was BARELY noticeable. if you are shooting for an exact rpm at XXX mph then by all means change it. my $$$ would borrow some tires and wheels from a buddy that are a smaller diameter to mimic the gear swap and see if you like it.

[Psycho71]

I agree, a 2.73 to 3.08 gear swap is really not worth the time or the money spent. The 3.73 is about the perfect gear ratio for the 700r4 transmission when running a "normal" tire size, and shouldn't have a detrimental effect on the overall engine rpm's.

[spacetransient]

Thanks for the feedback folks. The performance people like 3.42-3.73 ratio's. The economy people like the 3.08-3.23 ratio's. By the numbers getting the rpm's in the 1800-2000 range for max mpg's at 74 mph (to be safe from the Troopers at posted 65 mph), means for 235/75/15 tires @ 28.5" (my Gerneral Grabbers are 28.4) gives us final RPM's around 1881 for 3.08, 1973 for 3.23, and 1667 for 2.73!! No wonder she doesn't start to grab until 80 mph!!
   For me it is still a tough choice given I do as many miles in town as on the highway. Clearly the 3.08 favors optimal highway miles. Clearly the 3.23 is better for a mix. If I go with the 3.23 I can always move up to a larger rim of 16" which is more standard nowadays and gives me a much better choice of hubcaps. (no slight intended about our beauty rings but they make it tough to keep the rims clean and they are also not as fuel efficient as flat hub caps) At 16" tires I would still be in there at 1890 RPM/74 mph.
  *The reason all this makes sense is I forgot to add I will be changing out my standard rear to a posi.* Tired of trying to make headway on one wheel. That is why the $ change out for the gears is not as painful as it seems when I am taking apart the rear anyway. Get it done once and for all since I will be keeping this Classic! Trying to swap rears is just as tough and I don't know where its been. So I will be going with the 3.23 gear ratio. 

[ehjorten]

3.23s sounds like a good compromise.  Next step up would be 3.42s and you would be right at 2,000 RPM @ 70 MPH.