4:10 VS 3:73 VS 3:54 Gears

[captfrankie]

I have an 84 w350 Crewcab with a 360, Edlebrock 1406 I think,727 transmission, NP 205 transfer case with 4:10 gears.I love everything about it but the fuel economy8-9mpg.I need to improve fuel economy. Has anyone tried changing gear ratios?What are the pros and cons? How much bottom end will I lose? I've pondered a Cummins swap but I really don't have the time to swap.This is my daily driver. Thanks

[QuantumJo]

The easiest way to change your final ratio is with tire size. I have found that a taller, narrow tire increased my mpg on my 2000 XJ.

here is a link that may help you.
http://jeeptalk.org/gear_chart.php

[USN78W150]

gears

Leave the 4.10 gears in the truck. Use 3.54 gears if you swap to a Cummins engine. I have a truck identical to yours ('85 model). I get almost 12 mpg in stop and go city driving with 9.50/R 16.5 tires. Regearing would affect your performance adversely. The 3.54 gears work perfect with the diesels because they have a very low rpm power band and low redline speed.

[captfrankie]

Crewcab

I would love to get 12mpg. Is your truck auto or manual? I remember only getting a solid 10mpg in my 85 too. My tire size is 285/75/16. I'm open to any and all suggestions.

[USN78W150]

gears

My truck is automatic. Unless you have regeared your speedometer drive, your mileage on the odometer is not reading correctly with your 285 tires. When your odometer says you have traveled one mile, you have really gone more than one mile because your tires are over 1 1/2" taller than stock tires. Roughly a 5% difference. The effect of the taller tires rolling resistance will make a small difference in economy, as well as how you drive. When I need new tires for mine, which won't be for a long time as my tires are new, I will be going to a LT265/75R16. I expect my mileage will drop slightly when that happens.

[captfrankie]

Crewcab

I bought the truck about a year ago. It had 33/12.50R/16.5LT BFGoodrich Mud Terrains. There is a piece/unit between the speedo cable and transfer case.

[Chewie]

I would love to get 12mpg. Is your truck auto or manual? I remember only getting a solid 10mpg in my 85 too. My tire size is 285/75/16. I'm open to any and all suggestions.

I think most of your trouble is pushing those tires around. Like Q said, skinny tires will help mileage. The tires you have now are about 32X11s. Look for a 235/85/16 (if you want to stay metric). That's about 32X9.25 or so. Hard to convert from standard to metric, but you get the idea. The height conversion formula is:

width (235mm) * aspect (85%) / 2540 * 2 + 16 (rim size) = 31.7

Tire width is easy: 235mm / 25.4 = 9.251"

Otherwise, look for an OD tranny!