Gear ratio for 700r4???

[K10 DELUXE]

I am running 38" tires on the street with mainly in town driving and not sure what gears I should swap in. I currently have 3.08 gears and am getting 5 mpg lol. THANKS!!

 

[oneluckypops]

bare minium would be 4.56's but honestly 4.88's would be better.

anything less then 4.56's and your trans will be short lived

 

[K10 DELUXE]

Would those gears also be good if I swap in a turbo 350 eventually?

 

[oneluckypops]

yup

 

[bucket]

Boy, 38's and 3.08 has GOT to suck. Rather than a gear swap in a rear 10 bolt, money would be better spent swapping gears into a 14 bolt (6-lug semi-floater if you want to keep wheels) and bolting that under the truck.

 

[K10 DELUXE]

You mean a rear end out of an early 90's 6 lug 2500 truck?

 

[Driver4r]

Or he could get a 12Bolt out of the 70's.

When i step up to 35's and a 700r4, Im looking at regearing a set of axles to 4.88's

 

[bucket]

You mean a rear end out of an early 90's 6 lug 2500 truck?

Yup. Just needs the perches cut off and welded on in the correct spot.

Or he could get a 12Bolt out of the 70's.

When i step up to 35's and a 700r4, Im looking at regearing a set of axles to 4.88's

The 12 bolt truck rear is really no stronger than the 10 bolt.

 

[philjafo]

Rpm at 60 in od with 308 gears with stock tires should be in the neighborhood of 17-1800
With 38in tires probably about 1100 and with the added weight of the bigger wheel performance must be sucking. To restore rpm at 60 to that 17-1800 rpm and bring you back to stock performance 4.56 is the min. If your set on putting in a turbo 350 I'd say 4.11s just to keep cruising rpm down a little. Yes the turbo 350 is a little stronger then the 700r4 but the 700 has a lower first gear and overdrive and the later ones if properly maintained work well and last a long time. I would also say skip the semifloat 14 bolt if your running 38s you should have a 14 ff and a Dana 44 minimum probably a Dana 60. Just my two cents, how bout some pics of the truck.

 

[philjafo]

By the way there's lots of gear ratio tire size calculators on the net, google you one up and start playing around with different gears.

 

[Driver4r]

I Currently have 3.08's in my back axle on a set of 32" tires. At 2k im around 55ish, and at 2.5k im around 60-65

 

[K10 DELUXE]

Sorry I should have been more clear lol as of right now the truck has 31x10.50's but I HAVE a set of half wore out 38's to put on once I get time to install my lift and heavy axles. But I'm still only getting 5 mpg with the 3.08 gears and 31 inch tires.... Any idea why my mileage is so bad? Maybe the 4 speed auto?

 

[philjafo]

Are there other symptoms besides poor fuel mileage? It could be anything from the brakes dragging to a fuel leak.

 

[HotRodPC]

Sorry I should have been more clear lol as of right now the truck has 31x10.50's but I HAVE a set of half wore out 38's to put on once I get time to install my lift and heavy axles. But I'm still only getting 5 mpg with the 3.08 gears and 31 inch tires.... Any idea why my mileage is so bad? Maybe the 4 speed auto?

The 4 speed auto isn't hurthing your mpg, unless its got worn bushings and dragging drums, or has a valve body leak somewhere and hanging a servo up that's dragging a band. I much doubt that is the case though. If that was true, the trans would be overheating and would have wore it out by now.

I'd be looking for fuel leaks, tuning issues, and also as mentioned brakes dragging. How do you know you're getting 5mpg? What method of calculation are you using? Are you certain your odometer is accurate?

 

[K10 DELUXE]

The mileage is close because its 1 mile to work from my house so I drove 4 miles a day (coming home for lunch) and had to put in about 4-1/2 gallons per week. It runs great with tons of power, supposed to only have 10k on a rebuilt bottom end and I just put on fresh heads but it does run awfully rich. It doesn't help of course that I don't know much about carbs. I also put a new gas tank and sending unit along with fuel pump and new lines throughout. Hope this helps you guys help me.