Suggestion for final drive ratio

[GTP51]

looking for suggestions for final drive ratio.
Truck is 87 C10 Silverado turbo 700 R transmission diff is not LSD. With 2.73 gear set, tyres are 295/50/15
Which is 26.6'' high. Engine is 350 long rod , alloy heads TBI roller cam, roller rockers , Hooker headers etc etc. the thing is a slug as the moment was thinking of going to 3.55 gears. But after reading some posts here
maybe should be aiming higher.
I mainly just cruise in it so not looking for a burnout ratio set something that will give it some get up and go but not revving it's **** off at 60 mph (100 kph) and yep that slow but unless you wanna lose your licence here in OZ 60 mph is as fast we can go on most highways.
Any help / suggestions welcome

 

[bucket]

I think you are on the right track. If just cruising really is how the truck is used most, 3.73 should work well too while still being able to drive a bit faster without winding it out. Even though it's TBI, it should put it right in the meat of the power band.

 

[GTP51]

Appreciate the advice, now to try and find a gear set and LSD centre next week when I pass through Hawaii...

 

[HotRodPC]

Yep, having the 700r4 an OD, I myself would go with 3.73's. 3.42's would get the job done too. Do keep in mind, those 700r4's aren't the strongest transmissions on the planet so the lower the gear ratio, the more strain you're taking off the transmission. I think the 700r4 will love you for some 3.73's.

 

[HotRodPC]

BTW, with your tire size, and in OD with the converter locked up and 3.73 rear gears, you'd be at just under 2200rpm @ 60mph (100kph)

With 3.42's you'd be just under 2000rpm. That engine isn't even working hard at either one of those ratios. IIRC, 2200-2500rpm is the sweet spot for the optimal mpg/kpg for cruising down the freeway in a TBI small block.

Ohh and don't talk 3.55 rear gear ratio here. That's kind of a sin since 3.55's is a Ford thang, not GM.

 

[GTP51]

Ok 3.73 it is then , thanks guys.
My bad on the 3.55's got cornfused as just did the driveline on one of the other brand cars, can't say 65 mustang fastback here I suppose
Now to find the parts whilst enroute back home

 

[GTP51]

Anyone with a suggestion to get the gear set and centre in Honolulu feel free to chime in pleeeeeese..

 

[spanky55amg]

I dont know much about TBI, but...
In the world of 4th gen fbodies, from the factory, automatic cars came with either a series 2 carrier 2.73 or a series 3 carrier 3.24. My TA had the 3.24's and stock I got up to 24 or 26 mpg on one trip (this was rare as I have a lead foot). The 3.24 still got me a 12.6 in the 1/4 bone stock. (oh the DA that day was so nice.) That all said and done, most automatic guys that swapped gears went for 3.73's and saw marginally less mpg.

 

[MikeB]

Given your 26.6" tires, and assuming your axle is an 8.5" 10-bolt, I'd suggest 3.42. I think 3.73 with your short tires would make the 700R4 1st gear way too low.

Going from 2.73 to 3.42 will effectively make your engine feel like it has 25% more torque (!) at all RPMs, and will make a tremendous improvement in acceleration. I've been there & done that with a 55 Chevy car that came to me with a late 70s 10-bolt.

Also, if your truck has an 8.5" 10-bolt, the carriers are "2.73 up" and "2.56 down", so you don't need a thicker ring gear or a spacer for 3.43 or 3.73 gears.

Finally, having driven a 69 C10 for 24 years with a 350/TH350 and 3.08 axle, I can say 2100-2200 RPM at 60-65 MPH had me wishing for taller gears or OD trans. So, unless your engine has a big cam that isn't happy under 2000 RPM, I'd suggest you shoot for 1800-1900 when locked-up at 70 MPH. And when you need to scoot, let the TC unlock, or let the trans downshift to 3rd.

Edit: Just noticed you rarely drive faster than 60 MPH. In that case the 3.73s should work fine.

 

[MikeB]

Ohh and don't talk 3.55 rear gear ratio here. That's kind of a sin since 3.55's is a Ford thang, not GM

GM had 3.55 gears available for lots of vehicles, such as the old 8.2" axle in 55-65 cars, 8.2" 10-bolts in 67-up Camaros and mid-size cars, and 12-bolts in late 60s-70s cars with high performance engines.

 

[malibuman402]

have 3:73's in my 84 with a np833a manual 4speed. gear ratios are about the same as a 700r4. also have 295-50 15 tires. 60 miles an hour is about 2000 rpm.