Transmission question (700R4 vs TH350)

[stingray72]

Ok.. right now my 70 C-10 has a 290 HP crate 350 with the TH350 transmission. I have sitting in my garage a 700R4 transmission built by Maddog transmissions rated for 700HP. I bought it for hot rod plans on my Jimmy but that project has changed. Anyways I was considering putting it in the C-10 but is it overkill? I mean I think I could get $800 or so for it (retails for $1300 and was never installed). Is the change worth it? What kind of gas mileage change will I notice with the overdrive?

Once I get my 70 back from body guy I plan on driving it quite a bit

[mrein3]

Originally Posted by stingray72

Snip... Anyways I was considering putting it in the C-10 but is it overkill? I mean I think I could get $800 or so for it (retails for $1300 and was never installed). Is the change worth it? What kind of gas mileage change will I notice with the overdrive?

Once I get my 70 back from body guy I plan on driving it quite a bit

In a perfect world, you would see 30% better mileage in a .7 to 1 OD tranny like the 700r4 is. However the world isn't perfect.

What is nice about the 700r4 is that with the lock-up converter you won't be generating heat if you pull a lot. The more I pull with my th350 the hotter it runs. 700s lock up in 3rd too which is what you should be pulling in.

So if you are getting 10mpg now with the th350, guesstimate 13mpg with the 700r4. Gas is $3.00 - $3.09 around here BEFORE this BP issue up in Alaska. Fire up a spread sheet and start typing in numbers and making assumptions.

Assume you can get $800 for your tranny.
Assume you'd get 13 with the 700r4 installed verses 10 without (30%).
You can buy 258.9 gallons of gas with $800 at 3.09/gallon.
258.9 * 10mpg = 2588 miles worth of gas with that $800 if you don't sell the 700r4.
That same $800 will get you 3365 miles down the road with a 700r4 installed assuming 30% better fuel mileage.
3365-2588 = 777 miles further with the 700r4 verses the th350 with the above assumptions.

I usually turn it around to figure out savings.

I drive my truck approximately 1000 miles a year.
1000 miles per year at 10 mpg is 100 gallon of gas per year with a th350.
At 3.09/gallon that is $309 for fuel per year.
1000 miles at 13 mpg is 76.9 gallons of gas. At $3.09 that is $237.69.
I would save $71/year. So at $3.09/gallon it would take me 11+ years to save $800 in fuel at today's fuel price.

DON'T FORGET, ADJUSTING FOR INFLATION, gas is still cheaper now than during the fuel crunch when Carter was president.

NOTE: The above does not include other factors like running 30% less rpms at 70 mph which is a very noticable sound difference in the cab of my truck. 30% less rpms all around assuming you don't drive around in town all day will in the long run effect the life of your engine. 30% less r's with a 3.08 rear end may NEGATIVELY effect your gas mileage as some on this list have discovered.

You have to weigh all these things for YOUR truck and YOUR style of driving then make the decision.

[Longhorn Man]

A 30% increase is asking quite a bit.
The problem is, he probably has 3.07 gears being he has a small block with an automatic (standard ratio when equipped like so) which would put his RPMs way too low, I have seen MPG SUFFER in this configuration.

[67ChevyRedneck]

How much to ship to 29650

But seriously, Longhornman is right, with a 3.08 rear you might see little to no gain in mileage and lug your engine down. Most new trucks with OD come with at least a 3.40's something and most 5.3's have a 3.73 rear.

[67pro-street]

this doesnt answer your question, but i too bought a maddog transmission and it worked great for me. they have excellent customer service before AND after the sale. i bought mine off ebay and it is only rated for 450 hp, but that served my purposes very well. great buy

67pro-street


OH and PS, i would go with the 700 cause it would help with the resale value as well as the rpm's!!!!

[MISTERSHOTGUN]

30% Anything Looks Good On Paper... Or Does It? I Had A Powerglide In My 68 Chevy.. Switched To A 700r4. The Over Drive Made A Huge Difference. I Was Getting About 11 Mpg Now About 16 Mpg. I Would Also Like To Add That Depending On Rearend Ratio, You May Be Looking At An Overkill. In My Case, I Drive Interstate And Highway A Lot. I'm No Longer Paranoid About High Speeds Because My Engine Stays Cooler With The 700r4 At High Speeds Which Also Means It Is Better For My Engine. Lower Rpms. The Powerglide And 350 Should Be Comparable Because Of The 1 To 1. Just Remember That You Might Not Save Tones On Gas... Well We Never Know About Gas Prices.. But What Benifits Will Be Gained On Lower Rpms?
P.s. I Put In A Lokar Shifter And Only Putt'n Out About 340 Hp.. Love The Gears.
Gary

[Ackattack]

I put in a 700r4 when I put in my TPI 350 so I don't know its affect on gas mileage alone.

But with the 3.08 gears the RPMs were way too low and it was lugging the crap out of the engine on the highway at 60. I switched to 3.73 gears and the truck drives a lot better all around and I picked up about a 1.5 mpg.