Thread: Rearend gear calculations

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I found a formula to finding out what gears I should be running but need to know the RPM at crusing speed of 70 MPH. My engine is a stock 1980 Buick V6 which is rated and 115 HP @3800 RPM. The engine is for my full fendered 26 T coupe. I'll be using a 4 speed and don't want to stress out my little engine. Will be using a S10 rearend 3:42 or 3:73 and 28" high tires.

Formula: RPM X Tire height =
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Gear size X 336 = and divide into the above number equals speed

Sooooooooo

I'll take a guess at 3000 RPM X 28" high tire equals 84,000

Gear size 3.73 X 336 = 1,253.28 divided into 84,000 = 67.02 MPH
or
Gear size 3.42 X 336 = 1,149.12 divided into 84,000 = 73.09 MPH

Soooooooooo does anyone know the RPM of a 115 HP engine when crusing?
How about low HP Flat Heads?

Tall Tom ........... but now because of my stupid computer - Tom F

PS Sure would like HemiTcoupe to get a hold of me!!! I have lots of questions for him.

http://www.wallaceracing.com/Calculators.htm
calculate away.....

Hey this is fun!
However it assumes I know what my RPM will be, AND that's my main question.

Sooooooooo back to my question, how many RPM will my 115 HP (@3800 RPM) engine need for crusing without hurting the engine?

You were right on! I also used the "RPM/MPH from Gear Ratio Calculator" and put in my stock tire size (23.3937") and stock gears (2.81) and came up with lot of examples. I choose 3,000 RPM = 74.3 MPH and 2,500 RPM = 61.92 MPH; added them both together and divided by 2 and came up with 2,750 RPM @ 68 MPH.
Should be able to run all day and not hurt the engine at 2800 RPM.
Now I'll be able to decide if I want to be able to spin the tires a little with 3.73 rears or do the mileage thing and go with 3.42
Thanks a lot for all your help.

I started this project 1 1/2 years ago and am now just starting to build it because I ran into such good deals I just couldn't turn them down. Last fall I ended up with four 26 T coupes, all kinds of Stock parts and traded everything for a Stock 27 T Tudor which runs, has matching numbers, only one spot of rust and came with a title. The guy was asking $4500 for it and I traded all 4 of my Coupes and all of my T parts for it. My wife couldn't believe I traded all that JUNK for a running car in great shape. The only problem was, is that when he came to pick it up I realized that I didn't have a Coupe to Street Rod so I kept one and gave him $500 to boot.
If someone could help me on how to post pictures I'll show you where in SD I got 2 of the Coupes from. There's still all kinds of Vintage Tin out there guys!!!! He said he was going to have an auction; when he does I'll let you guys in on it.
Tom

Going to depend on the weight of the vehicle too. If its too heavy, around 3,000 lbs, I would think that the small engine/ big tire/ 3.42 gear will struggle to get moving and take more power to keep it moving. 115 hp AT 3,800 means even less HP at lower rpms.

Well I finally made up my mind and got 3:73 rears. It was tough to find, called the local junkyard and they wanted $300; went out to their other yard and made the deal with them on the same axle and got it for $100; picked it up today from the first place, tossed them a Hundred and walked out the door.
I even had them save the emergency brake cables. It's now setting under the coupe. Hope to have it up and running by Fall.