rpms at 70 calculation

KAWSAM62 · Jul 20, 2007

trying to figure out my rpms at 70 mph used a calculator off the web and came up with 2641@70, i have a 3.42 rear, 25(aprox) inch tall tires, 15 inch rim, 235s on the width and the circumfrance(sp) of the tires are 83 inchs measuring around the tire am i correct?
thx scott

GMPETE · Jul 20, 2007

KAWSAM62 said:
trying to figure out my rpms at 70 mph used a calculator off the web and came up with 2641@70, i have a 3.42 rear, 25(aprox) inch tall tires, 15 inch rim, 235s on the width and the circumfrance(sp) of the tires are 83 inchs measuring around the tire am i correct?
thx scott

I have a feeling something is not correct--I would guess
you are doing over 3000 with that combo. My data book shows 2646 with a
342 rear and 26" tires AT 60MPH--if your tires are 25" and you are checking
70 MPH you are way past 3000 at 70,with a 342 rear. Pete

Keith Tedford · Jul 20, 2007

Our '69 with the stock F70 X 14 tires and 4.10 gears was supposed to be about 3280 at 60 mph. I don't have a tire handy to measure right now but it would be in the 25-26" range I think.

bzack · Jul 20, 2007

If you have microsoft office or excell, you can use my thing I made, lets you type in tire size, gear ratios, rpm, and gives you the speeds. http://www.mediafire.com/?7g9xng0yx3y

If you don't, well I typed it in 235/60/15 for tire and at 3100rpm in 1:1 ratio gear you go 70mph. 235/60/15 is a 26.1 inch tire though. 235/50/15 is a 24.3 inch tire, so I'm not sure what 25 inch tire you have. Unless there is a 235/55/15

Either way those would all be even higher RPM.

65cayne · Jul 20, 2007

I am pushing about 3000 with a 3.36 rear, th400, and P215/70R14's.....my speedo might be off though (not the original tranny)....forget what I said...

65cayne · Jul 20, 2007

actually....shouldnt matter if the final gear is the same (1:1) and stock tires sizes are used on the car. I say that because my car does what the calculator says it should do.

Try this simplified example using a P205/75R14 (get this info off of your tire), 3.36:1 rear axle, 70 MPH cruising speed....

The formulas:
RPM = (168*MPH*GearRatio)/TireRadius
TireRadius = (RimDiameter/2) + AspectRatio*Width/2540

In this example (try it yourself using your own numbers):

TireRadius = (14/2) + (75*205)/2540 = 13.05" (which is about a 26" tire)
RPM = (168*70*3.36)/13.05 = 3027 rpm

Hope that helps....it's not that bad with a calculator and some paper.

BTW: This was gleaned from edelbrock's website. Kudo's to them, not me.

onovakind67 · Jul 20, 2007

Manual or automatic? None of these calculators makes any adjustments for converter slip.

bzack · Jul 20, 2007

True on the convertor slip, my original non posi 2.43 seemed like a 2.75 and my 4.11 seemed like a 4.33.

KAWSAM62 · Jul 23, 2007

its a 4 speed muncie m20 wide ratio tires are 235/60/15s
thx all

SWHEATON · Jul 23, 2007

I have 3:31's/m20/26" tires & motor turns approx 2650rpm @ 60mph & 3k rpms @ 70.

Scott

tedixon · Jul 24, 2007

My speedmometer must be way off. With an M20, at 65 mph, the rpm is about 3500. The rear axle gears are stock for a Powerglide automatic. It's supposed to be 2.73.

SWHEATON · Jul 24, 2007

65mph & 3500 rpms either your tack is off , the speedo gear in the trans is wrong,or someone changed the ring & pinion.

For a guess,m20 with 273's you should be @ approx 2,300 rpms+ - if you have 26" tires.

Scott

rpms at 70 calculation

KAWSAM62

GMPETE

Keith Tedford

bzack

65cayne

65cayne

onovakind67

bzack

KAWSAM62

SWHEATON

tedixon

SWHEATON

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