55 mph rpm?

1970F250 · Post by **1970F250** » Fri Feb 10, 2006 10:25 pm

update: i looked at my tag and it says 3.54. don't know where i got 4.10 from

CJ,s 67 F100 · Post by **CJ,s 67 F100** » Sat Feb 11, 2006 8:45 am

Ok Here's your new rpm's
3.54 X 55 X 336 Div by 33 = 1,954rpm's

jor · Post by **jor** » Sat Feb 11, 2006 9:53 am

i have a 390 with a c6 and 4.10 gears and im at 2300 rpm at 65 mph.

Gee, that seems awfully low for 4.10 gears! My 3.50 rear end delivers about 2,500 RPM at 65 mph. Seems as though yours would yield 2,300 or so. Are you sure that rear end is a 4.10?
jor

1970F250 · Post by **1970F250** » Sat Feb 11, 2006 11:05 am

1970F250 wrote:update: i looked at my tag and it says 3.54. don't know where i got 4.10 from

i really don't know where i got 4.10 from

if the equation says 1,954 but my tach says 2300 does that mean my tach is off or that my tag is wrong and i have different gears?

binder56jd · Post by **binder56jd** » Sat Feb 11, 2006 12:00 pm

http://www.4wheelparts.com/4wp/ORS/gear_calculator.asp

jd

1970F250 · Post by **1970F250** » Sat Feb 11, 2006 12:57 pm

that one says 2345 so that sounds better

CJ,s 67 F100 · Post by **CJ,s 67 F100** » Sat Feb 11, 2006 3:59 pm

That calculator gives 1982rpm. I came up with 1954. Your tach says 2300 so check your tire height again, measure it across from side to side and chock the front and jack up the rear end. Do this on a flat surface and be safe. Then put it in neutral, mark the bottom of the tire and the bottom of the drive shaft and turn the tire one full revolution and count how many turns the drive shaft turns. 4 to 1, 3 1/2 to 1, 3 1/4 to 1 what ever and then you'll have a better idea if your rear gears are correct with your axle tag

binder56jd · Post by **binder56jd** » Sat Feb 11, 2006 4:36 pm

another not so published fact- in relation to analog speedometers- law allows for them to be +/- 10% from the factory- law enforcement is aware of this- so cj's method is a good one- also the 4-wd calculator is assuming everything is new - no wear etc. jd

1970F250 · Post by **1970F250** » Sat Feb 11, 2006 6:10 pm

i'll do that tuesday and let you guys know what happens.

kani73 · Post by **kani73** » Wed Feb 15, 2006 5:03 pm

I am not sure what RPM I am at going 70 (no tach yet)but I know it is winding out pretty good. Enough I dont want to take it any faster. I have a 302 with a 3 speed. Not sure what rear end yet. Just got the truck last weekend.

Post by **FORDification** » Wed Feb 15, 2006 6:35 pm

kani73 wrote:I am not sure what RPM I am at going 70 (no tach yet)but I know it is winding out pretty good. Enough I dont want to take it any faster. I have a 302 with a 3 speed. Not sure what rear end yet. Just got the truck last weekend.

Compare the numbers on your VIN tag (on the driver's door) to the codes posted here:

http://www.fordification.com/data-1970.htm

The axle code will tell you what it came from the factory with....which of course means nothing if a previous owner has replaced it.

kani73 · Post by **kani73** » Wed Feb 15, 2006 9:02 pm

Will do that tomorrow. I am lucky because I know the original owner. He the father of a friend of mine(who i bought the truck from) Thx

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55 mph rpm?

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