What rear end gears do I have?

[Steve69]

Hey Guys,

Im running a 26.5 tires on the rear and when Im driving at 70 mph which doesn't
seem like im going that fast the Engine is reving at 3000 to 3100 rpms. My
tag on my vin says 2.80. Im running a 4sp transmission. I got under the car
and spun the wheel one turn and it appeared to me the driveshaft spun 2
complete turns. Is there a better way to determine gear ratio with out ripping the
rear end apart? Am I missing something or do I have 3.55 or 3.73 gears?
Like I said When Im going 70 mph it seems more like 55 or 60 mph.

Thank for the help.

 

[bigcat]

tape a shoe string tothe bottom of the driveshaft, then rotate. count the number of times the shoe string overlaps, and estimate from there.

 

[brianj5600]

Are you sure the speedo is right? 3000 rpm + 26.5" tires at 70 mph = 3.38 gear. 2500 rpm + 26.5" tire at 70 mph = 2.80 gear. Some thing is not adding up.

 

[Steve69]

Well the only thing I know for sure is the tire size which is 295 -50 15 and Im
reving about 3000 rpms. The speedo says 70mph but It seems slower than
that. I was thinking of following my wife in the Van and have her go 60mph
so I know exactly what my speed is and how far off it is. That way with all
the different calculators I can figure out what my rear end gears are or at least
pretty close.

 

[65ShelbyClone]

Something that works for me is putting a stripe of masking tape on the back of the tire so it lines up with something like the ground or frame. Then I turn the driveshaft and count how many revolutions it takes to line up the tape mark.

 

[Steve69]

alright I did the tape and string thing and I had 2 complete turns to one spine
of the tire. So is that 2.0? That doesn't seem right

 

[D.Hearne]

Jacking it up and spinning the tire doesn't work with an open diff rear. 3000 rpms at 70 with your tires works out to a 3.40 geared diff. That's a 70's 8" gear option. It's what's in my 77 Comet's 8". Sources say it likely came from a Pinto. 26.6" tire x 3.1416 (Pi) to get the circumference--83.56. Divide that into 63360 (inches in one mile) to get revolutions per mile. Multiply times gear ratio to get revs per mile @ 60 mph. Then mulitiply that times 1.166 (70 divided by 60 equals this figure) to get revs at 70 MPH. Ain't Algebra fun ?

 

[65ShelbyClone]

I forgot to mention to only jack up one side of the rear if an open differential is involved.......

 

[rbohm]

if your tach reads 3000 rpm, you have a 3.40 gear. if 3100 you have a 3.50 gear. that said, the first thing you need to do is drive the car on the freeway, and see what you odometer reads. this will tell you if your speedo is off or not.

 

[96 DOHC Cobra]

I forgot to mention to only jack up one side of the rear if an open differential is involved.......

Like he said. Jack up ontire only. Mark both the driveshaft and the raised wheel. Turn the wheel one full revolution and count the turns on the driveshaft. THEN, MULTIPLY BY TWO. This only works on an open diff.

 

[Steve69]

I had jacked up both axles. I try the one side and give that a try. If I multiply
by times 2 that would give me around 4.00 gears. Which seems about right.
Ill have my follow my wife at a set speed of 60 or 70 miles per hour to see
what my speedo says and what my rpms are to get a accurate reading.
Im thinking since most of miles are hwy miles maybe a Tko or T5 are in order
this winter.

Thanks again for all the info.
Steve