Gear ratio confusion

[Hopper]

I am confused using some gear ratio calculators.
I have a 1999 automatic 3spd and 275/75r16 tires. Running 65 mph I am at 2250 RPM. Any idea what ratio I may be at???

 

[jjvw]

Measure the height of the hub center from the ground. Double that number. That is very close to your true rolling tire size.

Enter that into the calculator of your choice. Report back.

 

[jjvw]

Got it worked out. Based on the information provided and assuming the 65mph is from GPS, I know the height of your wheel hub center and I know your gear ratio.

 

[Hopper]

Got it worked out. Based on the information provided and assuming the 65mph is from GPS, I know the height of your wheel hub center and I know your gear ratio.

Thanks for the help.
I will get on that after all this rain blows out by Wednesday. In the meantime what did you come up with?

 

[Hopper]

I will see if I can GPS it today

 

[freedom_in_4low]

275/75r16 tires

really? Discount tire and tire rack don't sell tires in that size.

3.07 is the ratio that comes closest for me but it's still off by about 70rpm, which probably comes down to speedometer error (about 2mph would cover it). Since a quick search didn't turn up a tire in that size for me to get the specs from, I'm guessing at 656 for your tire revolutions per mile because your tire size works out to 32.2" and (for a BFG KM3) a 33 is 640 and a 32 is 659.

 

[Jerry Bransford]

Look at the steel tag bolted onto the diff cover at its 9 o'click position, embossed into it will be the axle ratio. My bet, depending on which axle you look at, is it will will say either 3 07 or 3 08.

 

[jjvw]

really? Discount tire and tire rack don't sell tires in that size.

3.07 is the ratio that comes closest for me but it's still off by about 70rpm, which probably comes down to speedometer error (about 2mph would cover it). Since a quick search didn't turn up a tire in that size for me to get the specs from, I'm guessing at 656 for your tire revolutions per mile because your tire size works out to 32.2" and (for a BFG KM3) a 33 is 640 and a 32 is 659.

Based on the information provided, these are the two likely options.

32rh/29.8"/3.07=2250rpm@65mph
32rh/36.2"/3.73=2250rpm@65mph

Assuming the mph is accurate at the speedo, we can guess pretty well that his hub height is either very close to 15" or very close to 18".

 

[Hopper]

really? Discount tire and tire rack don't sell tires in that size.

3.07 is the ratio that comes closest for me but it's still off by about 70rpm, which probably comes down to speedometer error (about 2mph would cover it). Since a quick search didn't turn up a tire in that size for me to get the specs from, I'm guessing at 656 for your tire revolutions per mile because your tire size works out to 32.2" and (for a BFG KM3) a 33 is 640 and a 32 is 659.

You are correct. I sausage finger typed that in. 265/75r16 Cooper AT
I am comming up with 3.07?

 

[Hopper]

Look at the steel tag bolted onto the diff cover at its 9 o'click position, embossed into it will be the axle ratio. My bet, depending on which axle you look at, is it will will say either 3 07 or 3 08.

The tag is a bit worn. Hopefully this rain will blow out and I can get 8t cleaned of and get a better look.

 

[freedom_in_4low]

You are correct. I sausage finger typed that in. 265/75r16 Cooper AT
I am comming up with 3.07?

yeah I would say it's about 100% chance you have 3.07.

 

[freedom_in_4low]

if you want to calculate it on your own your engine RPM should be (speed/60)*tire revs/mile*axle gear ratio*top transmission gear ratio. Or if you can't find the revs/mile on your tires then you can sub that term with 5280/(.5236*hub height in inches).

 

[jjvw]

The tag is a bit worn. Hopefully this rain will blow out and I can get 8t cleaned of and get a better look.

Measure the hub height, too.