Quote:
Originally Posted by tracer
All right ... let's see... The diameter of the tires isn't as useful as the REVOLUTIONS PER MILE of each particular tire. Usually 22.5 LP tires do about 510, while11R 24.5 spin at 475 revs/mile. So, assuming you have a 3.73 axle ratio and 0.73 top gear
510 x 3.73 x 0.73 = 1388 RPM at 60 MPH
1388 / 60 * 63 = 1458 RPM at 63 MPH
If you switch to tall rubber (appr. 475 revs/mile) the formula above will now look like this:
475 x 3.73 x 0.73 = 1293 RPM at 60 MPH
or 1293 / 60 * 63 = 1358 RPM
You can multiply the result of 1293/60 by any speed number and this will give you your RPM at that speed. 1293 / 60 = 21.55
So, if you want to know your speed at 65 MPH, you multiply 21.55 by 65 and you get 1,400 RPM. Or: 21.55 x 70 gives you 1508 RPM at 70 MPH.
All this looks pretty good on paper but with taller rubber you will get:
- less torque at the wheels (you change your axle ratio!); and
- worse fuel mileage due to higher resistance and heavier weight of the tires.
When I was faced with a similar choice I went for axle ratio change (from 3.73 to 3.42) and I"m pretty happy cruising at 1,325 RPM at 62 MPH (I got a Cat).
Ok so, the diameter gives you the circumference and that gives you revolutions per mile. But, you made asumptions about the gear ratios that may or may not be correct and are unnecessary anyway given the situation he describes. And, your speed at 65 mph is...65 mph! ;-)
This is what he said: Now
instead of changing the gears, and seeing I'm due for new tires, I was debating on going to a 11R24.5 rim/tire..... any idea what the RPM would be (or speed if it's easier)
at 1,500rpm????
We don't even need to guess what his gear ratios are. If he says the number of wheel rotations in an hour is constant (1500 rpms with the transmission and differential that he has) and the low pros will take him 63 miles in one hour, just knowing the ratio of the diameters of the two different tires will tell us the ratio of the distance the truck will travel in an hour.
I mean, the stuff you said was fine, and the advice you gave was fine, but he said "this truck" and "this rpm" and that just makes it all easier.