Quote:
Originally Posted by Steel Horse Cowboy
I currently have 22.5LP tires on the back of my rig. At 63mph I am at 1,500rpm....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
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).