Rear tire size and gear ratios

Lew Ledyard · #1 (**permalink**) 02-10-2004, 08:48 PM

KMPs come stock with 275/60X15 rear tires. Would there be any advantages or disadvantages in going to a 295/50 size? The 295 is about 1 1/2 inches less in diameter (28" vs 26.5" I believe).

I was thinking of running the 295/50 tire with a 3.08 rear and a 2.87 (vs 2.78) first gear in a WR Toploader. That would seem to keep first gear at about the same effective ratio as a 275/60 tire, 3.31 rear with the stock WR Toploader combination, but give it a taller (numerically lower) 4th gear for cruising. Am I off base with my calculations?

Thanks

Lew Ledyard · #2 (**permalink**) 02-12-2004, 09:33 PM

Has anyone tried 295/50X15 rear tires on a Kirkham?

Ron61 · #3 (**permalink**) 02-13-2004, 06:27 AM

Lew,

The taller the tire the more it will effectively raise your gear ratio. Example per Isky Ratio Computer;; 25" tall tires with 3:10 gears would let your crusie at 60MPH at 2400RPM. 28" tall tires would lower that to just over 2100RPM. I run 235/60 which when mounted are 25" tall with 3:27 gears and in 5th gear I can cruise at 70 at 1900RPM. Should have gone to a .8 split in the transmission incstead of the .6.

Edit:: The above calculations are based on a straight 1 to 1 drive ration, not the overdrive 5th gear. My 60 MPH speed with the 3:27s is at 2900RPM. I have ran the car through the CHP
radar testing setup at several speeds ranging from 50 to 90 and all have been right on the mark to the exat RPM and MPH when we did the math.

Ron

Tom Kirkham · #4 (**permalink**) 02-13-2004, 10:15 PM

Lew,

Shelby American uses (or at least used to use) 295/50-15 tires on their cars. To calculate how the tires effect the speedo and gear ratios multiply the gear ratio by the ratio of the tire diameters.

1st gear in a C.R. toploader is 2.32 and 2.78 in a WR

The smaller diameter tire will make the car act like it has a higher number gear ratio in the diff. So you will be turning more RPMs with a shorter tire than you will with a taller tire.

Lew Ledyard · #5 (**permalink**) 02-14-2004, 09:03 AM

Thanks Thomas. I was not sure if running a 3.08 rear (as opposed to a 3.23 or 3.31) would offset the shorter diameter rear tire. Also whether,when taken in combination with the shorter tires and the 3.08 rear, a 2.87 first gear (which I think are availalble from David Kee) in a WT Toploader would allow the best of both worlds: a first gear drive ratio (combining first gear, final drive and tire size) that is "fun" while a 4th gear that is a little longer for cruising?

Bluesman · #6 (**permalink**) 02-14-2004, 09:25 AM

A formula you can have fun with:

Top speed =
336 (constant) divided by 27" (diameter of tire) X
3.31:1 (top gear of transmission X rear end ratio) =
RPM @ MPH.
example:
336 divided by 27 = 12.444
12.444 X 3.31 = 41.19 RPM at 1 mile per hour.

so...

6500 RPM divided by 41.19 = 157.8 MPH.

This is real data, however, I have tried to achieve 6500 RPM in
5th to get at 160 MPH and the laws of physics always offered me
something less. You will have to insert your own values. These
represent my car only.

The formula to predict your elapsed time and speed in the quarter
mile can be calculated also, knowing your HP and weight.