GEAR RATIO, REAR END ?

TomC · February 17, 2010, 03:07:26 PM

With 12R22.5 tires (486rpm), the 3.73 will have a top speed of 70, and the 3.36 will have a top speed of 78 at 2100rpm. Good Luck, TomC

5B Steve · February 17, 2010, 04:59:31 PM

TomC,

Thanks for that info!

Steve 5B.....

gus · February 17, 2010, 07:40:01 PM

Steve,

Jim and Lin are right, I remember doing it a few times with one wheel jacked.

I thought I divided by two but I think Jim is right, it is probably X2. It is for sure one or the other!

Mark the shaft with chalk and count the turns. If you do the math wrong it will be very obvious!!

bevans6 · February 18, 2010, 04:34:35 AM

Steve, just a thought but with your 8V-92 natural you may want a lower cruising rpm than I would with my 8V-71. My 5C has the 3.73 gear and I top out at about 72 mph by my watch and the mile posts. That seems fine for me, it gives me a bit better performance in lower gears than otherwise. You might do better with the 3.36 if you happen to have it.

Brian

JackConrad · February 18, 2010, 04:43:06 AM

Our MC-8 with 8V71 & 740 transmission has a 3.73 rear end ratio. I recently had a 102A3 at my place that had the 8V92 & 740 that had the 3.36 (or 3.33), I forget which. After driving both of them it seemed that our MC-8 had as good or better acceleration, but the 102A3 definately ran fast faster and cruised at a lower RPM for a given speed. This was simply seat of the pants feels like testing, no stopwatches or timers were used for this test. YMMV Jack

rv_safetyman · February 18, 2010, 06:08:47 AM

Gus, when I wrote the note about drive shaft rotation being twice with one wheel rotating, I sat a long time and tried to recall which way it was. So, maybe we should play it safe and say it will be off by a factor of two. How is that for "engineer speak"

Jim

Ed Hackenbruch · February 18, 2010, 07:56:44 AM

Hmmmm, my 5A has an 8V71 and Allison 644 and 3.36 gears and 11r 22.5 tires. My speedo only reads to 80 mph and i know that it reads 3mph fast. I have had it pegged at 80 running at about 1850 rpm in 4th and still had throttle to spare. After i let off of the pedal it took 6-7 seconds to drop below 80 mph. I have never hit top end. Would sure like to find a place where i could find out without getting a ticket!

luvrbus · February 18, 2010, 08:10:24 AM

I am not touching this one Ed

good luck

bevans6 · February 18, 2010, 08:34:14 AM

1850 rpm on 11R22.5 tires with a 3.36 diff is 62.8 mph. Perhaps your speedo starts to read high at higher speeds?

1850/3.36 is 550.6 axle rev. per minute or 33,036 axle rev per hour.
11R22.5 loaded diameter is 38.4", circumference is 120.576", or near enough 10 feet. (Bridgestone data page)
10 feet divided by 5280 is 0.0019 miles per revolution
0.0019 MPR times 33,036 RPH is 62.76 MPH

EDIT: I said 38.4 loaded diameter, when what I really meant was 19.2 loaded radius, times two, to fit into the PI D formula for circumference. Obviously it's not the actual loaded diameter, since a loaded tire is only flat on the bottom...

If you have the electric sensor speedo on the front brake drum, the sensor is too close to the drum and it's reading the bolts twice at high speeds. I had that problem too. But anyway, the math is the math...

Brian

Ed Hackenbruch · February 18, 2010, 08:39:57 AM

i have checked it with gps, and road mileage test areas with stop watch, and at 75 mph on the speedo i am actually doing 72.

bevans6 · February 18, 2010, 10:41:21 AM

No problem with that, Ed, but then you aren't running 1850 rpm, or you aren't on 11R-22.5 tires, or you don't have a 3.36 diff ratio, or your gearbox is overdrive (which is perfectly possible, I suppose...). Other than that it's pure math and math is math, there's only one way to do it.

Edit: So, for fun and because it's bus related and therefore more interesting than work, I calculated that at 80 mph, 1850 rpm and 3.36 diff ratio you could have a .78 overdrive 4th gear, or you could have 48.9" tall tires. I am still working on the HP required to get a bus going at 80 mph, but I am pretty sure an 8V71 would have enough to do it, no wind and on the flat.

Edit again: My bus tops out on the governor at 72 mph, near as I can tell, and with 12R22.5 tires (Firestone FS560 with rated 19.8 loaded radius) and 3.73 diff that is 2280 rpm. But I don't have a tach...

Brian

Ed Hackenbruch · February 18, 2010, 01:25:27 PM

Brian, My tach appears to be fine, it reads where it should at idle and fast idle, and when i accelerate, it does not jump around but climbs smoothly. 4th gear is 1.00, my tires are 11r 22.5 and are 41" in diameter. The tag on my rear end says i have the 3.36 gears. Going down the road last year i had my wife write down the following; In 4th gear @ 65 mph,-1700 rpm, 55psi oil and 180 degrees H2O temp. In 4th gear,@ 55 mph, 1500 rpm. I don't like to run it in 4th at 55 because it is lugging at 1500 rpm and will downshift itself at 1450-1480 rpm. Also i need to be turning 1700 rpms or my temp will start going up. If you look back to Jan 26-09 at the Re MC5 Fuel Economy post, you will see that Viento1 said that he had to feather the throttle on his old 5 to stay below 80. Also Steve5B posted that he got a ticket in Indiana for doing 104 in a 70 mph speed zone in his 5. "Math is Math"........ yes, it is. And math also tells us that neither the bumble bee or a helicopter is capable of flight but they both do.

I don't know what my top speed is, because i have never reached it yet, but i do know that it is not 68 mph.

bevans6 · February 18, 2010, 02:01:43 PM

I'll stop now, I know when I'm beat... I guess numbers add up differently wherever you are...

Brian

Ed Hackenbruch · February 18, 2010, 02:05:22 PM

Sometimes numbers don't tell the whole story. Many times i have seen that what looks good on paper doesn't work in the field.

bevans6 · February 18, 2010, 02:29:40 PM

Ed, the numbers never lie. They are what they are. Gauges lie, watches lie, GPS systems lie, all sorts of things lie, but numbers do not. People can and do interpret what the numbers may say wrong, but I don't think I am doing so in this case.

I challenge you to come up with a formula or calculation that backs your observed data. Show me how it can happen, then I'd agree with you. Easier yet, just show me where I made the mistake in my calculation. Since I've taken the step of checking my data with a known accurate professional spreadsheet designed to calculate exactly this sort of thing, I'll be surprised if there is a mistake.

Just for fun, how many engine rpm would it take to do 104 mph in a MC-5B, with either of the two stock gear ratio's that seemed to be available?

Brian