Author Topic: Help me with calculating speed (Read 3665 times)

desertoy · « **on:** Mar 04, 2009, 11:00:53 AM »

I am trying to see what the difference in speed would be if I switched to a higher R&P ratio. I found an on-line calculator but I need to know what the ratio is on a L-52HD in 5th gear.

blackdiamond · « **Reply #1 on:** Mar 04, 2009, 11:49:05 AM »

If nothing else is changing it should be a simple ratio (pardon the pun) of the old and new gear ratios.

For example, if you are going from stock 4.10 gears to 4.88 gears the ratio would be:

4.10 / 4.88 = 0.84 or 84%

This means that at any given speed you will be going 84% of the speed shown on the speedometer.

If the speedometer reads 60 mph then you're really going 60 x 0.84 = 50.4 mph.

Note: Higher number gears will make you go slower than what the speedometer shows because the drivetrain is turning more times per revolution of the tire. Larger tires will make you go faster than what the speedometer shows because the drivetrain is going farther (i.e. circumference of the tire) for each revolution of the drivetrain. This is why higher number gears make a great combination with larger tires.

The ratio calculation will also work with tires, but you have to calculate the circumference (3.14 x Diameter = circumference) and compare those numbers.

To answer the other part of your question, most people use 4th gear in calculators because they are almost always 1:1.

Hope this helps...

blackdiamond · « **Reply #2 on:** Mar 04, 2009, 11:51:27 AM »

P.s. whatever % your speedo is off applies to all speeds.

10% @ 30 mph = 3 mph off

10% @ 60 mph = 6 mph off

The faster you go the farther off your speedo is.

desertoy · « **Reply #3 on:** Mar 04, 2009, 12:41:06 PM »

Here is what I am trying to figure out. In the King of the Hammers race, I ended up racing most of the race with the front t-case in low and the rear in high. The reason for this was simply gear ratio. The jump in ratio between 1st gear and second and between second gear and third was more that my little 22re could handle. I found that instead of running with the t-cases both in high and jumping from second to third gear, I was able to go faster by putting the front case on low and using 3rd,4th, and 5th gears because the ratios are closer together. The bad part was when I got to a fairly fast section I found myself shifting the front case into high and putting the trans in 3rd. This worked ok but shifting back down took too much time.
I wondered if I went from a 5.29:1 ratio in the rear end to a 4.10:1 ratio, how much faster I would be able to go.
I currently have 5.29 R&P. 4.7 rear case, 2.28 front case, L-52HD trans, pushing 37" Krawlers.

My question is how fast am I going with the front case in low, rear case in high, 5th gear, 5500RPM? I am guessing 52-55MPH.

My second question is how fast am I going with the same transfer case and trans selection, same RPM with a 4.10:1 R&P ratio.

If you smart guys could answer that, it would save me some time.

THANKS!!

All_Set · « **Reply #4 on:** Mar 04, 2009, 12:46:47 PM »

5th gear is .85 ratio. 4th is 1:1 (1)

All_Set · « **Reply #5 on:** Mar 04, 2009, 01:25:05 PM »

Using this calculator: http://www.4lo.com/calc/gearratio.htm

I'm using 37" your actual diameter is probably smaller aired down..

Tire 37"
Tranny .85
Tcase 2.28
2nd tcase 1 (1:1 omit from calculation)
gear 4.10
@ 5500
=76.2

Tire 37"
Tranny .85
Tcase 2.28
2nd tcase 1 (1:1 omit from calculation)
gear 5.29
@ 5500
= 59.1mph

desertoy · « **Reply #6 on:** Mar 04, 2009, 01:33:26 PM »

Thank's ALL_SET, a 29% increase in speed would be great. We might just do that for KOH 2010.

blackdiamond · « **Reply #7 on:** Mar 04, 2009, 05:33:56 PM »

Quote from: desertoy on Mar 04, 2009, 01:33:26 PM

Thank's ALL_SET, a 29% increase in speed would be great. We might just do that for KOH 2010.

You might find that a 29% decrease in torque to the wheels is more than enough to negate the potential increase in speed.

KYOTA · « **Reply #8 on:** Mar 04, 2009, 05:42:40 PM »

Quote from: blackdiamond on Mar 04, 2009, 05:33:56 PM

You might find that a 29% decrease in torque to the wheels is more than enough to negate the potential increase in speed.

but does that 29% speed increase equal the same percentage in power loss at the wheels?

blackdiamond · « **Reply #9 on:** Mar 04, 2009, 06:11:11 PM »

Quote from: KYOTA on Mar 04, 2009, 05:42:40 PM

but does that 29% speed increase equal the same percentage in power loss at the wheels?

Yes, the differential gears multiply torque to the wheels. Reducing the gear ratio is no different than shortening a lever arm. A shorter lever arm requires less travel so it can be moved more quickly, but it requires more force.

79coyotefrg · « **Reply #10 on:** Mar 04, 2009, 06:22:55 PM »

which means if you run 4.10's you'll need 29% more horsepower to go the same speed in the same gear than you would need if running 5.29's

blackdiamond · « **Reply #11 on:** Mar 04, 2009, 06:42:45 PM »

Quote from: 79coyotefrg on Mar 04, 2009, 06:22:55 PM

which means if you run 4.10's you'll need 29% more horsepower to go the same speed in the same gear than you would need if running 5.29's

I'm not sure I agree, but let me do some figuring and get back to you...

kiyul4130 · « **Reply #12 on:** Mar 05, 2009, 09:57:32 AM »

i do the same thing in my truck only i have 42s. When i get on the highway (short distances to the trail) i almost always start off with the crawl box in and the rear case in high to push the tires. im pinned out at about 50-55mph in 5th gear with the 4.10s. i then rev match the tranny to the crawl box while the tranny is in neutral and slip the front case in high range and the tranny in second and drive it like shifting between 2nd and 3rd. im then able (on flat ground) able to get up to 65-70ish. with a smaller tire im certain you will be able to accomplish around 80mph with running those 37s on the FTOY. good job at the race. not sure if you saw the youtube clip i got of you guys just after the starting line.

All_Set · « **Reply #13 on:** Mar 05, 2009, 10:09:40 AM »

Quote from: 79coyotefrg on Mar 04, 2009, 06:22:55 PM

which means if you run 4.10's you'll need 29% more horsepower to go the same speed in the same gear than you would need if running 5.29's

eh, that assumes that top speed was limited by HP and not gearing.. 22re sucks above 5500 rpms.

blackdiamond · « **Reply #14 on:** Mar 05, 2009, 11:50:16 AM »

Quote from: blackdiamond on Mar 04, 2009, 06:42:45 PM

I'm not sure I agree, but let me do some figuring and get back to you...

After "geeking" around a bit in excel, I determined that I needed more information to make a good comparison. If I had a hp & torque curve for a 22RE I could compare the gearing combinations at different speeds to see where the engine was at.

I just realized that if you changed to differential gears from 5.29 to 4.10 you would be decreasing torque to the ground by 29%, but if you shift into low you're actually increasing the torque to the ground by 228%.

Assuming 100 lb-ft of engine torque, the comparison would be the following for 4th gear:

1 x 1 x 5.29 = 529 lb-ft

1 x 2.28 x 4.10 = 934.8 lb-ft (76% increase in torque, but speed is limited due to gearing reduction)

It would be fun to compare the various gearing combinations and verify available torque at matching speeds.

desertoy · « **Reply #15 on:** Mar 05, 2009, 02:01:55 PM »

In 5th gear with the front case in low tapped out at 5500 rpm, I found that I could use one more gear. I had plenty of torque in that gear. I just needed a little more speed at times. I'm a little worried about going back to 4.10's because I broke alot of high pinion front R&P with 4.10's in the past. I need to look into the cost of adding 40-50 more horse power.

desertoy · « **Reply #16 on:** Mar 05, 2009, 02:03:04 PM »

Quote from: kiyul4130 on Mar 05, 2009, 09:57:32 AM

not sure if you saw the youtube clip i got of you guys just after the starting line.

I don't know. Can you put it up here?

toyman80 · « **Reply #17 on:** Mar 05, 2009, 02:15:28 PM »

hey guys so i running 35's on stock 4.10's how far off would my speedo be?

blackdiamond · « **Reply #18 on:** Mar 05, 2009, 07:12:40 PM »

Geek Alert!

I can't explain why I would do something like this, but...

I started by building a HP & TQ curve based on a standard gas engine that I modified to match the factory 22RE ratings.
I then calculated the MPH in each gear throughout the RPM range and determined the optimal shift points based on available torque at equal speeds.
I did this for both configurations that have been discussed.
I used a trendline for the torque curve to calculate the torque at various speeds using the optimal transmission gears for comparison.

If you have to ask, then I probabaly can't explain it.

Draw your own conclusions, but it looks to me like the 4.10 gears with the transfer case in low range is a great option. Similar speeds can be achieved and the torque curve is better.

yota_krawler · « **Reply #19 on:** Mar 05, 2009, 08:15:57 PM »

Quote from: toyman80 on Mar 05, 2009, 02:15:28 PM

hey guys so i running 35's on stock 4.10's how far off would my speedo be?

well I am running 35s and 4.10s and I was on my way down the highway and I looked down at another car and it had a digital speedometer. He was going 60mph, I looked at mine and I was doing 45mph.

blackdiamond · « **Reply #20 on:** Mar 05, 2009, 09:17:41 PM »

Quote from: toyman80 on Mar 05, 2009, 02:15:28 PM

hey guys so i running 35's on stock 4.10's how far off would my speedo be?

Stock tire diameter = 28.3 (225x75x15) 89 inches around
New tire diameter = 34.8 (35x12.50x15) 109 inched around

34.8 / 28.3 = 1.23 or 123%

60 mph x 1.23 = 73.8 mph

Your speedo should be about 23% off.

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