Lexus GS450h RPM vs MPH Speedometer

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Gregski
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Lexus GS450h RPM vs MPH Speedometer

Post by Gregski »

Since my 1971 Chevy truck aka GMC used a mechanical speedometer which I am ditching I plan on using MG2_Speed to calculate my miles per hour MPH

so I punched some numbers into the good ol' Lotus 1-2-3 (pronounced: Excel) ha ha to correlate RPMs to Miles Per Hour based on my 3.73 rear end and 215/65R-17 wheels

I relied on the good ol' folks at x-engineer.org to provide the complicated formulas, (and our daughter to recite pi to the 57th digit):

How to calculate wheel and vehicle speed from engine speed

How to calculate wheel radius


Tire-size-and-aspect-ratio.jpg
IMG_7955.JPG
RPM MPH 3.73.jpg
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Gregski
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Re: Lexus GS450h RPM vs MPH Speedometer

Post by Gregski »

this made me consider Taller / higher differential gears like the 2.73 for my GM 12 bolt to drop the RPMs by approximately 2 grand, I'm pretty sure (97.1%) that electric motors don't have a traditional Power Band, so did the "Royal We" determine at what RPMs this Unicorn is most happy happy?

the-royal-we-man.jpg
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RPM MPH 2.73.jpg
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Gregski
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Re: Lexus GS450h RPM vs MPH Speedometer

Post by Gregski »

and let's not forget how simply changing our tire aka wheel size can impact RPMing and MPHing

below I use the exact same tire profile on three different size rims 16", 17", and 18"
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PatrcioEV-ATX
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Re: Lexus GS450h RPM vs MPH Speedometer

Post by PatrcioEV-ATX »

Gregski wrote: Wed Dec 08, 2021 5:44 pm and let's not forget how simply changing our tire aka wheel size can impact RPMing and MPHing

below I use the exact same tire profile on three different size rims 16", 17", and 18"

Wheel Size.jpg
Don't forget to account for the gear reduction from MG2 speed.
2008 DC EV conversion (sold)
2012 Leaf (sold after 95k miles)
2016 B250e

Current plan: 1964 Rambler Classic 2 door sedan with GS450h transmission/inverter. 36kwh Tesla batteries from B250e.
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Gregski
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Re: Lexus GS450h RPM vs MPH Speedometer

Post by Gregski »

PatrcioEV-ATX wrote: Wed Dec 08, 2021 6:22 pm
Gregski wrote: Wed Dec 08, 2021 5:44 pm and let's not forget how simply changing our tire aka wheel size can impact RPMing and MPHing

below I use the exact same tire profile on three different size rims 16", 17", and 18"

Wheel Size.jpg
Don't forget to account for the gear reduction from MG2 speed.
is it 1.900 ?
talk-to-me-goose.jpg
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PatrcioEV-ATX
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Re: Lexus GS450h RPM vs MPH Speedometer

Post by PatrcioEV-ATX »

Yes, these are the pieces of my mph calc, though others have done it slightly differently:

float speedCalc;
int tireRpm=737; //revolutions per mile for tire size
int MPH;
float diff=3.31; //your diff ratio

void Calc_MPH()
{
speedCalc=((abs(mg2_speed/1.9))/diff);
MPH=((speedCalc*60)/tireRpm);
}
2008 DC EV conversion (sold)
2012 Leaf (sold after 95k miles)
2016 B250e

Current plan: 1964 Rambler Classic 2 door sedan with GS450h transmission/inverter. 36kwh Tesla batteries from B250e.
xp677
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Re: Lexus GS450h RPM vs MPH Speedometer

Post by xp677 »

I covered this a while back in another thread: viewtopic.php?f=14&t=396&p=32431#p32431

Here it is again to save you clicking:

-----------------------------------------

I saw someone mention speed calculation from MG2 rpm, here's how I did it:

I have this at the top

Code: Select all

//for 255/40/R17 tyre
#define TYRE_WIDTH 255
#define TYRE_RATIO 40
#define TYRE_SIZE 17 //inches
#define DIFF_RATIO 4.1 //x:1

#define TYRE_CIRCUMFERENCE (TYRE_SIZE*25.4+TYRE_WIDTH*TYRE_RATIO/50)*3.14159265359/1000 //m travels in one wheel revolution
#define DIST_PER_MG2_REV TYRE_CIRCUMFERENCE/1.9/DIFF_RATIO/1000 //km car travels in 1x MG2 revolution
#define LOWGEAR_RATIO 2.05263157895 //how much lower the low gear is compared to high
Then these lines after mg2_speed = mth_data[31] | mth_data[32] << 8;

Code: Select all

      veh_speed_kph = mg2_speed * 60 * DIST_PER_MG2_REV;
      if (veh_speed_kph < 0)veh_speed_kph *= -1; //for reverse
      //if (put your "low gear" check in here)veh_speed_kph /= LOWGEAR_RATIO; //low gear  
      veh_speed_mph = veh_speed_kph/1.60934;
Change your tyre specs (printed on the tyre) and the differential ratio to suit your application.

For MPH you can just divide the speed by 1.60934. I run the car in kph and just do the conversion in the gauge cluster if needed.

If you never want to use kph then just do the /1.60934 on the lines above.

This code isn't for Damiens VCU so adapt it to suit your needs.

The advantage of this code is that it makes it easy to enter your tyre size and differential ratio. The #define's are calculated by the compiler at compile-time and therefore those calculations do not use any processor or memory.
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PatrcioEV-ATX
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Re: Lexus GS450h RPM vs MPH Speedometer

Post by PatrcioEV-ATX »

xp677 wrote: Sat Dec 11, 2021 12:49 pm I covered this a while back in another thread: viewtopic.php?f=14&t=396&p=32431#p32431

Here it is again to save you clicking:

-----------------------------------------

I saw someone mention speed calculation from MG2 rpm, here's how I did it:

I have this at the top

Code: Select all

//for 255/40/R17 tyre
#define TYRE_WIDTH 255
#define TYRE_RATIO 40
#define TYRE_SIZE 17 //inches
#define DIFF_RATIO 4.1 //x:1

#define TYRE_CIRCUMFERENCE (TYRE_SIZE*25.4+TYRE_WIDTH*TYRE_RATIO/50)*3.14159265359/1000 //m travels in one wheel revolution
#define DIST_PER_MG2_REV TYRE_CIRCUMFERENCE/1.9/DIFF_RATIO/1000 //km car travels in 1x MG2 revolution
#define LOWGEAR_RATIO 2.05263157895 //how much lower the low gear is compared to high
Then these lines after mg2_speed = mth_data[31] | mth_data[32] << 8;

Code: Select all

      veh_speed_kph = mg2_speed * 60 * DIST_PER_MG2_REV;
      if (veh_speed_kph < 0)veh_speed_kph *= -1; //for reverse
      //if (put your "low gear" check in here)veh_speed_kph /= LOWGEAR_RATIO; //low gear  
      veh_speed_mph = veh_speed_kph/1.60934;
Change your tyre specs (printed on the tyre) and the differential ratio to suit your application.

For MPH you can just divide the speed by 1.60934. I run the car in kph and just do the conversion in the gauge cluster if needed.

If you never want to use kph then just do the /1.60934 on the lines above.

This code isn't for Damiens VCU so adapt it to suit your needs.

The advantage of this code is that it makes it easy to enter your tyre size and differential ratio. The #define's are calculated by the compiler at compile-time and therefore those calculations do not use any processor or memory.
I even added tire RPM and rear diff (since I’m not entirely sure what mine is) to the serial menu for easy updating.
F5FAD249-FCEF-4312-AA9D-01E1213C60A3.png
2008 DC EV conversion (sold)
2012 Leaf (sold after 95k miles)
2016 B250e

Current plan: 1964 Rambler Classic 2 door sedan with GS450h transmission/inverter. 36kwh Tesla batteries from B250e.
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slanceley
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Re: Lexus GS450h RPM vs MPH Speedometer

Post by slanceley »

I'm not sure this is the right place to ask, but does anyone have a way for a simpleton to work out my expected top speed etc?
(ie a formula rather than code please?)

The rear diff on my Reliant Scimitar is 3.58.1 and I'm wondering if I'll need to change that to use the GS450H transmission.
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ZooKeeper
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Re: Lexus GS450h RPM vs MPH Speedometer

Post by ZooKeeper »

It's actually fairly simple...

(Motor RPM*60)/[trans ratio/final ratio tire revs/(mi or km)]

The word version: Motor turns per hour divided by the total gear ratio over tire revolutions per unit measure (miles or km) = Distance Per Hour
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xp677
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Re: Lexus GS450h RPM vs MPH Speedometer

Post by xp677 »

MG2 max speed is 10000rpm, divide by 1.9 which is the high gear internal ratio gives 5263rpm out the back of the transmission.

Divide by 3.58 gives 1470rpm at the wheels at top speed.

The Scimitar came with various tyre options, a common one was the 185 HR 14 tyre which has a diameter of 650mm, therefore a circumference of 2.04m.

Therefore, the distance travelled at full speed in one minute is 2.04m * 1470rev. Which is 3000.18 metres per minute, or 180 kilometres per hour.

180kph is 112mph.
nkiernan
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Re: Lexus GS450h RPM vs MPH Speedometer

Post by nkiernan »

xp677 wrote: Sat Jan 08, 2022 3:10 pm MG2 max speed is 10000rpm, divide by 1.9 which is the high gear internal ratio gives 5263rpm out the back of the transmission.
xp677, is this the transmission output speed (5263rpm) with the locked input shaft yes?
xp677
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Re: Lexus GS450h RPM vs MPH Speedometer

Post by xp677 »

Yes, it's for the typical application.
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Re: Lexus GS450h RPM vs MPH Speedometer

Post by nkiernan »

Re: Connecting a GS450H to an existing Transfer Case?

Post by sfk » Tue Jan 05, 2021 9:13 pm
Worth pointing out that MG1 and MG2 could be operating at different RPM depending if you use the Lock Input Shaft method or Lock Planetary Gearset method.

Locking the planetary gearset will cause them to spin at same speed and direction with a max RPM around 11,000ish due to the limits of the bearings. In High gear 1.9 reduction this will result in about 5,250rpm coming out the back of the transmission.

Locking the input shaft will cause MG1 to spin 2.2x in the opposite direction to MG2. Thereby MG1 will reach the bearing speed limits much earlier than MG2 and limit the total output RPM. This will result in a max RPM of about 2,600.

Divide that by your 3.36 axle ratio and you have your rear wheel RPM and it's just a final step to factor in your tyre circumference to find your max road speed.
That suits my project much better, I had been working off the info guoted from another thread up to now which was giving a low top speed with the locked input shaft option, but is it true this pushes the MG1 bearing limits?
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