Stock Speedometer Gear, Mph Reading in Ecu, Bigger Tires & Calculating Slippage?

[GNVYUS 1]

This just hit me, I'm trying to calculate slippage accurately but I've been using the stock speedometer gear mph reading in the ECUGN, but this can't be accurate on a 28" tire. I know it's off but I wasn't sure about the ECU side of things and feel real slow. LOL

Stock tire 215/65/15 = 26"
Current Tire 275/60/15 = 28"
Difference = 7.65%

So when I see 50mph on the speedo and in the Scanmaster etc it's really 54mph.

The ECU can't possibly get the real and accurate mph of 54 correct?

It's not a GPS based reading, it's right out of the trans using the stock speedo gear so it's off for all of us using the bigger tires, correct?

I need to use this tire size to mph calculator then...

Tire and wheel speed calculator based on tire size

calculator that shows effect of different size tires on wheel and vehicle speed

www.agcoauto.com

When I saw 84mph in 2ng gear in the ECUGN log at 6000rpm it was really around 90mph, so my slippage calculations should've been...

6000rpm
90mph
28"
3.42
1.57 (2nd gear)
3.5% slippage.

Dyno pull corrected was
6050rpm
142mph
28"
3.42
1
3% slippage

Is this on the right track or am I missing something and the ECU is reading the correct mph on a larger tire?

I'm trying to calculate slippage accurately. Thanks for any replies, it'll save me the trouble of pulling the converter if I'm on the right track.

 

[Mr.Spool]

I have you slightly under 3.5 % in 2nd gear based on the info you provided.
Changing your tire ratio will affect the slip numbers.

 

[Mr.Spool]

How the gnecu reads mph is a question for eric or whomever is supporting your purchase.
However if its solely based on the trans gear and you didnt program in the tire aspect ratio change the speedo wont be correct.

 

[GNVYUS 1]

I have confirmed with those road mph signs and phone GPS my speedo is off for sure (low). I'll post back after hearing from Eric in case there's a way to change the tire size in ECUGN on the log readings. There might be but I bet mine is reading the uncorrected number due to how high the slippage is.

I'd be pumped over 3.5% slippage on a 17 blade, wow I wouldn't even swap it for the tighter 16 blade.

 

[Mr.Spool]

I have confirmed with those road mph signs and phone GPS my speedo is off for sure (low). I'll post back after hearing from Eric in case there's a way to change the tire size in ECUGN on the log readings. There might be but I bet mine is reading the uncorrected number due to how high the slippage is.

I'd be pumped over 3.5% slippage on a 17 blade, wow I wouldn't even swap it for the tighter 16 blade.

The blade changes the rpm around 200.
The converter will eventually fluid lock.
I didnt think you were slipping around 10+% like you posted in the other thread,but I'm not looking at the data,and there can be a host of other things wrong that may lead to slippage,for instance a slipping 2nd gear
Like I posted up we usually dont see 10+% slippage in 2nd gear on a properly speced non slipping trans unless that is what's needed to run quicker on the surface we race.

 

[GNVYUS 1]

I'm gonna try..

GPS on my phone and then in the software change the pulse rate, found in Speed and Gears, from 2000 to whatever is closest.

Trans speedo gear change might work.

GPS into ECUGN can also work. Not sure which one or how but it's an option.

 

[Nigel]

I'm gonna try..

GPS on my phone and then in the software change the pulse rate, found in Speed and Gears, from 2000 to whatever is closest.

Trans speedo gear change might work.

GPS into ECUGN can also work. Not sure which one or how but it's an option.

This may help:

 

[GNVYUS 1]

Thanks, really helpful.
It's definitely not take 2000 pulses and take away tire diameter difference of 7.65%.
153 pulses

Anyone know which teeth combo could work so the dash speedo will show the real mph?

 

[Nigel]

Thanks, really helpful.
It's definitely not take 2000 pulses and take away tire diameter difference of 7.65%.
153 pulses

Anyone know which teeth combo could work so the dash speedo will show the real mph?

11 and 29 would be close for stock tire size.
11 and 27 for a 28" tire.

 

[Chuck Leeper]

This?

200-4R Speedometer Driven Gear. Ranges From 26 to 31 tooth gear counts.

200-4R Speedometer Driven Gear. Comes in 26 to 31 tooth gear count.

transmissioncenter.net

 

[Nigel]

This?

200-4R Speedometer Driven Gear. Ranges From 26 to 31 tooth gear counts.

200-4R Speedometer Driven Gear. Comes in 26 to 31 tooth gear count.

transmissioncenter.net

That would do it. I came up with the same answer as that website, so nice to see corroboration.

 

[GNVYUS 1]

Awesome info, thanks guys, I'm sure this will help many others throughout the years as I didn't find posts like yours in the search.

I ordered the 27 gear for my 28" tire for better dash speedo readings and I'll burn 1806.5 pulses into ECUGN for the more accurate mph logging.

 

[Nigel]

Awesome info, thanks guys, I'm sure this will help many others throughout the years as I didn't find posts like yours in the search.

I ordered the 27 gear for my 28" tire for better dash speedo readings and I'll burn 1806.5 pulses into ECUGN for the more accurate mph logging.

Just to state for clarity, if you change to a 27 tooth driven gear, change the ecugn back to 2000 pulses per mile.

 

[TurboBob]

I presume you really only care about the converter slip, and the actual MPH reading is not important?

Is your ecuGN set for 2000 pulses, and does your 200-4R have the original speedo drive parts?

Were you dynoing in 2nd, or 3rd gear?

For converter slip, we only care about output shaft speed, relative to engine RPM. I'm working out how to calculate it from datalogs, but the speedo drive parts and pulse/mile settings affect it. (not rear gear or tire size).

Let me know on the above, and we can look at a log and analyze the data.

Bob

 

[GNVYUS 1]

I care about converter slippage and without getting a Driveshaft sensor was trying to get an accurate mph to run the calculation.

My ECUGN was 2000 pulses and I have the stock speedo gear.

Dyno was in 3rd

I'll PM you the log

 

[TurboBob]

his converter slip (CS) is really low by my calculations

 

[GNVYUS 1]

Damn if I'm reading it correctly that's looking like 3% from like 5500rpm on and maybe high 2s at 6k. Really nice graph, you guys are always improving the data, nice.

Ok I guess I'm not looking for a 16 blade stator. LOL
This 17 flashes instantly and if its coupling that tight how can I complain.

 

[Mr.Spool]

Damn if I'm reading it correctly that's looking like 3% from like 5500rpm on and maybe high 2s at 6k. Really nice graph, you guys are always improving the data, nice.

Ok I guess I'm not looking for a 16 blade stator. LOL
This 17 flashes instantly and if its coupling that tight how can I complain.

Your seeing the ptc converter basically lock up and almost take the slip fully out
They all work that way it's just getting the right power applied and getting the motor rpms on point.

 

[Mr.Spool]

his converter slip (CS) is really low by my calculations

View attachment 388665

Mine too