37" Speed and MPG

Recently put on 37" tires on 18" wheels. I've noticed my speedometer runs about 5mph behind the GPS mph on my phone during travel. I'm thinking the phone is accurate.

MPG on the computer used to average around 13.6 now my average is 12.3.

I'm thinking that my odometer is probably off then too. Any math genius' out there figure out how to calculate MPG given the different tire size?

2017 TRD PRO Tundra

 

Yes, you need to calculate the % bigger your tires are vs stock. You are actually traveling farther than the computer thinks. You need to do a hand calculation for MPG, truck system is no longer accurate because it is set up based on stock tire size.

Stock tire is 32 inches tall, you are now at whatever your 37s are, check the tire specs, guessing somewhere around 35.9 to 37 depending.

Add the % increase in tire size to the miles you traveled on the odometer. Example, you drove 200 miles. If new tires are 8% larger than stock- so you actually traveled 216 miles, not 200. Divide that 216 by the number of gallons of gas used based on your fillup = your true MPG.

 

Nice! Thanks. That is easy to follow, I can do that.

 

did you add a Speedo calibrator?

 

My actual percentage was 12%. Use 10% for now because the mental conversion is easy as you're driving. Get a speed calibrator so your odometer reads correctly. All tires obviously vary in size.

If you're at all tech inclined, I've done a self-hosted form of the Hypertech on a Raspberry Pi. Two others are now running Arduino Teensies. But both projects are for more than speed correction. So you'd generally have another desire to get on the CAN bus than just this. Far easier to buy an off-the-shelf item.

 

Do you have the Pi installed on your setup?

 

Yes. Go here for the RPi and interceptor links. Currently have the RPi out to make v2 of the system and trial a different software. v2 is a step closer to a product and less a prototype, but first learning how to mill PCBs.

 

My RC speedo calibrator isn't doing much for accuracy so I'm down to try the Pi

 

The downside is ~25sec bootup. The others are using a Teensie since it's a microcontroller and instantly boots. That's a better application for redirecting cluster messages or speed correction.

I wanted an actual computer in the truck. Something to tether my phone and use a VPN service, VPN to my home network, create custom apps, etc. Eventually it'll also have a 13.3" screen in portrait orientation and replace the HVAC controls. I installed a remote start and just live with the delay.

What I'm getting at is it comes down to what you want it to do. Send me a PM if you want to chat more so we don't hijack.

 

Nope. I'd have to look that up to see what that is. I don't even know.

 

I used a Hypertech on my 2.5 , still working good. I was also just wondering why someone hasn't tried moving the pickup/sending unit or adding another pickup in each hub of the wheels to change the final speed reading.

 

I added the rc unit and it’s pretty accurate to the numbers I added. Just need to adjust the stability control settings

 

ABS already has a tone ring per wheel end. Changing the tooth count to trick the system would be nearly impossible. Far easier to recalculate the CAN bus message and resend it to the network. Even better would be if we could find a register holding the wheel radius and rewrite that. I've been told Techstream doesn't have this, unlike other mfgs, making me think Toyota didn't put that in as a variable.

 

The ABS ecu reads the number of “pulses” aka each time a tooth goes by the wheel speed sensor. The ABS ecu has a value hardcoded to tell it the number of pulses per mile traveled. Adjusting this value inside the Toyota abs ecu is the way to properly fix it. However no one has the ability to access the internal code and programming in the ABS ecu.

With the ABS ecu reprogrammed to an adjusted number of pulses per mile, your CAN bus and factory communication would be right with no need for modification

 

I'm gonna rip my RC calibrator out and use it for target practice.

 

Open it up and take pictures. I am a little curious to see what’s in there.

 

The module itself?

 

Yes if you ever do end up pulling it out. I would be curious to see the circuit board and what all they have going on in there.

 

I'd be interested in that Pi project if you've a link or something other you can point me at

 

Sure. Go to post #7 here. That has a hyperlink to my build page. There are the other two hyperlinks pertaining to the RPi and speed interception.

 

How much money for a solution? I'm not good with science projects though.

 

Go here and use the Tire Comparison. Put in the old tire size and the new and it will tell you how much faster you are at various speeds. It's not a simple 5 MPH faster or slower type of thing, it's a percentage so the discrepancy varies based on speed. Also, keep in mind that it is also throwing off your Odometer and fuel computer. You are travelling further than the computer thinks, so change your oil sooner and don't think your MPGs are as bad as the computer tells you.

 

$230

https://sdhqoffroad.com/collections...undra-hypertech-speedometer-calibrator-730119

 

I just came to see the 37s

 

This x2

 

That's part of while I would be hesitant to buy a lifted truck on big tires. Is that 30K miles actually 33K? Was maintenance kept up? Too many questions and I don't trust people.

 

Are these units accessible for future changes without opening up the dash to get to them?

 

I don't own one, but from what I've read, yeah, just be sure to leave the USB cable plugged in so you don't have to take the dash apart to access again.

 

I threw this together quickly. Should be correct but if anyone sees a mistake don't hesitate to point it out. I did all this is about 10 mins but it should give some people a better idea what's going on.

The 37" in tire is about 13.35% larger than the stock 18. This means your speedometer will be off by about 13.35% as it works on stock tire size. When the Speedometer display shows 70 Mph your real speed is 79 ish Mph (13.35% higher)

Same for Odometer, it might say you have traveled 1 mile but in reality you have gone 1.13 miles. Not a huge difference but it adds up over time. After 10,000 miles on the odo, you have really traveled 11,335 miles. Do this for 100,000 miles and your truck really has 113,350 ish miles on it.

 

a little cheaper on Amazon for anyone looking $188.72

$199 Hypertech 730119 Speedometer Calibrator

https://www.amazon.com/Hypertech-730119-Speedometer-Calibrator/dp/B00GUYCNMS