Speedo correction for larger tires?

ah64id wrote:
Many people have mentioned calibrating the speedometer, but I prefer to calibrate the odometer.

I have never driven a rig where the speedo and odometer match, some are off by 2-3 mph. My DD VW is off by 2-3 mph at 70 but the odo is dead accurate.

For calculating mileage, maintenance, and true odometer readings I suggest calibrating the odometer over a 100-200 mile stretch and learning the small speedo error.

My sig truck is calibrated within 0.3%, and reads about 1mph fast at 70. It's more accurate IMHO, and keeps the wife from speeding and I just set it based on the speedo and I'm not going as fast.

Many manufacturers post rev/mile data, this is far more accurate to start with than tire math.

wintersun wrote:
Check your speed with a GPS or smartphone with GPS and see how accurate your speedometer/odometer are with the stock tires. I did not realize it but with a camper load the actual speed was 2 MPH less than indicated thanks to the tires squatting and not being as tall. Only takes a 1/2" of squat to affect the speedometer accuracy by 4%.

If you are getting a 1/2" of squat your tires are overloaded or under inflated.

I will see a small change empty to loaded, but never even 1%.

You really think that the speedometer and the odometer function differently and that one uses the rotation of the tires and the other uses what instead? Talk about magical thinking.

To have 3,000 lbs. of weight on a tire with a 3,750 load rating is not overloading it. At 80 PSI the weight results in flexing of the sidewall. Why do you think that the air temperature inside tires increases as the vehicle is driven? It is the flexing that generates heat which warms up the air inside the tire. Not rocket science but common knowledge that somehow escaped your notice.

I went from 265/70/17 to 315/70/17 about 4 mph not worth worrying about, I always have my GPS on anyway

One of the pleasures of testifying in court, on speeding cases, speedometer error often was a defense. Lots of things impact a speedometer's accuracy, such as your tires change the speedometer as they wear, as does a change in air pressure. For some people, a 2 to 3 mph error +/- is important, especially when they are pulled over for speeding.

When in doubt, stay with traffic. Or, follow someone with a functioning speedometer, and compare your speedometer at the speeds they are traveling. In the old days, they synchronized watches off of others, works for speedometers too.

Or, you mentioned a recent change in your MPG. Sometimes changes like this are due to change of travel area, Diesel fuel changing from winter to summer grades, then back. Or someone else driving your truck.

b

The auto Garmin GPS only give MPH in whole numbers so I used my expensive Garmin form my boat as it give MPH to the one tenth MPH. Our 2004.5 Chevy D/A CC LB 4X4 in cruise at 58 MPH on the speedometer on flat roads is actually going 60.7 MPH with the 265/75/16 "E" LTX Michelins with 80 PSI in them. Tires are new and put on in late October 2013. That means I need to add approx 5% to the odometer distance indicated to be accurate. 100 miles indicated means I'd actually gone approx 105 miles.

I didn't check the speedometer with a GPS when the truck was new with the OEM 245/75/16 "E" Bridgestone tires but the tire charts lead me to believe 58 MPH indicated would have been 58 MPH actual knowing what the 265's numbers acually are. The 245's were only on for 29,000 miles.

Worn down tires are approx 3/4"+ smaller in diameter and alters your speedometer and odometer also. So does having less than full inflation or overloading a tire as the tire squats more so the radius distance on the road contact side is less than if fully inflated. Radius X 2 = diameter X 3.16 equals the circumference or distance traveled with one complete tire revolution.

Max weight loaded underinflated worn down tread tires can make more than a 10% difference (error) on the speedo and odo and give false by 10+% MPG even hand calculated. More revolutions and less distance actually traveled. The GPS doesn't lie as it's accurate!

No idea how much they cost but the Jeep we bought last fall had 35" tires on it and we had to put on 265's to fit on the rollers at our emissions test, and I needed snow/ice tires for the winter canyon roads. The Jeep came with a device from Superlift that use can manually adjust it easily. Haven't gotten around to bothering but something like that might be a good option if someone changed back and forth a lot and didn't want to mess with the programming.

Oh yeh, the short tires mean I can get in the garage at work which is a big plus!

Americamper wrote:
My truck has the old style speedo so I'm thinking that it is not re-programmable. My MPG has recently went down a little, so If I understand some of the posts correctly then the tires may be the reason.

You may be able to reprogram it yourself. I know you can do it on a 97 F-250 using the ignition and the odometer button. Check on dieselstop.com I am sure there is info there.

Many people have mentioned calibrating the speedometer, but I prefer to calibrate the odometer.

I have never driven a rig where the speedo and odometer match, some are off by 2-3 mph. My DD VW is off by 2-3 mph at 70 but the odo is dead accurate.

For calculating mileage, maintenance, and true odometer readings I suggest calibrating the odometer over a 100-200 mile stretch and learning the small speedo error.

My sig truck is calibrated within 0.3%, and reads about 1mph fast at 70. It's more accurate IMHO, and keeps the wife from speeding and I just set it based on the speedo and I'm not going as fast.

Many manufacturers post rev/mile data, this is far more accurate to start with than tire math.

wintersun wrote:
Check your speed with a GPS or smartphone with GPS and see how accurate your speedometer/odometer are with the stock tires. I did not realize it but with a camper load the actual speed was 2 MPH less than indicated thanks to the tires squatting and not being as tall. Only takes a 1/2" of squat to affect the speedometer accuracy by 4%.

If you are getting a 1/2" of squat your tires are overloaded or under inflated.

I will see a small change empty to loaded, but never even 1%.

Check your speed with a GPS or smartphone with GPS and see how accurate your speedometer/odometer are with the stock tires. I did not realize it but with a camper load the actual speed was 2 MPH less than indicated thanks to the tires squatting and not being as tall. Only takes a 1/2" of squat to affect the speedometer accuracy by 4%.

On the ford f-series and excursions, the speed sensor is at the ring gear. Ford has different wheel and tire sizes but all trucks use the same PCM's. So a ford dealer for around $50.00 can reprogram your PCM to correct the tire size.

Also, if you are looking at the Overhead display for MPG, any change in tire size will throw this off. So you would need to hand calculate the MPG's. My truck is off by 5mph with 34" tires on an F350 with 4.10 gears. My MPG's are off as well by 3-4mpg's. But this is also and Average, so that has to be taken into account.

Americamper wrote:
When is it necessary to get a speedo correction for larger tires? I went from a 265/75 to a 285/75 both 16 inch. Would the height even change?

On your year truck, you will most likely find that with the 285's your speedo will read accurately, since most Ford superduty's actually read fast with 265's... That is what I have found with two 2000 Excursions, a 2001 F350 dually, a 2002 F350 SRW, and a 2005 Excursion.
Hope this helps!
Garry

On edit, the odometer and trip, both were affected, and require that I add 5% to the miles to adjust for the changes to the odometer when hand calculating mileage.

Garry