Just wondering so I could calculate my fuel mileage with the new tires.

 

Mine is off about 3-4 mph. This has been verified by RADAR.

 

I have the same size tires as you on my 02 super duty. I checked my speedo with a GPS and at 100KLMs per hour [ aproximately 63 miles per hour] on the GPS my speedo read 106 KLMs per hour. Thats the only way I could think of doing it to be accurate. hope this helps

 

sorry I got that backwards. At 100KLMs on the speedo my GPS read 106 KLMs per hour

 

You're speedo will read about 9% lower than your actual speed. Mine are almost worn out and its reading 6% lower now.

 

So I'd add 9% to my odometer reading to get my actual mileage to calculate MPG.

 

i always multiply my miles by 1.1, then divide by gallons

 

if you have 3.73s its 5 mph, gps verified.

 

Yes, that is correct.

Gears don't affect the speedo. Also, it will only be 5mph at one speed and will scale up or down from there as your speed changes.

 

whatever..... 5mph, no matter what speed I go, verified by magellan street pilot.

 

It changes by a percentage of your speed, not by a fixed amount. Either your magellan or the speedo have an offset if you are seeing a fixed number. Check it at low speed and high speed, both with cruise set and you should not see the same mph difference. For instance, if you see 3mph difference at 30mph, you should see 6mph difference at 60mph (if tires are 10% different size than stock).

 

Roughly 5mph then. I have a truspeed unit now, haven't used the GPS for that reason in a long time.

 

Tire Dimensions Made Simple - Discount Tire Co.

this calculator will give you a ball park setup when dealing with different speeds.

 

I agree 100%, lots of people seem to come in and say my speedometer is off by X MPH, the fact is - it is off by a percentage like Cart here is saying.

when somebody says their speedometer is off by 5 MPH I ask them - is it off by 5mph at 10 mph? Cause if it is then it is 50% off and thats nearly impossible, however if they are doing 100 then it is 5% off - lots more likely.



anyway to the original poster - consider getting an inline speed signal correction device, truspeed, speed wizard, accuspeed

I use a Speed Wizard and my speedometer agrees with my GPS by a factor of 1%, I know it is 1% cause at 100 on the GPS my speedometer looks like it is a touch under 100



as for calculating milege.... first figure out your multiply factor - new size / old size = multiply factor.

When you go to fill up use the multiply factor multiplied by miles traveled on odometer. Use that new number to get milege - but remember your speedometer may be reading incorrectly even when stock, mine did

I would suggest tracking mileage with the GPS - then you can get a better gas mileage guesstimate

thanks for reading

kenny

 

got a link for any of those?