weird question (1 Viewer)

[airon23]

We just did a soa on the 60 and put 35's on. Same springs for now. I figured, by just driving along side my wife's car that the speedometer is off by approx. 5-7miles over so if my speedo says 25 i'm really going like 30-32 mph.

Now I don't know how to figure out what that means when my mileage says I've gone 100miles. How much have I really gone?

 

[stonefly]

It's a little tricky isn't it? When your going 30 miles an hour but your speedometer says your going 25 that means that when your really going 60 your speedometer will say 50.

So your odometer is going to read approximately 5/6 of the actual distance traveled. So when you measure 100 miles you've acutally gone approximately 120 miles.

A better way to figure is out is with tire diameters.

 

[Mace]

it is a percentage of how fast you are going. it is not a constant.

You can easily be off 15% or so.

 

[airon23]

Is there any way to be exact? Just trying to calculate for gas mileage even though I know it's prolly pretty bad but just trying to figure it out anyway. Thanks for any help given.

 

[stonefly]

Yup! Take the ratio of tire diameters...

Assuming 27's are stock, you'd divide 35 by 27 and then muliply by how many miles you went.

 

[Fearnofish!]

It has to be constant becasue the change in tire diameter is constant. Stonefly is right on the money.

BTW, where did you get that bonerriffic avatar? Talk about POSTing a message!

 

[Mace]

so say at 60 you are actually going 65
Does that mean when the speedo says 10mph you are going 15??

Nope.

the change in tire size is constant, however, the distance traveled/time is not.

The faster you go the more distance you are going compared to the time spent geting there.


I have about a 10% loss in my superduty. That equates to going 55 when the speeds says 50mph. But when I am going 17 mph the speedo says 15mph.

It is the time factor that makes the difference.

 

[stonefly]

Mace you are correct and we're saying the same thing just a different way of figuring it out.

The difference in tire sizes is constant. That percent difference is constant. For your rig it's about 10% it seems that for airon23 it'ls more like 16-20%.

What size tire is stock on a 60 anyway?

 

[chitown40]

I concur with Mace. I run 31"s and I'm off just about 10%. I think stock was somewhere around 28-29" tires. 235/75/15. You need to figure out the PERCENTAGE you are off, and then you take your mileage and multiply by 1.1 (if you're 10% off), then divide by gallons.

 

[lovetoski]

Here's how to figure this out.

Use a long-ish set of measured mileposts on the freeway to determine actual distance vs odo distance.

Since we're all running larger tires, the odo distance will be shorter than the actual distance.

Now to the math part:
Divide actual distance by odo distance. Example 10 miles/8.5 miles. This =1.176, or approx 18% difference. That is, you are going 18% further than your odometer indicates.

When you next do a fillup, multiply your odo milage by 1.18 (this will give you the actual miles you travelled). Then divide this adjusted number by gallons.

 

[FJ40Jim]

Is there any way to be exact? Just trying to calculate for gas mileage even though I know it's prolly pretty bad but just trying to figure it out anyway. Thanks for any help given.

Yes. There is one way to be exact, compare the speedo to the GPS reading on a flat road at a steady speed.

The flat road requirement is because most GPS units only give speed on a horizontal plane. IOW, if you jumped off a cliff w/ the GPS, it would show a speed of 0MPH, because your Lat & Long. is not changing one bit. Neat stuff, huh?

Speedo error is inconsistent. It might be -1.5% at 30MPH and +1.8% at 60MPH. So for absolute accuracy, check the speedo/GPS at a couple of useful speeds, like 35MPH for in town and 60MPH for highway.

HTH

 

[Dirty Toad]

From what I understand there is a rather simple fix for this.