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Hope you can offer some help to a car problem

Paul1000RR said:
6-7km/h is caused by the tires, add in another 5-10% the speedos usually read lower and there is your answer.[/QUOT
Thanks Paul, I was kind of thinking that, apparently they are going to put new tires on it Friday, the right size, so hopefully it brings it mych closer to what it is supposed to be. It is just a pain in the *** to be seeing almost 140 and you are actually only doing 120.. I have had many many vehicles, and about 5 since I've owned a GPS and never had one so far off, it was bothersome. I will report back and let you guys know how much better it is with the proper tires on it. Thanks again for all the input.
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Jay72 said:
Paul1000RR said:
6-7km/h is caused by the tires, add in another 5-10% the speedos usually read lower and there is your answer.
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Thanks Paul, I was kind of thinking that, apparently they are going to put new tires on it Friday, the right size, so hopefully it brings it mych closer to what it is supposed to be. It is just a pain in the *** to be seeing almost 140 and you are actually only doing 120.. I have had many many vehicles, and about 5 since I've owned a GPS and never had one so far off, it was bothersome. I will report back and let you guys know how much better it is with the proper tires on it. Thanks again for all the input.
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Pls keep us posted.

If we are talking 225 70R-16 vs. 225 60r 16, then I am afraid the new tires are not going to fix anything. The problem may be less, but still be there.
 
If you really want to check out how your vehicle's speed sensor is working, measure distance. Travel a fixed distance as measured by GPS or highway distance markers, then compare it to your vehicle's odometer/trip meter (trip meter is usually better as it has a decimal place). The longer the distance, the better this works. If they match, then everything is working as designed.
 
I ran a set of 235 60 16 on my Liberty, was supposed to be 235 70 16. You do what you've got to do when the chips are down. I averaged about 5-6 KpH slow on the speedo and burnt a isht tonne of gas. A buddy of mine sold me a set of Wrangler take off wheels and tires for a song - 225 75 16. They ran 1 or 2 KpH faster than speedo according to GPS and picked up a couple of mpg too.
 
Paul1000RR said:
6-7km/h is caused by the tires, add in another 5-10% the speedos usually read lower and there is your answer.
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^I would guess this as well. The calculators say the tires you have on will give a speedo reading 6.2% too fast. Based on your post (going 106 when speedo reads 120) this accounts for about 6.5 km/h. This is only a theoretical number based on calculated tire diameter. Actual tire diameter will differ slightly from model to model so allow for some variance in this figure +/-. Now add to that the speedo's inherent inaccuracy and you should be pretty close to the 14 km/h difference you're seeing.

Inaccuracies in GPS positioning (and therefore velocity calculation) might also contribute but my feeling is the contribution would be small in most cases.
 
I'm hoping it's something as simple as changing the tires but have a suspecion there may be more of a problem. I could see if it was the 6 or so km across the board but it is such a huge difference when higher speeds. Anyway, thank you for all the great replies. I will post what the outcome is when I get home. And I will be having them take me for a ride when they are done to prove to me it's fun.

Jay
 
Just an update, So I took the van in on Friday and they ordered and put on brand new proper size tires. And sure enough that was the problem, the GPS only reads 3-4 kms off, which I am sure is normal for most vehicles.
So as much as I was worried it was a larger problem because of the difference, it wasn't and all is good.
Thanks again for the input.
 
Thanks, and I found a couple of those tire size calculators. Being on night shift for 14 days straight I wasn't reading the result it was giving me properly, it was giving me a result in mph, dumb *** me wasn't converting. I will say I was very surprised that that small of a difference in tire size would make that drastic of a change in the actual speed showing on the speedometer.
Thanks again for all the help...now back to bike stuff instead of boring minivan crap. lol
 
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