on the topic of mpg/speedometer/trip odometer

[jeep-N-montero]

Based on factory gearing for my 98 the revolutions per mile difference between the factory tire size of 265/70r15 and 33x10.50r15 tires is just under 11.5%, so this would indicate that when calculating a more accurate mpg/speed/trip odometer you should in theory add 11.5% to the number(s) you currently see. For example, if you just filled up and you got 310 miles out of that tank then you should add about 32 miles to that overall number giving you 342 miles. Or if you are driving at an indicated speed of 70 then in theory you are actually going 77-78. If my logic is flawed please feel free to say so, but it seems fairly straightforward from a common sense approach.

 

[plh]

Based on factory gearing for my 98 the revolutions per mile difference between the factory tire size of 265/70r15 and 33x10.50r15 tires is just under 11.5%, so this would indicate that when calculating a more accurate mpg/speed/trip odometer you should in theory add 11.5% to the number(s) you currently see. For example, if you just filled up and you got 310 miles out of that tank then you should add about 32 miles to that overall number giving you 342 miles. Or if you are driving at an indicated speed of 70 then in theory you are actually going 77-78. If my logic is flawed please feel free to say so, but it seems fairly straightforward from a common sense approach.

You got it correct in theory, although as factory your speedometer with the factory size tires is probably off by a couple % - I know all mine have been. If you have an Androoid phone, there are many GPS apps to get a more accurate comparison. I use "GPS Test", set your cruise at 60mph and take several seconds average from your phone and compare to the results on your speedo.

Its funny because our '93 with 33" tires is dead on now.

 

[jeep-N-montero]

Anyone else?

 

[PacS14]

I checked my speed on my 99 with 33" brand new tires right now (will decrease as they wear off) at 60 mph on the speedometer I'm actually doing 65 mph, at 65 I'm doing 70 mph (margin of error is less than 1 due to the needle on the cluster). I also tracked a short distance drive: for every 1.5 miles on the trip odometer, the truck actually moved 1.8 miles. After that I used simple algebra to calculate when my MPG.

I did not want to use more complex math and calculate the circumference percentage differential, that would require to actually measure the tire on the rim, make sure air pressure is the same in all tires, thread depth the same in all four, etc.

 

[jeep-N-montero]

I checked my speed on my 99 with 33" brand new tires right now (will decrease as they wear off) at 60 mph on the speedometer I'm actually doing 65 mph, at 65 I'm doing 70 mph (margin of error is less than 1 due to the needle on the cluster). I also tracked a short distance drive: for every 1.5 miles on the trip odometer, the truck actually moved 1.8 miles. After that I used simple algebra to calculate when my MPG.

I did not want to use more complex math and calculate the circumference percentage differential, that would require to actually measure the tire on the rim, make sure air pressure is the same in all tires, thread depth the same in all four, etc.

You can look up the revolutions per mile for most tires online that already has the tire pressure listed when tested, seems to be a more accurate measurement to me when calculating the percentage of variation between stock size tires and larger tires, in my head it does anyway.

 

[PacS14]

You can look up the revolutions per mile for most tires online that already has the tire pressure listed when tested, seems to be a more accurate measurement to me when calculating the percentage of variation between stock size tires and larger tires, in my head it does anyway.

The only problem I see is the variables used, you would have to have the same vehicle (weight), tire pressure, if it's a test done by a research company, they probably used brand new tires, etc. like plh said most vehicles already come with a margin of error.

Finding out how many miles you actually traveled before a fill up is the only problem, there are multiple ways of doing it, after that there is only one simple division to get the answer.

To be honest this is the first time I actually cared about mpg after buying new tires, the last few vehicles I had I did not care, I just drove them... I'm getting old.

 

[PA_JERO]

Buy one of these. http://www.dakotadigital.com/index....t_id=837/category_id=287/mode=prod/prd837.htm

Pretty nice unit. Easy install and dead on accurate. I was always 10mph off with 35" tires. Now it doesn't matter what tire size I run.

 

[PA_JERO]

This is also the same unit anyone will need when they swap in a rubicon 4:1 tcase into there monty. Gps speedo rules and 4:1 is just awesome.

 

[PacS14]

I'm liking that PA _ JERO, for a while I was looking at the scan gauge I might get both.
http://www.scangauge.com/

 

[TheMole]

I'm liking that PA _ JERO, for a while I was looking at the scan gauge I might get both.
http://www.scangauge.com/

I have an ultraguage, which is similar to the scangauge in that it can display the vehicle speed. Try downloading a free gps speedometer app on your smartphone to help with calibrating it in the settings. Just gotta find a nice open road to do this on. No need to spend extra cash on a separate gps unit if you're gonna get a scangauge.

 

[PacS14]

Thanks for the info TheMole. I still need to read a bunch about it, I'm pretty sure it will fit most my needs, I'm just liking some of the features of the Dakota too like not having to calibrate and get the actual accurate everything at all times, but to be honest I will buy the ultra gauge or scan gauge first and the decide.

 

[TheMole]

Thanks for the info TheMole. I still need to read a bunch about it, I'm pretty sure it will fit most my needs, I'm just liking some of the features of the Dakota too like not having to calibrate and get the actual accurate everything at all times, but to be honest I will buy the ultra gauge or scan gauge first and the decide.

Cool yea the Dakota looks like no hassle solution. Let us know what you go with!

 

[PA_JERO]

The reason I went with the dakota digital is it allowed my factory gauge to display what speed I was actually moving at. Also gives your odometer a true reading with this as well. And it fits in behind the gauge cluster so you don't even know it's there. The other part of this, is if you upgrade to a jeep rubicon transfer case in your monty you will need your speedo to work, otherwise you will no longer have OD. It uses the the vehicle speed sensor off of the rear output shaft on the t case to send the speed signal to the gauge, once a cretain speed is hit the trans control unit allows 4th gear to engage. When you eliminate the t-case with VSS, your trans becomes a 3 speed. If ever you plan on low range gears buy the dakota digital and save some money down the road.