larger wheel/tire effect on odometer?

[White Whine]

not sure if I'm way off base, but I posed this question in my low mpg thread but figured I'd ask directly here. I have an 03 L and searched but couldnt find how big the stock wheels and tires are, but the dealer slapped on 20" wheels and 295/50/R20 tires and I had a thought. Assuming these are bigger than the stockers, will these affect the speedometer and odometer readings in my car? will it lead the ecu to read I've traveled less distance than I really have or is that backwards? I have a predator and wonder if I should check to see what tire size its set for, whats the best way to measure diameter? tape measure? am I off base with this thought?

 

[White Whine]

so...none of the 10 people who read this knew or they were so baffled at my idiocy that they didn't want to comment?

 

[DEVOURER]

stock tire revolutions per mile is like 731 or close. call up discount tire and ask them the tire revolutions per mile of your brand/style of tire. hook up your predator and under axle ratio is tire revolutions per mile. change it to whatever they said your number is!

 

[Dzznttz]

Stockers I believe are 295/45/R18

 

[johnny-longtors]

http://www.powerdog.com/tiresize.cgi

On a 17" tire every change in height of 1" = ~3MPH on the speedo, and comensurate effect on odo; obviously, this ratio changes with base height (based on pi)

 

[White Whine]

ok...so I'm not crazy. thanks guys, ill look into it further now!

 

[RidnRodsLightng]

so...stockers are 18" and we have 20's....so Johnny Longtors - are you saying tht we are actually going 6MPH faster than our speedometer says....or slower...or maybe i am the crazy one?

 

[White Whine]

stock tire revolutions per mile is like 731 or close. call up discount tire and ask them the tire revolutions per mile of your brand/style of tire. hook up your predator and under axle ratio is tire revolutions per mile. change it to whatever they said your number is!

in the predator user manual under tire size it says "to get revs per mile use overall tire diameter in inches. 20168 divided by tire diameter = revs per mile." That said, is the correct way to measure diameter just using a tape measure from the ground in the center of the tire to the highest point on the top? if there isn't a more sophosticated way to do it, my "overall tire diameter" is 31.5", which in the forumula would then be 20168/31.5 = 640.25 revs per mile. does that sound right? thats a lot less that the stockers if thats the case, so maybe this really is why i only got 11.75mpg, I actually got more but couldnt calculate it due to incorrect odo/speedo reading.

 

[johnny-longtors]

so...stockers are 18" and we have 20's....so Johnny Longtors(o) - are you saying tht we are actually going 6MPH faster than our speedometer says....or slower...or maybe i am the crazy one?

No - you have to have the actual tire sizes and run them through the calculator I pointed out. It's a tool to find overall tire dimensions based on specs.

You can have a 20" tire that has the same overall dimensions as a 17" - I know, I have them on my Mustang. My point was that the site will help you find out the possible differences in upsizing/downsizing the rubber you run.

Take a look at the link - it's fairly self explanatory

Note the site gives revs/mile

Also note that it's all relative - different tire MFGRs have different specs. TireRack does a good job of showing actual "per brand" specs

 

[White Whine]

looks about right to me...
Specification 295/50-20 vs 295/45-18
Sidewall 5.8" vs 5.2"
Radius 15.8" vs 14.2"
Diameter 31.6" vs 28.5"
Circumference 99.3" vs 89.4"
Revolutions 638/mi vs 709/mi

so, I need to go change it with the predator, cause larger tire = less revs to go equal distance as would with smaller stockers, therefore my speedo is reading slower than I'm going (and I thought I was crazy!) thus odo is reading less mileage than I'm doing and lower mpg...It would seem, anyway.

 

[DEVOURER]

looks about right to me...
Specification 295/50-20 vs 295/45-18
Sidewall 5.8" vs 5.2"
Radius 15.8" vs 14.2"
Diameter 31.6" vs 28.5"
Circumference 99.3" vs 89.4"
Revolutions 638/mi vs 709/mi

so, I need to go change it with the predator, cause larger tire = less revs to go equal distance as would with smaller stockers, therefore my speedo is reading slower than I'm going (and I thought I was crazy!) thus odo is reading less mileage than I'm doing and lower mpg...It would seem, anyway.

i'm pretty sure that the stocks are ~730. it could be the math or just tire brand. either way... i'd just call the tire manufacturer or distributor and find the exact Revolutions for that size and brand of tire. example. my stock f1s were set at 733 my 305/40/20 is set at 702. hope that helps :burnout:

 

[White Whine]

I did a quick measure myself this morning and not getting too exact it looked like 31.5" when I entered the tire info it spit out 31.6...probably more correct, I'll go with their figure and see what happens.

 

[BMWBig6]

i'm pretty sure that the stocks are ~730. it could be the math or just tire brand. either way... i'd just call the tire manufacturer or distributor and find the exact Revolutions for that size and brand of tire. example. my stock f1s were set at 733 my 305/40/20 is set at 702. hope that helps :burnout:

How were you able to set it at 702? My Predator has a "fake" slider and the increments are huge. The next one down below 733 (stock) is like 685. Nothing available in between. Did you have your tuner program it directly into the tune, or do I have an older firmware version in my Predator?

 

[simshahh]

How were you able to set it at 702? My Predator has a "fake" slider and the increments are huge. The next one down below 733 (stock) is like 685. Nothing available in between. Did you have your tuner program it directly into the tune, or do I have an older firmware version in my Predator?

once i set mine with the predator i was unable to get it back to 733 again. the number changes every time i make a change. for instance, if i set it at 600 (random number, not actual), do the programming, then set it at 700, the chances of me getting it back to exactly 600 probably wont happen. but when i did a complete reset, it was back to 733 again no problem.

another reason he might be able to get his to 702 is he has a 2000, and you have an 03. the gearing is different on those trucks and the DSP has the 2000 set at 3.55 and your truck at 3.73.

 

[ducman996]

I did a quick measure myself this morning and not getting too exact it looked like 31.5" when I entered the tire info it spit out 31.6...probably more correct, I'll go with their figure and see what happens.

When I calculate your tire height based on tire size I get 31.6"

As mentioned above adjust your tune with the predaor to the closeset rev/mile setting to 638 rev/mile

If your speedo is on stock settings it will be showing your speed about 10% slower than actual, same with the odometer.

((295x0.50)/25.4)x2+20"=31.61"

20168/31.6= 638 rev/mile

Stock F1's = 28.45" dia = 709 rev/mile

 

[CowlSS]

I did a quick measure myself this morning and not getting too exact it looked like 31.5" when I entered the tire info it spit out 31.6...probably more correct, I'll go with their figure and see what happens.

Damn, those are some tall tires.