I have a 2002 dodge 2500 with a 5 speed. I have 315/75/R16 tires on the truck. The speedometer is off about 8 mph. I filled up yesterday 26.8 gallons and 346 miles. I think the sticker on the door said this truck came with 245/75/R16 tires. Does anyone know how to figure the correct MPG?

 

Just go to the dealer and let them change it in the truck. My dealer did it for free. It only takes about 10 min. that way there is no guessing.

 

by my calculations, the new tires increase your rolling distance by ~28.57% so I figure your mileage at about 16.6mpg for that tank...

346 + 28.57% = 444.85

444.85 / 26.8 = 16.598


I also found that your speedo should be a bit more off than 8mph (of course you didn't list at what speed that is... it would be off by a percentage, meaning the fast you go, you more it will be off...). I think that at 70 you would actually be doing 90 for example, and at 50 you would be doing 64... of course all this assuming I'm not a total moron... which I can't really be sure

 

Do you think 16.6 MPG is good for that truck? It has 4:10 gears. I have a set of 3:73. I am still kicking around the thought of swapping them out...

 

do you need the extra towing power (read: are you towing MASSIVE weight)? 3.73's will get you a bit better mileage... personally I love my 3.54's and find them plenty even for towing the toy around.

 

I will probably never tow more than 6K pounds. I am thinking about selling the tires and wheels that are on it now. I would like to go with a aluminum wheel that is closer to the stock size and a good set of hwy tires. I just don't know if the mpg difference is going to worth the change. I wonder what the average mpg on a stock 2002 dodge 2500 xtra cab diesel 4X4 5sp manual with 3:55's or 3:73's is?

 

Little-bit I think thats ok for that set up. I have an 01 and the best mpg is around 20, when towing 8500 to 9000lbs high 14's to high 15's depending on terrain.

Gary

 

Go to http://www.dakota-truck.net/TIRECALC/tirecalc.html and you can get all the info on tire size changes.

 

Does anyone know what the stock tire size was on the 2002 dodges?

 

Ratio of revs/mile is all you need. You can get the tire's revs/mile from the spec sheets or from the maker's websites. A 315 is typically ~600revs/mile, and a 245 is closer to 670ish.

So, basic ratio says: new revs/old revs=variance.

Plugging in the above guesses, you get (600revs/670revs)= 89%, so your speedo will be 11% low.

Conversely, you'll need to add 11% to your mpg calculation.

PM me with any questions. You can use the above variance to calculate the effect of different tire sizes on gear ratio, RPM, and such.



BTW-- you CANNOT calculate "rolling resistance" using the above method! The math to actually calculate rolling resistance (which involves variables of tire design, tread pattern, size, road surface texture, etc) are VERY COMPLEX and beyond our needs.

jlh

 

The stock size was 245 unless you got the tow package in which case it comes with 265's even thought the door label says 245. Just for reference, I get ~26 mpg (imperial) with 3:55 gears at 100 kph. This translates to about 21.6 mpg US.

 

Care to elaborate on your math? I could be wrong, but it looks WAY OFF.

 

[QUOTE=rfcom] with 3:55 gears at 100 kph.

Hi rfcom what weight are you pulling at 100 kph?.I`m on 4.10s at the moment and have a set of 3.55s on the garage shelf,but was thinking that at 60mph pulling 4000lbs max 3.55s might make the engine lug to much.Also in Norway and Spain there is a 50mph speed limit towing,would the 3.55s be too high a gear.Your thoughts on this please

 

That's assuming it was accurate when new. I know of two '97s that where not. Basically, check your speed error at several MPH and you can figure the % and calc as Hohn described.

Randy

 

The numbers I quoted were for driving empty. I do tow a 22' trailer that weighs around 7K and agree with yamroamer that 50 mph in 5th gear will make the engine lug too much. I say this because I know that it turns 2000 rpm at 110 kph so 80 kph is about 1450 rpm. If you don't go over 50 mph, you are probably better off with the 4:10s and 315 tires. BTW, my speedo and odo are dead on with my GPS and measured test sections on the highway, respectively.

Jim