Effect of gear ratio on tow capacity ratings

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Author Topic:   Effect of gear ratio on tow capacity ratings

Buckda posted 11-02-2003 06:17 PM ET (US)
This may be a simple question, but I am hoping that someone with greater mechanical knowledge can help me out.
Why are the tow ratings for a given vehicle different based on rear axle gear ratios?
For instance, I have a 2002 Ford Explorer V-6. First, I am unsure of which gear ratio I have, because I purchased the vehicle as a "used/new" from the dealer. (titled new but it had "dealer miles" on it.)
The Ford Web site lists the capacities as follows:
for the 3.55 axle - rating is 3,400 pounds
for the 3.73 axle - rating is increased to 5,780 pounds
This is important to me (and hopefully to others on this site) because it determines how large of a boat I can tow and remain inside the ratings (i.e. am I limited to an 18' Outrage because of my rating?)
Second, how do I determine what axle I have? All Explorers come with a Class II Hitch as standard (a sad situation!) so there is no apparent way to tell at first glance. My owners manual does not specify which model that I have - it simply covers all the bases for all vehicles, including the V-8.
Thanks in advance to anyone who has a greater knowledge or insight in this area - you will help me narrow down (or perhaps widen) my search.
Dave
diamondjj posted 11-02-2003 07:20 PM ET (US)
Assuming rear wheel drive. Number of drive shaft revolutions per one complete revolution of the rear wheel(tire) making sure both wheels turn at the same time . So a 3.55 axle would revolve 3.55 times per one complete revolution of your rear wheel.
"Why are the tow ratings for a given vehicle different based on rear axle gear ratios?"
Because the higher the number, ie more revolutions per one complete revolution of the drive wheels, the easier it is for your engine and transmission to move the load while putting less strain on the drive train allowing higher towing capacities. The higher the ratios, the less mileage you get, because your engine is turning higher rpms per one revolution of your wheel.
"Second, how do I determine what axle I have?"
For the GM trucks, on the inside of the glove compartment door, you will find the order codes for your particular vehicle. You can give them to the dealer and he can interpret the codes and tell you. The Fords may have the same thing. Other than that, you have to count the number of revolutions of the drive shaft per one complete revolution of the wheel or remove the rear axle cover and count the teeth for a more accurate count.
Moe posted 11-02-2003 09:25 PM ET (US)
Perhaps this will help:
http://www.drivetrain.com/Fordtrkratio_posidata.html
Lower axle ratios (higher numerically) not only multiply the driveshaft torque more, but for any given road speed, they put the engine rpm higher, and closer to its torque peak, which is pretty high on a gasser.
--
Moe
Moe posted 11-02-2003 09:40 PM ET (US)
Let me add this... the tow rating is the actual vehicle weight subtracted from its Gross Combined Weight Rating (GCWR). I don't have the 2002 data, but I believe the 2004 data is the same. You'll find it here:
https://www.fleet.ford.com/products/rv_trailer_towing/2004/2004RVTrailer_Escape_Explorer.pdf
As you can see, the GCWR of the 3.55:1 V6 is 8,000 lbs, and a "tow rating" of 3400 lbs assumes the Explorer doesn't weigh more than 4600 lbs. If it does, when fueled and loaded with passengers and cargo, the tow rating is LESS than 3400 lbs. Read the note at the top of the page. Tow rating is reduced by the weight of options, passengers and cargo in the vehicle.
The 3.73:1 V6 has a GCWR of 10,000 lbs, and a tow rating of 5380, which assumes the Explorer weighs no more than 4620. Again, if the Explorer weight exceeds that, towing capacity is reduced.
"Tow ratings" are determined with the lowest optioned vehicle possible, with an empty fuel tank, usually, only a 150 lb driver in the vehicle.
Fuel your Explorer up, load it with the passengers and cargo you expect to be in it, and take it to a certified scale, such as a CAT scale.
http://www.catscale.com
That way you'll know how much you have left out of your GCWR for gross trailer weight, as well as how much you have left out of your Gross Vehicle Weight Rating (GVWR) and rear Gross Axle Weight Rating (GAWR) for trailer tongue weight. Keep in mind when considering the rear GAWR, that besides carrying the tongue weight, it will also be carrying some weight that is leveraged off the front axle onto the rear axle, by the tongue weight.
--
Moe
jimh posted 11-03-2003 08:30 AM ET (US)
On cars assembled by General Motors there is usually a "build sheet" (often in the glove compartment or on the trunk lid) which contains all of the Regular Production Order codes (RPO codes) that have been used to assemble the car. You can usually deduce the rear end ratio from interpretation of these codes.
On Ford vehicles, I have been told that such a list also exists, but I have not been able to locate it on the one Ford vehicle that I own. I have been told that it is sometimes located under the rear passenger seat bench.
One solution to determining your actual rear axle gear ratio is this:
--attach a piece of tape to the drive shaft; attach a piece of tape to a rear wheel. These will be markers used to count revolutions.
--jack up one rear wheel of the vehicle;
--rotate the rear wheel while observing the drive shaft, noting the number of rotations of both. A higher number of rotations will provide more accurate results;
--compute the numerical gear ratio from your observations.
I have not actually tried this, but I assume it would work.
jimh posted 11-03-2003 08:31 AM ET (US)
Also, it is often possible to deduce the rear end axle ratio from interpretation of the Vehicle Identification Number (VIN). Check for this on your car's VIN. I believe this is possible with certain Ford vehicles.
BillD posted 11-03-2003 08:48 AM ET (US)
Buckda:
On the inside of the doorjamb should be the axle ratio.
I have a 2003 Explorer and when I bought it my understanding was that if the truck was assembled with the 3.73 axle they put the Class III hitch on. So if you have the Class II hitch you have the 3.55 axle in all likelihood.
Most Ford dealers' websites will allow you to look at the vehicle "sticker" for any vehicle for sale. If you look at the stickers for all of the ones with the class III hitch I bet they will all have the 3.73 axle and vice versa.
I suspect that the real reason they do this is for gas mileage. The 3.55 gets better gas mileage and in order for Ford to meet the Federal fleet gas mileage statistic requirements they can not afford to give up a higher mileage rate on one of their most popular vehicles.
I bought my truck on one of the last days of 5 year 0% financing. Several dealers I contacted did not even have any of the 3.73 vehicles in stock.
Crabby Mike posted 11-03-2003 11:12 AM ET (US)
Buckda,
As a novice boater I recently took a �safe boater course� from the local power squadron and picked up a copy of �Your Guide to Towing� published by �Trailer Boats� in conjunction with Ford Trucks, Sea Ray and Jayco. They have a pretty good explanation of axle ratio as are the forum replies, but what I found interesting is the discussion on the need for Oil and Transmission Coolers to reduce the risk of major damage to your tow vehicle.
The Towing Guide is on-line at: www.iboats.com/sites/trailerboats/site_page_1483/article_page_162.html

Mike
Perry posted 11-03-2003 12:25 PM ET (US)
BillD has it correct. Fords have the axle code located in the driver's side door jamb. My F150 has it listed as H9 which is a 3.55 limited slip. Hope this helps.
Tom2697 posted 11-03-2003 12:40 PM ET (US)
Contact a dealer with your VIN number. This will allow them to access ALL the build information (including the knickname of the person who assembled the 3rd to last lug nut on the right rear tire.) Seriously, the FMVSS (Federal Motor Vehicle Safety Standards) require that all info be tracked on every vehicle in the event that a safety defect is found. My company can track the time, date, and operator for any vehicle system that we made back to at least 10 years.
djahncke posted 11-03-2003 01:15 PM ET (US)
Dave,
If you email your VIN I will look it for you.
-Don
triblet posted 11-03-2003 09:46 PM ET (US)
jimh, that method of computing the axle ratio by jacking
ONE wheel will be off by a factor of two on an open
differential (the free wheel will turn twice the revs it
would have if both wheels are turning), and won't work for a
limited slip at all (no slip).

Chuck
cmarques posted 11-03-2003 10:09 PM ET (US)
GM vehicles have a 'spid' label either in the glove box or on the spare tire cover in trunk with all the production codes. Usually look for the 'G' codes g60 or g80 usually show a limited slip differential and a code like gu6 is the axle ratio- if I remember right a gu6 is a 3.42 on GM vehicles.
Buckda posted 11-04-2003 09:26 AM ET (US)
Thank you for the replies -
I will look for the code on the inside of the door jam -
Mine is a 4-wheel-drive that auto-engages, so I'd have to jack up the whole vehicle in order to get a "spin test" as suggested.
Thank you for the thorough explanation of the effects (and mechanics) of the different gear ratios - it is very helpful.
Moe posted 11-04-2003 02:59 PM ET (US)
Dave, once you get the axle ratio, fuel it, load it, weigh it. That's the only way you'll really know how much capacity you have left if you want to stay under the ratings.
--
Moe

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Author	Topic: Effect of gear ratio on tow capacity ratings
Buckda	posted 11-02-2003 06:17 PM ET (US) This may be a simple question, but I am hoping that someone with greater mechanical knowledge can help me out. Why are the tow ratings for a given vehicle different based on rear axle gear ratios? For instance, I have a 2002 Ford Explorer V-6. First, I am unsure of which gear ratio I have, because I purchased the vehicle as a "used/new" from the dealer. (titled new but it had "dealer miles" on it.) The Ford Web site lists the capacities as follows: for the 3.55 axle - rating is 3,400 pounds for the 3.73 axle - rating is increased to 5,780 pounds This is important to me (and hopefully to others on this site) because it determines how large of a boat I can tow and remain inside the ratings (i.e. am I limited to an 18' Outrage because of my rating?) Second, how do I determine what axle I have? All Explorers come with a Class II Hitch as standard (a sad situation!) so there is no apparent way to tell at first glance. My owners manual does not specify which model that I have - it simply covers all the bases for all vehicles, including the V-8. Thanks in advance to anyone who has a greater knowledge or insight in this area - you will help me narrow down (or perhaps widen) my search. Dave
diamondjj	posted 11-02-2003 07:20 PM ET (US) Assuming rear wheel drive. Number of drive shaft revolutions per one complete revolution of the rear wheel(tire) making sure both wheels turn at the same time . So a 3.55 axle would revolve 3.55 times per one complete revolution of your rear wheel. "Why are the tow ratings for a given vehicle different based on rear axle gear ratios?" Because the higher the number, ie more revolutions per one complete revolution of the drive wheels, the easier it is for your engine and transmission to move the load while putting less strain on the drive train allowing higher towing capacities. The higher the ratios, the less mileage you get, because your engine is turning higher rpms per one revolution of your wheel. "Second, how do I determine what axle I have?" For the GM trucks, on the inside of the glove compartment door, you will find the order codes for your particular vehicle. You can give them to the dealer and he can interpret the codes and tell you. The Fords may have the same thing. Other than that, you have to count the number of revolutions of the drive shaft per one complete revolution of the wheel or remove the rear axle cover and count the teeth for a more accurate count.
Moe	posted 11-02-2003 09:25 PM ET (US) Perhaps this will help: http://www.drivetrain.com/Fordtrkratio_posidata.html Lower axle ratios (higher numerically) not only multiply the driveshaft torque more, but for any given road speed, they put the engine rpm higher, and closer to its torque peak, which is pretty high on a gasser. -- Moe
Moe	posted 11-02-2003 09:40 PM ET (US) Let me add this... the tow rating is the actual vehicle weight subtracted from its Gross Combined Weight Rating (GCWR). I don't have the 2002 data, but I believe the 2004 data is the same. You'll find it here: https://www.fleet.ford.com/products/rv_trailer_towing/2004/2004RVTrailer_Escape_Explorer.pdf As you can see, the GCWR of the 3.55:1 V6 is 8,000 lbs, and a "tow rating" of 3400 lbs assumes the Explorer doesn't weigh more than 4600 lbs. If it does, when fueled and loaded with passengers and cargo, the tow rating is LESS than 3400 lbs. Read the note at the top of the page. Tow rating is reduced by the weight of options, passengers and cargo in the vehicle. The 3.73:1 V6 has a GCWR of 10,000 lbs, and a tow rating of 5380, which assumes the Explorer weighs no more than 4620. Again, if the Explorer weight exceeds that, towing capacity is reduced. "Tow ratings" are determined with the lowest optioned vehicle possible, with an empty fuel tank, usually, only a 150 lb driver in the vehicle. Fuel your Explorer up, load it with the passengers and cargo you expect to be in it, and take it to a certified scale, such as a CAT scale. http://www.catscale.com That way you'll know how much you have left out of your GCWR for gross trailer weight, as well as how much you have left out of your Gross Vehicle Weight Rating (GVWR) and rear Gross Axle Weight Rating (GAWR) for trailer tongue weight. Keep in mind when considering the rear GAWR, that besides carrying the tongue weight, it will also be carrying some weight that is leveraged off the front axle onto the rear axle, by the tongue weight. -- Moe
jimh	posted 11-03-2003 08:30 AM ET (US) On cars assembled by General Motors there is usually a "build sheet" (often in the glove compartment or on the trunk lid) which contains all of the Regular Production Order codes (RPO codes) that have been used to assemble the car. You can usually deduce the rear end ratio from interpretation of these codes. On Ford vehicles, I have been told that such a list also exists, but I have not been able to locate it on the one Ford vehicle that I own. I have been told that it is sometimes located under the rear passenger seat bench. One solution to determining your actual rear axle gear ratio is this: --attach a piece of tape to the drive shaft; attach a piece of tape to a rear wheel. These will be markers used to count revolutions. --jack up one rear wheel of the vehicle; --rotate the rear wheel while observing the drive shaft, noting the number of rotations of both. A higher number of rotations will provide more accurate results; --compute the numerical gear ratio from your observations. I have not actually tried this, but I assume it would work.
jimh	posted 11-03-2003 08:31 AM ET (US) Also, it is often possible to deduce the rear end axle ratio from interpretation of the Vehicle Identification Number (VIN). Check for this on your car's VIN. I believe this is possible with certain Ford vehicles.
BillD	posted 11-03-2003 08:48 AM ET (US) Buckda: On the inside of the doorjamb should be the axle ratio. I have a 2003 Explorer and when I bought it my understanding was that if the truck was assembled with the 3.73 axle they put the Class III hitch on. So if you have the Class II hitch you have the 3.55 axle in all likelihood. Most Ford dealers' websites will allow you to look at the vehicle "sticker" for any vehicle for sale. If you look at the stickers for all of the ones with the class III hitch I bet they will all have the 3.73 axle and vice versa. I suspect that the real reason they do this is for gas mileage. The 3.55 gets better gas mileage and in order for Ford to meet the Federal fleet gas mileage statistic requirements they can not afford to give up a higher mileage rate on one of their most popular vehicles. I bought my truck on one of the last days of 5 year 0% financing. Several dealers I contacted did not even have any of the 3.73 vehicles in stock.
Crabby Mike	posted 11-03-2003 11:12 AM ET (US) Buckda, As a novice boater I recently took a �safe boater course� from the local power squadron and picked up a copy of �Your Guide to Towing� published by �Trailer Boats� in conjunction with Ford Trucks, Sea Ray and Jayco. They have a pretty good explanation of axle ratio as are the forum replies, but what I found interesting is the discussion on the need for Oil and Transmission Coolers to reduce the risk of major damage to your tow vehicle. The Towing Guide is on-line at: www.iboats.com/sites/trailerboats/site_page_1483/article_page_162.html Mike
Perry	posted 11-03-2003 12:25 PM ET (US) BillD has it correct. Fords have the axle code located in the driver's side door jamb. My F150 has it listed as H9 which is a 3.55 limited slip. Hope this helps.
Tom2697	posted 11-03-2003 12:40 PM ET (US) Contact a dealer with your VIN number. This will allow them to access ALL the build information (including the knickname of the person who assembled the 3rd to last lug nut on the right rear tire.) Seriously, the FMVSS (Federal Motor Vehicle Safety Standards) require that all info be tracked on every vehicle in the event that a safety defect is found. My company can track the time, date, and operator for any vehicle system that we made back to at least 10 years.
djahncke	posted 11-03-2003 01:15 PM ET (US) Dave, If you email your VIN I will look it for you. -Don
triblet	posted 11-03-2003 09:46 PM ET (US) jimh, that method of computing the axle ratio by jacking ONE wheel will be off by a factor of two on an open differential (the free wheel will turn twice the revs it would have if both wheels are turning), and won't work for a limited slip at all (no slip). Chuck
cmarques	posted 11-03-2003 10:09 PM ET (US) GM vehicles have a 'spid' label either in the glove box or on the spare tire cover in trunk with all the production codes. Usually look for the 'G' codes g60 or g80 usually show a limited slip differential and a code like gu6 is the axle ratio- if I remember right a gu6 is a 3.42 on GM vehicles.
Buckda	posted 11-04-2003 09:26 AM ET (US) Thank you for the replies - I will look for the code on the inside of the door jam - Mine is a 4-wheel-drive that auto-engages, so I'd have to jack up the whole vehicle in order to get a "spin test" as suggested. Thank you for the thorough explanation of the effects (and mechanics) of the different gear ratios - it is very helpful.
Moe	posted 11-04-2003 02:59 PM ET (US) Dave, once you get the axle ratio, fuel it, load it, weigh it. That's the only way you'll really know how much capacity you have left if you want to stay under the ratings. -- Moe