Did my neighbor get ripped off??

Facts - He has a 2001 F250SD 4x4 with the 7.3. Stock gear ratio 3.73 with 31 inch tires. He did some lift work and we lifted it 14 inches. He placed 37" SuperSwamper tires on it (going to get bigger tires). He took it to a drive train specialist and had (allegedly) 4.56's placed in the differentials. The truck just doesn't seem to have the umph it should so we took it out on the highway with my GPS and here is what we got.

30mph speedometer= 35 actual
40 mph speedometer = 47 mph actual
50 mph (1800 rpm) = 58.7 mph actual
55 mph speedometer (2000 rpm) = 64.4 mph actual
60mph speedometer (2200 rpm) = 70.3 mph actual

While the shifting points seemed okay, the speedometer does not seem to reflect a correct gear ratio change.

What gears do you all think he actually got installed. He was charged $1300.00 for the work and gears.

Need opinions please...

 

speeds are only off about about 1-3 mph according to my calculations. This can be +/- 1-2 mph error with the speedometer in the actual vehicle and measurinng veh or device.

 

im not sure about the speedo but his rpm and and speed seam to be in the correct range.

Tony

 

Well changing the gear ratio in the differentials will not make the speedometer correct. To correct the speedometer you need to get the gears for the transmission speed sensor or you can do it with a computer tune.

As far as if the correct gears were put in. From the RPM's you gave me at 65 m.p.h. NO, that does not sound like 4.56 gears.

TO be sure. Jack up the rear end of the truck so BOTH rear tires are off the ground. Put a piece of masking tape on the side wall of the tire. Also put one on the rear drive shaft. Now, count the rotations of the rear drive shaft for ONE turn of the tire. The rear drive-shaft should turn 4.5 times per rotation of the rear tire.

Let me know!! Hope this helps.

 

He'll need a Superlifts Tru Speed unit to correct the speedo.

 

The speedo will be off because of the change in tire size. I don't know where the speed sensor is on SuperDuty trucks, but on my '92 F350 with 10.25" rear axle, the speed sensor is in the axle housing and reads from a tone ring on the ring gear. I changed from 3.55 to 4.56, so it was a noticeable difference in power. My gear change did not affect my speedometer, as the ring gear (and tone ring) turns with the axleshaft. My 33" tires did cause the speedometer to be a little off though.

You didn't state what transmission your friend has in his truck or what gear you were in when you made those measurements (overdrive or direct high gear) so it's really not possible to figure out what the rpms should be, as overdrive ratios are not the same on every transmission and some automatics have a lockup torque converter and others dont, which effects engine rpms.

Here is the formula for figuring engine rpms per tire size and gear ratio, assuming a manual transmission in 1:1 high gear:

RPM = (MPH X Gear Ratio X 336) / TireDia.

Using your information the equation should look like: (60mphx4.56x336)/37"=2485rpm. A .78:1 overdrive ratio would make the rpms = 1939rpm. A .80:1 OD would be 1988rpm. Torque converter slip could raise the engine speed by 200-400rpms. A difference in actual tire loaded radius will also affect rpms, as most tires are not actually the exact size that is stated on the sidewall and air pressure and loaded truck weight also affect the loaded radius.

One can re-arrange the equation to solve for gear ratio as well:

GearRatio = (RPM X Tire Dia) / (MPH X 336)

Using your numbers: (1800rpm X 37) / (58.7 X 336) = ~3.38 ..and.. (2000 X 37) / (64.4 X 336) = ~3.42. Obviously there was an overdrive gear involved here and also the engine rpms were not recorded exactly, as both equations should have resulted in the same gear ratio otherwise. Assuming an OD ratio of .76:1.., 3.42 / 0.76 = 4.50. It appears he (could) have 4.56 gears in that truck.

 

Sorry I didn't say what tranny he has. When we did the tests his truck has an auto tranny and we were in over dirve at the time the rpm were recorded. I did notice that his shift points corrected out nicely though.

He says he wished he would have gotten the 4.88's now instead of the 4.56's.

I used the formula (new tire diameter / old tire diameter x the gear ratio = new gear ratio the vehicle should have)

37's/ 31's x 3.73 = 4.45 ratio needed to bring back to stock performance. Since there is not 4.45's we went with the 4.56's.

I am in the same situation he is but I haven't done my gear change yet. I tow a fairly heavy boat and think I should do the 4.88's. I am afraid that 4.88's will be much worse on gas milage than the 4.56's though. Although with my 3.73's and 37's SS tires my milage is about 9.4 so anything should be an improvement.

Today we also plan on lifting the rear axle off the ground and turing the rear tires one revolution and counting how many times the drive shaft turns. If it has 4.56's then the dire shaft will(should) turn 4.56 times for each revolution of the rear tire.

Thanks for everyones help and opinions...

Doug D

 

Stupid double post button.

 

That sounds right to me. I had 37's when I first did my gears and I ran about 2200 at 60 or so. I would say that with a 14" lift he is going to be putting on 40's-44's so yeah,he should have put on 4.88's or even 5.13's.

 

Do you think that 4.88's are too much if I stay with the 37's on my truck? Or should I go with the 4.56's to get back, or as near as possible to stock spec's??

Thanks

DD

 

4.88's will put you at about a 3.79 final drive ratio, just a bit higher than stock.

 

Then that is where I want to be - 4.56's would be too little...

What formula do you use to figure that?

Thanks

DD

 

Don't try to do it yourself, it's a pain. Go to intercotire.com and the go to calculators and use the gear ratio calculator.Use 31"s for the tire(stock sd tires)Or go to 4lo.com and use his calcs. They are the same.

 

Thanks -

This site - (http://www.4lo.com/) shows that to keep it at a stock setting I would need 4.56's and 4.88's would be equivalent to stock 4.10's

When I enter 31's as the stock tire size and 37's as the new tire size with current gear ratio of 3.73. It comes out to 4.45's

They are just using the formula of new tire size divided by old tire size times the current gear ratio...

 

Well I am so bad at math i could not even get it right when I have someone to do it for me!. No seriously, I did that from memory so I am glad you double checked. Sorry.