Gearing and Tire Size advice

smokshwn · January 3, 2005

I have an 01 burb with the 8.1 in it and 3.73 axle gearing. I am considering going to either 295/75/16 or 315/75/16 tires. (33.4in and 34.6in respectively). I do very little towing, at most a trailer of 4 wheelers or motorcycles, mainly I using it for a 40/60 mix of city and highway driving (3000 mi a month). With 265/75s the power is plenty for me, a true flow intake, magnaflow exhaust and PCMforless is on its way.

In changing tire sizes I would like some opinions on the need to regear to 4.10s. Will the tire change dramatically effect either Power or MPG? Any and all opinions are welcomed.

Thanks Craig

dewfpo · January 3, 2005

You can calculate your effective gear ratio by calculating the difference in size between the "stock" tire size and the new tire size as a percentage. Then apply that percentage to your gear ratio and you will see what gear ratio you are affectively running with the new tires. You can then back calulate what ratio would give you the stock gearing "level of performance".

DEWFPO

vhato · January 3, 2005

Well, the issue of noticing Power will be in the tranny. Since you have the Suburban you could not get the Allison 1000, so you were stuck with the 4L80-E. The 4L80-E tranny is a good unit, but the ratios are built for EASY towing and transmission life, not performance and economy. That being said, combined with the 33" tires you want to run and the 3.73:1 rear axle we can move on.

The 2 important transmissions ratios are 1st and 4th (OD):

1st gear is a SLUGGISH 2.48:1. Add 3.73's and 33-35" larger tires and your Torque Monster Engine will snail along the road trying to build speed while sucking up the fuel (Won't be that bad, but you will feel it). Ever drove a 1500 with a V-8 and an Automatic? Feels quick right?? It isn't the engine, it is the 1500's 3.06:1 First gear ratio.

Next is the Overdrive or 4th gear. It is a worker ratio of .75:1. This works to your advantage with 33-35" tires and your rear axle. On long trips you will notice a Fuel Increase and your engine Torque will be better managed at higher speeds.

The 1500's have a OD .69:1 ratio and 33-35" Tires and 3.73's would require the transmission to constantly downshift to maintain speed.

So you will have a love/hate relationship between low end acceleration and highway cruising.

Since you are looking at both 295/75r16 and 315/75r16 I assume a set of BFG AT KO's are in your future. (IMO) Without any lift a 295 will appear skinny due to its increase in height and a 315 will look stuffed. I would run a 285/75r16 or a 305/70r16 (Yes it is available from BFG).

496 CUBIC INCHES · January 3, 2005

Will the tire change dramatically effect either Power or MPG? Any and all opinions are welcomed.

T

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If you stay with the 3.73s the effective gear ratio with 295s will be about 3.41,and with the 315s it will be about 3.28s. IMO your overall driving experience will suck. Gear ratios like 3.28s belong in a car not a Burb with a BB gas engine. One of the reasons I like my truck is because its fun to drive,its not a sports car,but it moves well for weighing 6400lbs. If you don't regear you'll be driving a dog.

Another question is what to regear to. IMO going to 4.10s wouldn't be enough,I'd look at 4.56s.