Rear End Gear ratio

[Bfrank]

I am trying to find out what my rear end gear ratio is I have a 1979 24 ft
Excella motorhome and I installed a new engine and tranny last year. I am trying to figure which set of spedo gears I need to install in the tranny.
The transmission is a 700R4 overdrive which is working well I think I'm getting
around 12 mpg with this setup and am able to keep from getting blown off the road by everyone else. So far I have traveled about 2000 miles with it and just about have all the bugs worked out of it but in order to get the speedo working correctly I need to have the rear end gear ratio which no chevy dealer has been able or interested in helping me find out so I am turning to the experts here. If anyone happens to know this info or knows who to ask that can offer assistance please chine in and Thanks in advance!

[7xp]

If my memory serves me correct, the ratio is based on how many rotations the driveshaft makes for one complete rotation of the tire. I belive the gear ratios available for Chevy trucks (14 bolts for the rear cover on the differential) would be 3.42, 3.73, 4.10, 4.56, 4.88, and 5.13.
If you place the rear of your motorhome on jackstands, block the front tires, and place the transmission in neutral with the emergency brake off, you should be able to rotate the rear tires. Now make a mark on the driveshaft where it enters the differential and on the differential so they line up. (a peice of white tape on each marked with a black line works well) Now place a block of wood next to the rear tire you are going to rotate (this is to get your reference mark close to the tire) with the marks lined up on the driveshaft and differential, make a mark on the tire and the block of wood. You will now rotate your tire one complete turn while counting the number of rotations of the drive shaft. A little less then 3 1/2 turns equals 3.42 ratio, 3 3/4 = 3.73, a little over 4 turns = 4.10 etc.
Don't forget to account for any change in tire diameter for your speedometer drive. Hope this helps. "Mac"

[Bfrank]

Thanks Mac, I will give it a try, it has the origional rims which are 16.5 inch I was thinking someone had maybe done some research on these P-30 chassis's and had the info which would save me a bunch of work but all those P-30 guys must be retired by now.

[HowieE]

Only jack up one rear tire so the other is locked to the ground. Mark the driveshaft and count the number of times the mark passes the starting point per one rev. of the rear tire. Since it will be 3 or 4 plus some fraction, that may be hidden behind the shaft, it may be more accurate to rotate the rear tire 5 or 10 times to increace the accuracy.