Gearing Question

WildWilly

True Classic
Trying to figure out the gearing of my X. If I'm turning at 3500 RPMs at 55 MPH, what final drive ratio do I have? From what I can determine, the options are 4.42, 4.08 and 3.76. To the best of my knowledge, I have a 74 tranny connected to a 74 1300 engine. Thanks.
 
Trying to figure out the gearing of my X. If I'm turning at 3500 RPMs at 55 MPH, what final drive ratio do I have? From what I can determine, the options are 4.42, 4.08 and 3.76. To the best of my knowledge, I have a 74 tranny connected to a 74 1300 engine. Thanks.
Are your RPM and MPH numbers from the tachometer and speedometer in the car? It's a bit dicey to back calculate from these numbers if that is the case, as neither the tachometer nor the speedometer are particularly accurate.
 
my bad....

I guess those details would be good to have:) The speed is from my GPS, so I'm pretty sure that's pretty well exact. The RPM??? Who knows, the tach says 3500 and one would hope its reasonably close to correct. The tire size is: 185/60-13. 1300/4-spd. Thanks....
 
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All 74 trannys

came with a 4:08 ring and pinion and a .96 4th gear. Who knows if it has been changed inside

Cheers
Charlie
 
55 mph and 185/60/13 tires with a 3.76:1 differential and 0.959:1 4th would give ~3334 RPM

55 mph and 185/60/13 tires with a 4.077:1 differential and 0.959:1 4th would give ~3615 RPM

55 mph and 185/60/13 tires with a 4.42:1 differential and 0.959:1 4th would give ~3919 RPM

So 4.077:1 is closest (3% off from what you are seeing). The 3.76:1 numbers are 5% off from your reading, and certainly possible. Do you have reason to believe the differential has been replaced with a non-stock ratio one?
 
Reasoning...

Thanks so much for the info, very helpful. The reason for the question is that I currently have 2 X1/9s and they run at different RPMs on the highway, one os a 74 and one a 78, the 74 tranny runs at the lower RPMs (more comfortable) and I am wanting to change the gearing in the 78 to be more comfortable as well. I have already ordered the 4.08 gears from MWB. My mechanic just wants to swap trannys, but I'd rather keep the 74 motor with the 74 tranny and the same for the 78, so we will install the 4.08 gears in the 78 tranny. My 78 must have the 4.42 gears at it runs at a higher RPM on the highway, cruising at 75 mph, we are over 5,000 rpm which is not exactly comfortable. Just an FYI for others reading, I had the 74 up to 95 MPH yesterday at about 6,000 RPM. She loves to fly:)
 
bill the 3rd gear synchro on the 74 is beginning to show signs of wear as it doesn't quite mesh perfectly all the time from my experience of driving it.

rev matching was important to preserve it. so just think about that as another factor in your decision making process. so i wouldnt just dump it in with your new motor becuase i'm sure with the enhanced juice you have it will chew it up in no time.

other than that it is a stock unit with 4.08 final.

if you are putting the 4.08 in your 78 trans and using that i would inspect the dogs and synchros and look at doing those since you're opening it up for the gears. might be best to do the 3rd gear ones regardless...
 
3rd Gear

Thanks Andrew, we have noticed the 3rd has to be handled gently in the 74 tranny. The 78 tranny has already had the 4.08 final installed and all the gears were inspected and seem to be fine. Hoping to be back on the road before the end of May. Cheers....
 
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