Taller tires changing the shift point

Status
Not open for further replies.

Yav8

Master Mechanic
Aug 19, 2014
275
114
43
Manitowoc wis.
I have been helping a new gbody owner with some upgrades. First its a 87 El Camino with the 200R4 trans. Motor is a vortec 350 with a cam headers carb and HEI. Wanted to change gears but was told that with that trans the shift points will be way off. So the stock tires are 195x14s and I want to up the tire size to 215/70/15s. Not sure where the shift points are right now (its in storage for the winter) but it was a 305 with the 200r4 trans. Some one must know about where it shifts. (could use help with that one) Motor is a copy of the GM 350 Ram Jet so it make peak power to 5200rpms. Not sure what the rear gear is but wanting to know what or if the biger tires will change the shift point.
 

CopperNick

Comic Book Super Hero
Supporting Member
Feb 20, 2018
3,347
3,011
113
Canada
Might also bugger the mph reading showing on the speedometer to some degree; probably not all that much. if you believe that has happened you can calculate by how much by using a stop watch and pair of fixed points like a couple of street light posts to calculate time lapse and then use the indicated speed in mph and the time from the watch to determine actual speed, then get a percentage of difference from that. Grade school math.



Nick
 

Yav8

Master Mechanic
Aug 19, 2014
275
114
43
Manitowoc wis.
Won’t change the rpm it shifts at, just the road speed
I know the speedo will be off, not what I am intrested in the most. I would like to know what the shift points might be the way it came form the factory. 305 and a 200r4 trans. Not sure on the rear gear ratio. That is what I am more intrested in to know. Also if any one know about what the El Cameno weights that would also help with the planning of the project. Those three things give me a starting point and somethint to do during these winter months. The young man is full of question on whitch way to go for his ride. Last fall the motor went in and headers 21/2 ehaust x pipe and good mufflers so thats done. So now it weight shift points tire size and gear ratio X HP. and TQ. = more questions.
 

565bbchevy

Geezer
Aug 8, 2011
9,590
12,611
113
Michigan
You can use one of these calculators to figure out the difference in MPH at a given RPM when changing tires sizes, as already stated tire size does not change the shift points in the trans only the MPH that they occur.
 
Oct 14, 2008
8,806
7,746
113
Melville,Saskatchewan
My bet is 4000 rpm full throttle, give or take a few hundred rpm. It could also be one of the transmissions that shift at 3000 rpm, I have ran across two, my 88 Cutlass with a 307 and 2.56 gears factory and a 87 model. You either need to grind the small weight or find a Monte SS or Buick GN governor. I grind the small weight and stake the spring by tapping, Lock Tite and grind the head, so it doesn't fly out. I make the weight pretty thin on sides, gives 5000+ rpm shifts. Once you decide on tire size and rear gears, swap the speedo gear, it is part of the governor. Here is a good calculator. https://www.tciauto.com/speedometer-gear-calculator
I have only seen 27 teeth black and brown 26 teeth driven gears. Both drive and driven gears along with new bullets and seals are available on Ebay. While in there, the pan needs dropped anyways, throw in a CK Performance or Transgo shift and servo to help it live behind the extra power. A bottom feed 700R4 filter and spacer from CK or deep pan with that filter or an early 4L60E filter, help keep it from uncovering on hard launches. Good luck.
 
Last edited:
  • Like
Reactions: 1 users

bracketchev1221

Royal Smart Person
Jan 18, 2018
1,386
1,599
113
I know the speedo will be off, not what I am intrested in the most. I would like to know what the shift points might be the way it came form the factory. 305 and a 200r4 trans. Not sure on the rear gear ratio. That is what I am more intrested in to know. Also if any one know about what the El Cameno weights that would also help with the planning of the project. Those three things give me a starting point and somethint to do during these winter months. The young man is full of question on whitch way to go for his ride. Last fall the motor went in and headers 21/2 ehaust x pipe and good mufflers so thats done. So now it weight shift points tire size and gear ratio X HP. and TQ. = more questions.
That’s not what I am saying. If the tire is taller, and the engine shifts at 3000 rpm it will always shift at 3000 rpm. But instead of shifting at 30 mph your gear ratio is higher so it’ll shift at 31-32 mph now. This may be extreme numbers but the point I’m trying to make.
 
  • Agree
  • Like
Reactions: 4 users

pagrunt

Geezer
Sep 14, 2014
9,122
15,255
113
Elderton, Pa
This pdf is from the GM '87 resto pack. It'll show the LG4/TH200 4R max up shift & max kick down speeds in KPH & MPH. Base rear gear was 2.41, optional was 3.08.
 

Attachments

  • Pg 66 shift points.pdf
    75.2 KB · Views: 109
  • Like
Reactions: 2 users

Yav8

Master Mechanic
Aug 19, 2014
275
114
43
Manitowoc wis.
Getting closer to what I am trying to find out. I can see two thing coming out of the El Cameno. One it will stay a nice looking driver and a mid 14 second simple bracket car. Two it get a built th350 and some 3.73 gears along with the posi and moser axrls and a cam change. Its already got the full worked vortec heads with the springs to handle .550 lift. The block is also cleranced for a stroker kit if he need the extra cubes. Should run mid 12s if its anything like the other one I have done. Still would like to hear the weight of one of these. Some one must have fun one on the scales some time. Weight with the driver would be the best help.
 

565bbchevy

Geezer
Aug 8, 2011
9,590
12,611
113
Michigan
Status
Not open for further replies.

GBodyForum is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com. Amazon, the Amazon logo, AmazonSupply, and the AmazonSupply logo are trademarks of Amazon.com, Inc. or its affiliates.

Please support GBodyForum Sponsors

Classic Truck Consoles Dixie Restoration Depot UMI Performance

Contact [email protected] for info on becoming a sponsor